Okie dokie! Here it is...we are finally at the end of this torture and suffering! So first off here is a picture of my babies:
Here is little James and Tony on their first day home. You can’t really see in this photo but this plant is already pretty tall and big and I’m fairly certain that it isn’t going to grow much anymore.
Look at their stems and leaves and axillary buds! What healthy boys!
James lost some of his flowers but that’s okay because he can always grow more!
This was on a particularly rough day for James and Tony - after a week of neglect from their owner, their cells were no longer turgid and they started to wilt.
But alas, Jesus shined his light upon us and said “Water your gosh dang plant, Emily.” So I did. And now James and Tony are upright and perky again.
This is a picture I took seven hours ago. I had to put an Instagram filter on it because as a last picture, I think Tony and James deserve to feel a little more beautiful.
So, when I first brought James and Tony home, they were 17 cm tall and their leaves were 5 cm long and 3 cm wide. After nine weeks, they are 20 cm tall and their leaves are, on average, 6 cm in length and 3.5 cm wide. Although James and Tony didn’t grow much. I’m still incredibly proud of them for surviving my horrible caretaking. I feel like my growth is similar to Tony and James’s in that my knowledge of biology is slowly expanding and that the suffering will be over soon and bring about a new, happier day.
🌸Briefly explain Signal Transduction Pathways and plant hormones.
Plants receive specific signals and respond to them in ways that enhance survival and reproductive success. Signals are first detected by receptors, proteins that undergo changes in shape in response to a specific stimulus. The receptor involved in de-etiolation is a type of phytochrome, a membrane of a class of photoreceptors. Unlike most receptors, which are built into the plasma membrane, the type of phytochrome that functions in de-etiolation is located in the cytoplasm. Researchers demonstrated the requirement for phytochrome in de-etiolation through studies of the tomato, a close relative of the potato.
Receptors can be sensitive to very weak environmental or chemical signals. Some de-etiolation responses are triggered by extremely low levels of light, in certain cases as little as the equivalent of a few seconds of moonlight. The transduction of these extremely weak signals involves second messengers. These are small molecules and ions in the cell that amplify the signal and transfer it from the receptor to other proteins that carry out the response. Changes in Ca^2+ levels play an important role in phytochrome.
Ultimately, second messengers regulate one or more cellular activities. In most cases, these responses involve the increased activity of particular enzymes. There are two main mechanisms by which a signaling pathway can enhance an enzymatic step in a biochemical pathway: post-transitional modification and transcriptional regulation. Post-translational modification activates preexisting enzymes. Transcriptional regulation increases or decreases the synthesis of mRNA encoding a specific enzyme. In most signal transduction pathways, preexisting proteins are modified by the phosphorylation of specific amino acids, which alters the protein's hydrophobicity and activity. Many second messengers activate protein kinases directly. Often, one protein kinase will phosphorylate another protein kinase, which then phophorylates another and so on. Such kinase cascades may link initial stimuli to responses at the level of gene expression, usually via the phosphorylation of transcription factors. Signal transduction pathways must also have a means for turning off when the initial signal is no longer present. At any particular moment, a cell's functioning depends on the balance of activity of many types of protein kinases and protein phosphatases.
A hormone, in the original meaning of the term, is a signaling molecule that is produced in tiny amounts by one part of an organism's body and transported to other parts, where it binds to a specific receptor and triggers responses in target cells and tissues. In animals, hormones are usually transported through the circulatory system, a criterion often included in definitions of the term. The hormone concept originate from studies of animals and was adopted by plant physiologists in the early 1900s. Many modern plant biologists, however, argue that it is too limiting to describe plant physiological processes using the narrow definitions established by animals physiologists. Moreover, some signaling molecules that are considered plant hormones act only locally. Finally, there are some signaling molecules in plants that typically occur in plants at concentrations that are hundreds of thousands times greater than a typical hormone. Nevertheless, they are transported through plants and activate signal transduction pathways that greatly alter the functioning of plants in a manner similar to a hormone. Thus, many plant biologists prefer the broader term plant growth regulator to describe organic compounds, either natural or synthetic, that modify or control one or more physiological processes within a plant. Virtually every aspect of plant growth and development is under hormone control to some degree.
The idea that chemical messengers exist in plants emerged from a series of classic experiments on how stems respond to light. Any growth response that results in a plant organ curving toward or away from stimuli is called a tropism. The growth of a shoot toward light or away from it is called phototropism; the former is positive phototropism, and the latter is negative phototropism. The classic hypothesis for what causes grass coleoptiles to grow towards light is that an asymmetrical distribution of auxin moving down from the coleoptile tip causes cells on the darker side to elongate faster than cells on the brighter side.
Plant hormones are produced in very low concentrations, but a tiny amount of hormone can have a profound effect on the growth and development of a plant organ. Signal transduction pathways amplify the hormonal signal and connect it to a cell's specific responses. In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells. Some hormones mediate shorter-term physiological responses of plants to environmental stimuli. Each hormone has multiple effects, depending on its site of action, its concentration, and the developmental stage of the plant.
The term auxin is used for any chemical substance that promotes elongation of coleoptiles, although auxins have multiple functions in flowering plants. The major natural auxin in plants is indoleacetic acid, although several other compounds, including some synthetic ones, have auxin activity. Auxin is produced primarily in shoot tips and is transported from cell to cell down the stem at a rate of about 1 cm/hr. It only moves from tip to base, not in the reverse direction. This unidirectional transport of auxin is called polar transport. Auxin has a variety of effects, including stimulating cell elongation and regulating plant architecture. One of auxin's chief functions is to stimulate elongation of cells within young developing shoots. As auxin from the shoot apex moves down to the region of cell elongation, the hormone stimulates growth. Auxin also rapidly alters gene expression, causing cells in the region of elongation to produce new proteins within minutes. The polar transport of auxin is a central element controlling the spatial organization, or pattern formation, or the developing plant. Auxin transport also plays a key role in establishing phyllotaxy, the arrangement of leaves on the stem.
