Entry Nine: Conclusion and Reflection

Unfortunately, I have not acquired a pictorial history of my plant on a weekly basis. I do not possess a smartphone nor did I took or found the opportunity to do so.

However, I have feel that the growth of my plant is indeed a good metaphor for my growth as a student of Biology. Biology is an interesting subject to me, in which we learn and acquire knowledge about our life and existence on this planet. From the smallest atom to biggest galaxies in our universe, it is amazing how humans like us have evolved and prosper for the past few hundred years, since the birth of the universe nearly fourteen billion years ago. As Penny grows, my interest and acquisition of knowledge in Biology has grown with her. Even after this project, I will forever be interested in what life will bring to me.

Entry Eight: Signal Transduction Pathway in Plants and Plant Hormones

Carolus Linnaeus, the father of taxonomy, was keen naturalist, noting that each plant species opened and closed its flowers at a characteristic time of the day. Why does the time vary? Well, the time at which flowers open presumably reflects the time when their insect pollinators are most active. This is just an example of the numerous environmental factors that a plant must sense to compete successfully. In this entry let’s talk a look at the mechanisms by which plants sense and respond to external and internal cues. Because, you see, unlike animals, plants are stationary and generally respond to environmental cues by adjusting their individual patterns of growth and development.

Like animals, plants receive specific signals and respond to them in ways that enhance survival and reproductive success. Here, we will explore the de-etiolation, the changes a plant shoot undergoes in response to sunlight, or plant as an example of how a plant cell’s reception of a signal – in his case, light – is transduced into a response.

Reception:

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 member of a class of photo receptors. Unlike most receptors, which are built in the plasma membrane, they type of phytochrome that functions in de-etiolation is located in the cytoplasm.

Transduction: 

Receptors can be sensitive to very weak environmental or chemical signals. Some de-etiolation responses are triggered by extremely low levels of light. The transduction of these extremely weak signals involves second messengers – 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. In de-etiolation, two second messengers, calcium ions and cyclic GMP must be produced for a complete de-etiolation response.

Response: 

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-translational modification and transcriptional regulation. Post-translational modification activates preexisting enzymes. Transcriptional regulation increases or decreases the synthesis of mRNA encoding a specific enzyme.

Post-translation Modifications:

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, including calcium ions and cyclic GMP, activate kinases directly. Often, one protein kinase will phosphorylate another protein kinase, which then phosphorylate another, causing a phosphorylation cascade. Such kinase cascades may link initial stimuli to responses at the level of gene expression.

Transcriptional Regulation:

The proteins we call transcription factors bind to specific regions of DNA and control the transcription of specific genes. In the case of the phytochrome-induced de-etiolation, several such transcription factors are activated by phosphorylation in response to the appropriate light conditions.

Hormones:

A hormone 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. We will explore four separate plant hormones and their effects on plants.

Auxin (IAA) is a plant hormone produced in the apical meristems and young leaves. Developing seeds and fruits contain high levels of auxin, but it is unclear whether it is newly synthesized or transported from maternal tissues. Auxin primarily stimulates stem elongation, promotes the formation of lateral and adventitious roots, regulates development of fruit, enhances apical dominance; functions in phototropism and gravitropism, promotes vascular differentiation, and retards leaf abscission.

Cytokinin is a plant hormone that are synthesized primarily in roots and transported to other organs. It helps regulate cell division in shoots and roots, modify apical dominance and promote lateral bud growth, promote movement of nutrients into sink tissues, stimulate seed germination, and delay leaf senescence.

Gibberellins are primarily produced in meristems of apical buds and roots, young leaves, and developing seeds. They play a role in stem elongation, pollen development, pollen tube growth, fruit growth, seed development and germination, regulation of sex determination and transition from juvenile to adult phases.

Abscisic acid (ABA) is a plant hormone found and synthesized in almost all plant cells and in every major organ and living tissue. It may be transported in the phloem or xylem. The acid helps inhibit growth, promote stomatal closure during drought stress, promotes seed dormancy and inhibits early germination, promote leaf senescence, and promote desiccation tolerance.

Entry Seven: Selective Breeding and Genetically Modified Crops

Humans have intervened in the reproduction and genetic makeup of plants since the dawn of agriculture. In this entry, we will discuss the differences between crop domestication through selective breeding and genetically modified crops.

The art of recognizing valuable traits is important in plant breeding. Breeders scrutinize their fields carefully and travel to other countries searching for domesticated varieties or wild relatives with desirable traits. Such traits occasionally arise spontaneously through mutation. While most breeders cross-pollinate plants of a single species, some breeding methods rely on hybridization between two distant species of the same genus. Less commonly, hybridization is carried out on members of two different genera.

Plant biotechnology has two meanings. In the general sense, it refers to innovations in the use of plants, or substances obtained from plants to make products of use to humans. In a more specific sense, biotechnology refers to the use of genetically modified (GM) organisms in agriculture and industry. Unlike traditional plant breeders, modern plant biotechnologists, using genetic engineering techniques, are not limited to the transfer of genes between closely related species or genera. In traditional breeding, breeders were not able to insert a desired gene into their domesticated plants through hybridization and cross-breeding methods. With genetic engineering, however, such gene transfers can be done more quickly, more specifically, and without the need for intermediate species.

The commercial use of transgenic crops has been one of the most dramatic examples of rapid technology adoption in the history of agriculture. These crops include varieties and hybrids of cotton, maize, and potatoes that contain genes from the bacterium Bacillus thuringiensis. These “transgenes” encode a protein called Bt toxin that is toxic to insect pests but harmless to humans, greatly reducing the need for chemical insecticides. Considerable progress has also been made in developing transgenic crops that tolerate certain herbicides. Cultivation of these plants may reduce production costs by enabling farmers to “weed” crops with herbicides without destroying the transgenic crop plants. Researchers also engineered plants with enhanced resistance to disease and improved nutritional quality. Scientists also concluded that the use of plant biotechnology could reduce fossil fuel dependency. By utilizing the polymers in cell walls and breaking them down into sugars by enzymatic reactions, these sugars would be fermented into alcohol and distilled to yield biofuels. The use of biofuels from plant biomass would reduce the net emission of carbon dioxide. Whereas burning fossil fuels increases atmospheric carbon dioxide concentration, biofuels crops reabsorb by photosynthesizing the carbon dioxide emitted when biofuels are burned, creating a cycle that is carbon neutral.

Much of the debate about GMOs in agriculture is political, social, economical, or ethical and therefore reaches beyond the scope of Biology. However, let us take a look at the biological concerns about GM crops. Many GMO opponents worry that genetic engineering may inadvertently transfer allergens, molecules to which some people are allergic, from a species that produces an allergen to a plant used for food. So far, there is no credible evidence that GM plants