Cytokinins are named so because they stimulate cytokinesis, or cell division. The most common natural cytokinin is zeatin. Cytokinins are produced in actively growing tissues, particularly in roots, embryos, and fruits. Cytokinins produced in the roots reach their target tissue by moving up the plant in the xylem sap. Cytokinins, auxin, and strigolactones interact in the control of apical dominance, the ability of the apical bud to suppress the development of axillary buds. Cytokinins slow the aging of certain plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues.
In 1930s, it was determined that a fungus caused hyperelongation of rice by secreting a chemical. It named gibberellin. The major sites of gibberellin production are young roots and leaves. Gibberellins are best known for stimulating stem and leaf growth by enhancing cell elongation and cell division. In many plants, both auxin and gibberellina must be present for fruit to develop.
Brassinosteroids are steroids that are similar to cholesterol and the sex hormones of animals. They induce cell elongation and division in stem segments and seedlings. They also slow leaf abscission and promote xylem differentiation.
In the 1960s, one research group studying the chemical changes that precede bud dormany and leaf abscission in deciduous trees and another team investigating chemical chages preceeding abscission of cotton fruits isolated the same compound, abscisic acid. ABA often antagonizes the actions of growth hormones, and the ratio of ABA to one or more growth hormones determines the final physiological outcome. Many types of dormant seeds germinate when ABA is removed or inactivated. ABA also plays a major role in drought signaling.
The hormones called strigolactones are upwardly mobile signls that stimulate seed germination, help establish mycorrhizal associations, and help control apical dominance. In 1901, the gas ethylene was demonstrated to be the cause of trees dropping prematurely. Plants produce ethylene in response to stress such as drought, flooding, mechanical pressure, injury, and infection. Ethylene is also produced during fruit ripening and programmed cell death and in response to high concentrations of externally applies auxin.
🌸Explain the difference between crop domestication through selective breeding and genetically modified agricultural crops.
The life cycles of plants are characterized by an alternation of generations, in which multicellular haploid and diploid generations take turns producing each other. The diploid plant, the sporophyte, produces haploid spores by meiosis. These spores divide by mitosis, giving rise to the multicellular gametophytes, the male and female haploid plants that produce gametes. Fertilization, the fusion of gametes, results in diploid zygotes, which divide by mitosis and form new sporophytes. In angiosperms, the sporophyte is the dominant generation: It is larger, more conspicuous, and longer-lived than the gametophyte. Over the course of seed plant evolution, gametophytes became reduced in size and wholly dependent on the sporophyte for nutrients. Angiosperm gametophytes are the most reduced of all plants, consisting of only a few cells.
Flowers, the reproductive shoots of angiosperm sporophytes, are typically composed of four whorls of modified leaves called floral organs. Unlike vegetative shoots, flowers are determinate shoots; they cease growing after the flower and fruit are formed. Floral organs – sepals, petals, stamens, and capels – are attached to a part of a stem called the receptacle. Stamens and carpels are reproductive organs, whereas sepals and petals are sterile. Sepals, which enclose and protect unopened floral buds, are usually more leafy in appearance than the other floral organs. Petals are typically more brightly colored than sepals and advertise the flower to insects and other pollinators. A stamen consists of a stalk called the filament and a terminal structure called the anther; within the anther are chambers called microsporangia that produces pollen. A carpel has an ovary at its base and a long, slender neck called the style. At the top of the style is a generally sticky structure called the stigma that captures pollen. Within the ovary are one or more ovules; the number of ovules depends on the species. In most species, two or more capels are fused into a single structure. The term pistil is sometimes used to refer to a single carpel or two or more fused carpels.
Complete flowers have all four basic floral organs. Some species have incomplete flowers, lacking sepals, petals, stamens, or carpels. Some incomplete flowers are sterile, lacking functional stamens and capels; others are unisexual, lacking either stamens or carpels. Flowers also vary in size, shape, color, odor, organ arrangement, and time of opening. Some are borne singly, while others are arranged in showy clusters called inflorescences.
Each anther contains four microsporangia, also known as pollen sacs. Within the microsporangia are many diploid cells called microsporocytes, or microspore mother cells. Each microsporocyte undergoes meiosis, forming four haploid male gametophytes. Each microspore then undergoes mitosis, producing a male gametophyte consisting of only two cells: the generative cell and the tube cell. Together, these cells and the spore wall constitute a pollen grain. The spore wall, which consists of material produced by both the microspore and the anther, usually exhibit an elaborate pattern unique to the species. During maturation of the male gametophyte, the generative cells passes into the tube cell, and the spore wall is completed. The tube cell now has a completely free standing cell inside it. After the microsporangium breaks open and releases the pollen, a pollen gran may be transferred to a receptive surface of a stigma. There, the tube cell produces the pollen tube, a long cellular protuberance that delivers sperm to the female gametophyte. Pollen tubes can grow very quickly.
In angiosperms, pollination is the transfer of pollen from an anther to a stigma. It is accomplished by wind, water, or animals. In wind-pollinated species, including grasses and many trees, the release of enormous quantities of smaller-sized pollen compensates for them randomness of dispersal by the wind. Most angiosperms depend on insects, birds, or other animal pollinators to transfer pollen directly from one flower to another.
At the time of pollination, the pollen grain typically consists of only the tube cell and the generative cell. After a pollen grain lands on a suitable stigma, it absorbs water and germinates by producing a pollen tube, which grows between the cells of the style toward the ovary. The nucleus of the generative cell divides by mitosis and forms two sperm. In response to chemical attractants produced by the synergids, the tip of the pollen tube grows toward the micropyle. Its arrival initiates the death of one of the two synergids, thereby providing a passageway into the embryo sac for the two sperm that are discharged from the pollen tube. Upon reaching the female gametophyte, one sperm fertilizes the egg, forming the zygote. The other sperm fertilizes the egg, forming the zygote. The other sperm combines with two polar nuclei, forming a triploid nucleus in the center of the large central cell of the female gametophyte. This large cell will give rise to the endosperm, a food-storing tissue of the seed. Th union of the two sperm cells with different nuclei of the female gametophyte is called double fertilization.
After doubler fertilization, each ovule develops into a seed, and the ovary develops into s fruit enclosing the seeds. Endosperm usually develops before the embryo does. The first mitotic division of the zygote splits the fertilized egg into a basal cell and a terminal cell. The terminal cell eventually gives rise to most of the embryo. The basal continues to divide, producing a thread of cells called the suspensor, which anchors the embryo to the parent plant. The suspensor helps in transferring nutrients to the embryo from the parent plant and, in some species of plant, from the endosperm.
The embryo, which surrounded by a food supply, enters dormancy; that is, it stops growing and its metabolism nearly ceases. The embryo and its food supply are enclosed by a hard, protective seed coat formed from the integuments of the ovules. Below where the cotyledons are attached, the embryonic axis is called the hypocotyl. The hypocotyl terminates in the radicle, or embryonic root. The portion of the embryonic axis above where the cotyledons are at epicotyl. The epicotyl, young leaves, and shoot apical meristem are collectively called the plumule. The embryo of a grass seed is enclosed within two protective sheathes: a coleoptile, which covers the young shoot, and a coleorhiza, which covers the young root.
Germination depends on imbibition, the uptake of water due to the low water potential of the dry seed. While the seeds are developing from ovules, the ovary of the flower is developing into a fruit, which protects the enclosed seeds and, when mature, aids in their dispersal by wind or animals. Fruits are classified into several types, depending on their developmental origin. Most fruits are derived from a single carpel or several fused carpels and are called simple fruits. An aggregate fruit results from a single flower that has more than one separate carpel, each forming a small fruit. A multiple fruit develops from an inflorescence, a group of flowers tightly clustered together. In some angiosperms, other floral parts contribute to what we commonly call the fruit. Such fruits are called accessory fruits.
🌸How long ago did flowering plants come into dominance and what was their evolutionary precursor?
The life cycles of plants are characterized by an alternation of generations, in which multicellular haploid and diploid generations take turns producing each other. The diploid plant, the sporophyte, produces haploid spores by meiosis. These spores divide by mitosis, giving rise to the multicellular gametophytes, the male and female haploid plants that produce gametes. Fertilization, the fusion of gametes, results in diploid zygotes, which divide by mitosis and form new sporophytes. In angiosperms, the sporophyte is the dominant generation: It is larger, more conspicuous, and longer-lived than the gametophyte. Over the course of seed plant evolution, gametophytes became reduced in size and wholly dependent on the sporophyte for nutrients. Angiosperm gametophytes are the most reduced of all plants, consisting of only a few cells.
Flowers, the reproductive shoots of angiosperm sporophytes, are typically composed of four whorls of modified leaves called floral organs. Unlike vegetative shoots, flowers are determinate shoots; they cease growing after the flower and fruit are formed. Floral organs – sepals, petals, stamens, and capels – are attached to a part of a stem called the receptacle. Stamens and carpels are reproductive organs, whereas sepals and petals are sterile. Sepals, which enclose and protect unopened floral buds, are usually more leafy in appearance than the other floral organs. Petals are typically more brightly colored than sepals and advertise the flower to insects and other pollinators. A stamen consists of a stalk called the filament and a terminal structure called the anther; within the anther are chambers called microsporangia that produces pollen. A carpel has an ovary at its base and a long, slender neck called the style. At the top of the style is a generally sticky structure called the stigma that captures pollen. Within the ovary are one or more ovules; the number of ovules depends on the species. In most species, two or more capels are fused into a single structure. The term pistil is sometimes used to refer to a single carpel or two or more fused carpels.
Complete flowers have all four basic floral organs. Some species have incomplete flowers, lacking sepals, petals, stamens, or carpels. Some incomplete flowers are sterile, lacking functional stamens and capels; others are unisexual, lacking either stamens or carpels. Flowers also vary in size, shape, color, odor, organ arrangement, and time of opening. Some are borne singly, while others are arranged in showy clusters called inflorescences.
Each anther contains four microsporangia, also known as pollen sacs. Within the microsporangia are many diploid cells called microsporocytes, or microspore mother cells. Each microsporocyte undergoes meiosis, forming four haploid male gametophytes. Each microspore then undergoes mitosis, producing a male gametophyte consisting of only two cells: the generative cell and the tube cell. Together, these cells and the spore wall constitute a pollen grain. The spore wall, which consists of material produced by both the microspore and the anther, usually exhibit an elaborate pattern unique to the species. During maturation of the male gametophyte, the generative cells passes into the tube cell, and the spore wall is completed. The tube cell now has a completely free standing cell inside it. After the microsporangium breaks open and releases the pollen, a pollen gran may be transferred to a receptive surface of a stigma. There, the tube cell produces the pollen tube, a long cellular protuberance that delivers sperm to the female gametophyte. Pollen tubes can grow very quickly.
In angiosperms, pollination is the transfer of pollen from an anther to a stigma. It is accomplished by wind, water, or animals. In wind-pollinated species, including grasses and many trees, the release of enormous quantities of smaller-sized pollen compensates for them randomness of dispersal by the wind. Most angiosperms depend on insects, birds, or other animal pollinators to transfer pollen directly from one flower to another.
At the time of pollination, the pollen grain typically consists of only the tube cell and the generative cell. After a pollen grain lands on a suitable stigma, it absorbs water and germinates by producing a pollen tube, which grows between the cells of the style toward the ovary. The nucleus of the generative cell divides by mitosis and forms two sperm. In response to chemical attractants produced by the synergids, the tip of the pollen tube grows toward the micropyle. Its arrival initiates the death of one of the two synergids, thereby providing a passageway into the embryo sac for the two sperm that are discharged from the pollen tube. Upon reaching the female gametophyte, one sperm fertilizes the egg, forming the zygote. The other sperm fertilizes the egg, forming the zygote. The other sperm combines with two polar nuclei, forming a triploid nucleus in the center of the large central cell of the female gametophyte. This large cell will give rise to the endosperm, a food-storing tissue of the seed. Th union of the two sperm cells with different nuclei of the female gametophyte is called double fertilization.
After doubler fertilization, each ovule develops into a seed, and the ovary develops into s fruit enclosing the seeds. Endosperm usually develops before the embryo does. The first mitotic division of the zygote splits the fertilized egg into a basal cell and a terminal cell. The terminal cell eventually gives rise to most of the embryo. The basal continues to divide, producing a thread of cells called the suspensor, which anchors the embryo to the parent plant. The suspensor helps in transferring nutrients to the embryo from the parent plant and, in some species of plant, from the endosperm.
The embryo, which surrounded by a food supply, enters dormancy; that is, it stops growing and its metabolism nearly ceases. The embryo and its food supply are enclosed by a hard, protective seed coat formed from the integuments of the ovules. Below where the cotyledons are attached, the embryonic axis is called the hypocotyl. The hypocotyl terminates in the radicle, or embryonic root. The portion of the embryonic axis above where the cotyledons are at epicotyl. The epicotyl, young leaves, and shoot apical meristem are collectively called the plumule. The embryo of a grass seed is enclosed within two protective sheathes: a coleoptile, which covers the young shoot, and a coleorhiza, which covers the young root.
Germination depends on imbibition, the uptake of water due to the low water potential of the dry seed. While the seeds are developing from ovules, the ovary of the flower is developing into a fruit, which protects the enclosed seeds and, when mature, aids in their dispersal by wind or animals. Fruits are classified into several types, depending on their developmental origin. Most fruits are derived from a single carpel or several fused carpels and are called simple fruits. An aggregate fruit results from a single flower that has more than one separate carpel, each forming a small fruit. A multiple fruit develops from an inflorescence, a group of flowers tightly clustered together. In some angiosperms, other floral parts contribute to what we commonly call the fruit. Such fruits are called accessory fruits.
🌸 How do plants move water and other nutrients throughout the organism?
Plant tissues may be viewed as having two major compartments – the apoplast and the symplast. The apoplast consists of everything external to the plasma membrane of living cells and includes cell walls, extracellular spaces, and the interior of dead cells such as vessel elements and tracheids. The symplast consists of the entire mass of cytosol of all the living cells in a plant, as well as the plasmodesmata, the cytoplasmic channels that interconnect them.
The compartmental structure of plants provides three routes for transport within a plant tissue or organ: the apoplastic, symplastic, and transmembrane routes. In the apoplastic route, water and solutes move along the continuum of cell walls and extracellular spaces. In the symplastic route, water and solutes move along the continuum of cytosol. This route requires substances to cross a plasma membrane once, when they first enter the plant. After entering one cell, substances can move from cell to cell via plasmodesmata. In the transmembrane route, water and solutes move out of one cell, across the cell wall, and into the neighboring cell, which may pass them to the next cell in the same way. The transmembrane route requires repeated crossings of plasma membranes as substances exit one cell and enter the next. These three routes are not mutually exclusive and some substances may use more than one route to varying degrees.
In plants, the selective permeability of the plasma membrane controls the short-distance movement of substances into and out of cells. Both active transport and passive transport mechanisms occur in plants, and plant cell membranes are equipped with the same general types of pumps and transport proteins that function in other cells. Hydrogen ions play the primary role in basic transport processes in plant cells. Also, hydrogen ions are most often cotransported in plants. During cotransport, plant cells use the energy in the hydrogen ion gradient and membrane potential to drive the active transport of many different solutes. The membranes of plant cells also have ion channels that allow only certain ions to pass.
Diffusion is an effective transport mechanism over the spatial scales typically found at the cellular level. However, diffusion is too slow to function in the long-distance transport within a plant. Although diffusion from one end of a cell to the other takes just seconds, diffusion from the roots to the top of a tree would take much longer. Instead, long-distance transport occurs through bulk flow, the movement of liquid in response to a pressure gradient. The bulk flow of material always occurs from higher to lower pressure. Unlike osmosis, bulk flow is independent of solute concentration. Long distance bulk flow occurs within the sieve-tube elements of the phloem. The structures of these conducting cells facilitates bulk flow. Diffusion, active transport, and bulk flow act in concert to transport resources throughout the whole plant.
Although all living plant cells absorb nutrients across their plasma membranes, the cells near the tips of roots are particularly important because most of the absorption of water and minerals occur there. In this region, epidermal cells are permeable to water, and many are differentiated into root hairs, modified cells that account for much of the absorption of water by roots. The root hairs absorb the soil solution, which consists of water molecules and dissolved mineral ions that are not bound tightly to soil particles. The soil solution is drawn into the hydrophilic walls of epidermal cells and passes freely along the cell walls and the extracellular spaces into the root cortex. This flow enhances the exposure of the cell of the cortex to the soil solution, providing a much greater membrane surface area for absorption than the surface area of the epidermis alone.
Water and minerals that pass from the soil into the root cortex cannot be transported to the rest of the plant until they enter the xylem of the vascular cylinder, or stele. The endodermis, the innermost layer of cells in the root cortex, functions as a last checkpoint for the selective passage of minerals from the cortex into the vascular cylinder. These minerals were already screened by the plasma membrane they had to cross in order to enter the symplast. Those minerals that reach the endodermis via the apoplast encounter a dead end that blocks their passage into the vascular cylinder. This barrier, located in the transverse and radial walls of each endodermal cell, is the Casparian strip, a belt made of suberin, a waxy material impervious to water and dissolved minerals. Thus, water and minerals cannot cross the endodermis and enter the vascular cylinder via the apoplast. The Casparian strip forces water and minerals that are passively moving through the apoplast to cross the plasma membrane of an endodermal cell before they can enter the vascular cylinder. The endodermis, with its Casparian strip, ensures that no minerals can reach the vascular tissue of the root without crossing a selectively permeable plasma membrane. It also prevents solutes that have accumulated in the xylem from leaking back into the soil solution. The last segment in the soil-to-xylem pathway is the passage of water and minerals into the tracheids and vessel elements of the xylem.
Water and minerals from the soil enter the plant through the epidermis of roots, cross the root cortex, and pass into the vascular cylinder. From there the xylem sap, the water and dissolved minerals in the xylem, gets transported long distances by bulk flow to the veins that branch throughout each leaf. The process of transporting xylem sap involves the loss of an astonishing amount of water by transpiration, the loss of water vapor from leaves and other aerial parts of the plant.
Xylem sap rises to heights of more than 120 m in the tallest trees. Water flows in from the root cortex, generating root pressure, a push of xylem sap. The root pressure sometimes causes more water to enter the leaves than is transpired, resulting in guttation, the exudation of water droplets that can be seen in the morning on the tips or edges of some plant leaves.
The transport of the products of photosynthesis, known as translocation, is carried out by another tissue, the phloem. In angiosperms, the specialized cells that are conduits for translocation are the sieve-tube elements. Arranged end to end, they form long sieve tubes. Between these cells are sieve plates, structures that allow the flow of sap along the sieve tube. Phloem sap, the aqueous solution that flows through sieve tubes, differs markedly from the xylem sap that is transported by tracheids and vessel elements. The most prevalent solute in phloem sap is sugar, typically sucrose in most species.
A sugar source is a plant organ that is a net producer of sugar, by photosynthesis or by breakdown of starch. A sugar sink is an organ that is a net consumer or depository of sugar. Growing roots, buds, stems, and fruits are sugar sinks. Although expanding leaves are sugar sinks, mature leaves, if well illuminated, are sugar sources. A storage organ, such as a tuber or a bulb, may be a source or a sink, depending on the season. Sinks usually receive sugar from the nearest sugar sources. A growing fruit may monopolize the sugar sources that surround it. Sugar must be transported, or loaded, into sieve-tube elements via the symplast, passing through plasmodesmata. In other species, it moves by symplastic and apoplastic pathways.
In many plants, sugar movement into the phloem requires active transport because sucrose is more concentrated in sieve-tube elements and companion cells than in mesophyll. Sucrose is unloaded at the sink end of a sieve-tube. The process varies by species and organ. However, the concentration of free sugar in the sink is always lower than in the sieve tube because of the unloaded sugar is consumed during growth and metabolism of cells of the sin or converted to insoluble polymers such as starch. As a result of this sugar concentration gradient, sugar molecules diffuse from the phloem into the sink tissues, and water follows by osmosis.
Many key traits of land plants also appear in some protists, primarily algae. Charophytes are the only algae that share four distinctive traits with land plants, strongly suggesting that they are the closest relatives to plants. They both have rings of cellulose-synthesizing proteins. The cells of both land plants and charophytes have distinct circular rings of proteins in the plasma membrane. These protein rings synthesize the cellulose microfibers of the cell wall. In contrast, noncharophyte algae have linear sets of proteins that synthesize cellulose. They also both have peroxisome enzymes. The peroxisomes of both land plants and charophytes contain enzymes that help minimize the loss of organic products resulting from photorespiration. In species of land plants that have flagellated sperm, the structure of the sperm closely resembles that of charophyte sperm. Particular details of cell division occur only in land plants and certain charophytes, including the genera Chara and Coleochaete.
In charophytes, a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out. A similar chemical adaption is found on sporopollenin walls that encase the spores of plants. The accumulation of these traits by at least one population of charophyte ancestors probably enabled their descendents to live permanently above the waterline.
Many of the adaptions that appear to have emerged after land plants diverged from their algal relatives facilitated survival and reproduction on dry land. Early plants lacked true roots and leaves. They formed symbiotic relationships with fungi similar in structure to beneficial associations observed today between plants and fungi. Many land plants produce molecules called secondary compounds that side branches off the primary metabolic pathways that produce the lipids, carbohydrates, amino acids, and other compounds called alkaloids, terpenes, tannins, and flavonoids.
One way to distinguish groups of plants is whether or not they have an extensive system of vascular tissue. Plants that do not have an extensive transport system are described as “nonvascular” plants, even though some mosses do have simple vascular tissue. Nonvascular plants are not informally called bryophytes. Although the term bryophyte is is commonly used to refer to all nonvascular plants, molecular studies show that bryophytes do not form a monophyletic group. Vascular plants, which form a clade that comprises about 93% of all extant plant species, can be categorized further into smaller clades. Two of these clades are lycophytes and the pterophytes. The plants in each of these clades lack seed, which is why collectively the two clades are often informally called seedless vascular plants. However, groups such as the seedless vascular plants are sometimes referred to as a grade, a collection of organisms that share a biological key feature. Grades can be informative by grouping organisms according to features, such as lack if seeds. Members of a grade do not share the same ancestry. A third clade of vascular plants consists of seed plants, which represent the vast majority of living plant species. A seed is an embryo packed with a supply of nutrients inside a protective coat. Seed plants can be divided into two groups, gymnosperms and angiosperms, based on the absence or presence of enclosed chambers in which seeds mature. Gymnosperms are grouped together as “naked seed” plants because their seeds are not enclosed in chambers. Angiosperms are a huge clade consisting of all flowering plants. Angiosperm seeds develop withing flowers and mature into ovaries, which originate withing flowers and mature into fruits.
Unlike vascular plants, in all three bryophyte phyla the haploid gametophytes are the dominant stage if the life cycle. They are usually larger and longer-living than the sporophytes. Sporophytes are typically only present for part of the time. When bryophyte spores are dispersed to a favorable habitat, such as moist soil or tree bark, they may germinate and grow into gametophytes. Germinating moss spores characteristically produces a mass of green, branched, one-cell-thick filaments known as protonema. A protonema has a large surface that enhances absorption of water and minerals. In favorable conditions, a protonema produces one or more buds. Each of these bud-like growths has an apical meristem that generates a gamete-producing structure known as a gametophore.
Bryophyte gametophytes generally form ground-hugging carpets, partly because their body parts are too thin to support a tall plant. A second restraint on the height of many bryophytes is the absence of vascular tissue, which would enable long-distance transport of water and nutrients. The gametophytes are anchored by delicate rhizoids, which are long, tubular cells or filaments in cells. Unlike roots, rhizoids are not composed of tissues. They also lack specialized conducting cells and do not play a primary role in water and mineral absorption.
Alternative forms of photosynthesis are used by specific types of plants, called C4 and CAM plants, to alleviate problems of photorespiration and excessive water loss. Photosynthesis is the physiological process whereby plants use the sun's energy to produce organic molecules. The backbone of all such organic compounds is a skeleton composed of carbon atoms. Plants use carbon dioxide from the atmosphere as their carbon source. Researcher discovered a biochemical pathway that involved incorporation of carbon dioxide into organic products at two different stages. In C4 plants, this photosynthetic pathway is tied to a unique leaf anatomy known as Kranz anatomy. In C4 plants the cells that surround that water are packed very tightly together and are called bundle sheath cells. In C4 plants, the initial fixation of carbon dioxide from the atmosphere takes place in the densely packed mesophyll cells. CAM exists in succulents such as cacti and other desert plants. CAM plants have both carbon dioxide-fixing enzymes within the same cell. The biological pathway of photosynthesis in CAM plants begin at night. When the stomata is open, carbon dioxide diffuses into the leaf and into mesophyll cells, where it is fixed by the C4 enzyme PEP carboxylase. The vacuoles will accumulate malic acid through most of the night. A few hours before daylight, the vacuole fills up and malic acid will accumulate outside the vacuole.
🌸 Explain how plants grow. Explain primary and secondary growth.
Growth occurs throughout a plant's life, a process that is known as indeterminable growth. At any given time, a typical plant has embryonic, developing, and mature organs. Except for dormant periods, most plants grow continuously. Plants are capable of indeterminable growth because they have perpetually undifferentiated tissues called meristems that divide when conditions permit, leading to new cells that can elongate. There are two main types of meristems, apical meristems and lateral meristems. Apical meristems, located at the tips of roots and shoots and in axillary buds of shoots, provide additional cells that enable growth in length, a process known as primary growth. Primary growth enables roots to extend throughout the soil and shoots to increase their exposure to light. Woody plants also grow in circumference in the parts of the stems and roots that no longer grow in length. This growth in thickness, known as secondary growth, is caused by lateral meristems called the vascular cambium and cork cambium. The vascular cambium adds layers of vascular tissue called secondary xylem and secondary phloem. The cork cambium replaces the epidermis with the thicker, tougher periderm.
The tip of a root is covered by a thimble-like root cap, which protects the delicate apical meristems as the root pushes through the abrasive soil during primary growth. The root cap also secretes a polysaccharide slime that lubricates the soil around the tip of the root. Growth occurs just behind the tip in three overlapping zones of cells at successive stages of primary growth. These zones are cell division, elongation, and differentiation. The zone of cell division includes the root apical meristem and its derivatives. New root cells are produced in this region, including cells of the root cap. Typically, a few millimeters behind the tip of the root is the zone of elongation, where most of the growth occurs as root cells elongate. Cell elongation in this zone pushes the tip farther into the soil. Meanwhile, the root apical meristem keeps adding cells to the younger end of the zone of elongation. In the zone of differentiation, or zone of maturation, cells complete their differentiation and become distinct cell types.
The primary growth of a root produces its epidermis, ground tissue, and vascular tissue. Water and minerals absorbed from the soil must enter through the root's epidermis. Root hairs, which account for much of this absorption, enhance this process by greatly increasing the surface area of the epidermis. The ground tissue of roots, consisting mostly of parenchyma cells, fills the cortex, the region between the vascular cylinder and epidermis. Cells within the ground tissue store carbohydrates and absorb water and minerals from the soil. The innermost layer of the cortex is called the endodermis, a cylinder one cell thick that forms the boundary with the vascular cylinder. Lateral roots arise from the pericycle, the outermost cell layer in the vascular cylinder, which is adjacent to and just inside the endodermis. A lateral root pushes through the cortex and epidermis until it emerges from the established root.
A shoot apical meristem is a dome-shaped mass of dividing cells at the shoot tip. Leaves develop from the leaf primordia, finger like projections along the sides of the apical meristem. Within a bud, young leaves are spaced close together because the internodes are very short. Shoot elongation is due to the lengthening of internode cells below the shoot tip. Branching arises from the activation of axillary buds. The dormancy of the axillary bud's shoot apical meristem depends on its proximity to an active apical bud. The closer an axillary bud is to active apical bud, the more inhibited it is.
The epidermis covers stems as a part of the continuous dermal tissue system. Vascular tissue runs the length of a stem in vascular bundle. Unlike lateral roots, which arise from vascular tissue deep within a root and disrupt the vascular cylinder, cortex, and epidermis as they emerge, lateral shoots develop from axillary bud meristems on the stem's surface and disrupt no other tissues.
In leaves, the epidermis is interrupted by pores called stomata, which allow for the exchange of carbon dioxide and oxygen between the surrounding air and the photosynthetic cells inside the leaf. In addition to regulating carbon dioxide uptake for photosynthesis, stomata are major avenues for the evaporative loss of water. The pore is flanked by two guard cells, which regulate the opening and closing of the pore.
The ground tissue of a leaf, s region called the mesophyll, is sandwiched between the upper and lower epidermal layers. Mesophyll consist mainly of parenchyma cells specialized for photosynthesis. The mesophylls of many eudicots have two distinct layers, palisade mesophyll and spongy mesophyll. Palisade mesophyll consists of one or more layers of elongated parenchyma cells on the upper part of the leaf. Spongy mesophyll is below the palisade mesophyll. These parenchyma cells are more loosely arranged, with a labyrinth of air spaces through which carbon dioxide and oxygen circulate around the cells and into the palisade region. The air spaces are particularly large int the vicinity of the stomata, where carbon dioxide is taken up from the outside air and oxygen is discharged. The vascular tissue of each leaf is continuous with the vascular tissue of the stem. Veins subdivide repeatedly and branch throughout the mesophyll. This network brings xylem and phloem into close contact with the photosynthetic tissue, which obtains water and minerals from the xylem and loads the sugars and other organic products into the phloem for transport.
Secondary growth consists of the tissues produced by the vascular cambium and cork cambium. The vascular cambium adds secondary xylem and secondary phloem, thereby increasing vascular flow and support for the roots. The cork cambium produces a tough, thick covering consisting mainly of wax-impregnated cells that protect the stem from water loss and from invasion by insects, bacteria, and fungi. All gymnosperm species and many eudicot species undergo secondary growth, but it is rare in monocots.
The vascular cambium is a cylinder of meristematic cells, often only one cell thick. It increases in circumference and also adds layers if secondary xylem to its interior and secondary phloem to its exterior. Each layer have a larger diameter than the previous layer. In this way, vascular cambium thickens roots and stems. In a typical woody stem, the vascular cambium consists of a continuous cylinder of undifferentiated parenchyma cells, located outside the pith and primary xylem and to the inside of the cortex and primary phloem. In a typical woody root, the vascular cambium forms to the exterior of the primary xylem and interior to the primary phloem and pericycle.
The vascular cambium appears to be a ring of stem cells. As these meristematic cells divide, they increase the circumference of the vascular cambium and also add secondary xylem to the inside and secondary phloem to the outside. Some stem sells are elongated and are oriented with their long axis parallel to the axis of the stem or root. They produce cells such as the tracheids, vessel elements, and the fibers of the xylem. The other stem cells are shorter and are oriented perpendicular to the axis of the stem or root. They produce vascular rays, radial files of mostly parenchyma cells that connect the secondary xylem and phloem. The cells of a vascular ray move water and nutrient between the secondary xylem and phloem, store carbohydrates, and aid in wound repair. The walls of secondary xylem cells are heavily lignified and account for the hardness and strength of wood. The structures of secondary xylem cells during the spring, also known as early wood, are designed to maximize delivery of water to new leaves. Wood produced during the rest of the growing season, also called late wood, is composed of thick-walled cells that do not transport much but provide much more support.
During the early stages of growth, the epidermis is pushed outward, causing it to split, dry, and fall off the stem or root. It is replaced by two tissues produced by the first cork cambium, a cylinder of diving cells that arises in the outer layer of the pericycle in roots. The other tissue, called phelloderm, is a thing layer of parenchyma cells that forms to the interior of the cork cambium. As cork cells mature, they deposit a waxy, hydrophobic material called suberin in their walls and then die. It then serves as a barrier that helps protect the stem or the root from water loss, physical damage, and pathogens. Most of the periderm is impermeable in water and gases. Dotting the periderm are small, raised areas called lenticels where there is more space between cork cells, enabling living cells within a woody stem or root to exchange gases with the outside air. Bark includes all the tissues external to the vascular cambium. Its main components are the secondary phloem, the most recent periderm, and all the older layers of periderm.
🌸 What are the basic plant structures? What are the different types of plant cells?
The basic plant organs are roots, stems, and leaves. These organs form a root system and a shoot system, which consists of stems and leaves. A root is an organ that anchors a vascular plant in the soil, absorbs the minerals and water, and often stores carbohydrates. Most eudicots and gymnosperms have a taproot system, consisting of one main vertical root, the taproot, which develops from an embryonic root. The taproot gives rise to lateral roots, also called branch roots. Taproot systems generally penetrate deeply and are therefore well adapted to deep soils, where the ground water is not close to the surface. In most monocots, the embryonic root dies early on and does not form a taproot. Instead, many small roots emerge from the stem. Such roots are said to be adventitious, a term that describes a plant organ that grows in an unusual location such as the stem or leaves. Each small root forms its own lateral roots. The result is a fibrous root system, a mat of generally thin roots spreading out below the soil surface. Fibrous root systems usually do not penetrate deeply and re therefore best adapted to shallow soils or regions where rainfall is light and does not moisten the soil much beneath the surface layer. In most plants the absorption of water and minerals occurs primarily in the tips of the roots, where vast numbers of root hairs emerge and increase the surface area of the root enormously. Root hairs, unlike lateral roots, contribute little to plant anchorage. Their main function is absorption.
A stem is an organ that raises or separates leaves, exposing them to sunlight. Stems also raise reproductive structures, facilitating dispersal of pollen and fruit. Each stem consists of an alternating system of noes, the points at which leaves are attached, and internodes, the stem segments between nodes. In the upper angle formed by each leaf and the stem is an axillary bud, a structure that can form a lateral shoot, commonly called a branch. Most of the growth of a young shoot is concentrated near the shoot tip, which consists of an apical bud, or terminal bud, that is composed of developing leaves and a compact series of nodes and internodes. The proximity of the axillary buds to the apical bud is party responsible for dormancy. The inhibition of axillary buds by an apical bud is called apical dominance. A growing axillary bud gives rise to a lateral shoot.
In most vascular plants, the leaf is the main photosynthetic organ, although green stems also perform photosynthesis. Leafs vary extensively in form but generally consist of a flattened blade and a stalk, the petiole, which joins the leaf to the stem at the node. Veins are the vascular tissue of leaves. Most monocots have parallel major veins that run the length of the blade. Eudicots have a branched network of major veins.
Each plant organ has dermal, vascular, and ground tissues. Each of these three categories forms a tissue system, a functional unit connecting all of the plant's organs. The dermal tissue system is the plant's outer protective covering. In nonwoody plants, it is usually a single tissue called the epidermis, a layer of tightly packed cells. In leaves and most stems, the cuticle, a waxy coating on the epidermal surface, helps prevent water loss. In woody plants protective tissues called periderm replace the epidermis in the older regions of stem and roots.
The vascular tissue system carries out long-distance transport of materials between the root and shoot systems. The two types of vascular tissues are xylem and phloem. Xylem conducts water and dissolved minerals, upward from roots into shoots. Phloem transports sugars, the products of photosynthesis, from where they are made to where they are needed. The vascular tissue of a root or stem is collectively called the stele. The arrangement of the stele varies depending on the species and organ.
Tissues that are neither dermal nor vascular are part of the ground tissue system. Ground tissue that is internal to the vascular tissue is called pith, and ground tissue that is external to the vascular tissue is called cortex.
A plant is characterized by cell differentiation, the specialization of ells in structure and function. Cell differentiation may involve changes both in the cytoplasm and its organelles and in the cell wall. The major types of plant cells are parenchyma cells, collenchyma cells, sclerenchyma cells, and the sugar-conducting cells of the phloem. Mature parenchyma cells have primary walls that are relatively thin and flexible, and most lack secondary walls. When mature, these cells generally have a large central vacuole. Parenchyma cells perform most of the metabolic functions of the plant, synthesizing and storing various organic products. Photosynthesis happens within the chloroplasts of the parenchyma cells in the leaf, some parenchyma cells retain the ability to divide and differentiate into other types of plant cells under particular conditions. It is even possible to grow an entire plant from a parenchyma cell.
Collenchyma cells are grouped in strands and help support young parts of the plant shoot. Collenchyma cells are generally elongated cells that have thicker primary walls than parenchyma cells, though the walls are unevenly thickened. Young stems and petioles often have strands of collenchyma cells just below their epidermis. Collenchyma cells provide flexible support without restraining growth. At maturity, these cells are living and flexible, elongating with the stems and leaves they support.
Sclerenchyma cells also function as supporting elements in the plant, but are much more rigid than collenchyma cells. The secondary walls of sclerenchyma cells are thick and contain large amounts of lignin. This relatively indigestible strengthening polymer accounts for more than a quarter of the dry mass of wood. Lignin is present in all vascular plants, but not in bryophytes. Mature sclerenchyma cells cannot elongate, and they occur in regions of the plants that have stopped growing in length. Sclerenchyma cells are mostly dead at functional maturity, but produce secondary walls before the protoplast dies. The rigid walls remain as a skeleton that continues to support the plant for many many years. There are two types of sclerenchyma cells known as sclereids and fibers. They are specialized entirely for support and strengthening. Sclereids are boxier than fibers and irregular in shape. They also have very thick, lignified secondary walls. Sclereids impart the hardness to nutshells and seed coats and the gritty texture to pears. Fibers, which are usually grouped in strands, are long, slender, and tapered.
The two types of water-conducting cells, tracheids and vessel elements, are tubular, elongated cells that are dead at functional maturity. Tracheids are in the xylem of nearly all vascular plants. In addition to tracheids, most angiosperms, as well as a few gymnosperms, have vessel elements. When the living cellular contents of a tracheid or vessel element disintegrate, the cell's thickened walls remain behind, forming a non-living conduit through which water can flow. The secondary walls of tracheids and vessel elements are often interrupted by pits, thinner regions where only primary walls are present. Water can migrate laterally between neighboring cells through pits. Tracheids are long, thin cells wit tapered ends. Water moves from cell to cell mainly through the pits. Vessel elements are generally wide, shorter, thinner walled, and less tapered than tracheids. They are aligned end to end, forming long micropipes know as vessels. The end walls of vessel elements have perforation plates that enable water to flow freely through the vessel.
Unlike the water-conducting cells of the xylem, the sugar-conducting cells of the phloem are alive at functional maturity. In seedless vascular plants and gymnosperms, sugars and other organic nutrients are transported through long, narrow cells called sieve cells. In the phloem of angiosperms, these nutrients are transported through sieve tubes, which consist of chains of cells called sieve-tube elements, or sieve-tube members. Though alive, sieve tube elements lack a nucleus, ribosomes, a distinct vacuole and cytoskeletal elements. This reduction is cell contents allows nutrients to pass more easily through cells. The end walls between sieve-tube elements, called sieve plates, have pores that facilitate the flow of fluid from cell to cell along the sieve tube. Alongside each sieve-tube element is a nonconducting cell called a companion cell, which is connected to the sieve-tube element by numerous channels called plasmodesmata. The nucleus and ribosomes of the companion cell serve not only the cell itself but also the adjacent sieve-tube elements. In some plants, the companion cells in leaves also help to load sugars into the sieve-tube elements, which then transport the sugars to other parts of the plants.
A year or two ago I bought a cute, tiny succulent and I swore I was going to keep it alive. For the first few weeks, I watched over the tiny plant, watered it when the soil was dry, and made sure it was getting enough sun. Then, for some reason, I just forgot to take care of it. I’ve always been incredibly forgetful, but I somehow forgot to take care of a plant that was literally sitting in front of me, on my work desk. Eventually, the plant was wilting so bad that my mom decided to put it outside, which honestly made the situation even worse because once it was out of sight, I completely forgot it existed. And so, it died. I somehow killed a succulent, which are supposedly “impossible to kill”. So... I would say that I definitely do not have a green thumb. Everything dies under my care.
The assignment is to keep our plant alive for the rest of the year and to measure its growth, along with learning how plants function and thrive. We have to make blog entries or just write about the different topics written on our handy dandy green piece of paper and each topic (except for this one) should be about two (2) pages long.
I’m honestly hoping to learn how to keep my plant from dying. I want to be one of those people who has a balcony full of cool plants. Or just have little plants or cacti that I can grow on my windowsill. I also hope that through this assignment I can prove to myself that I do not kill everything I touch. Also learn about plants. Cool.
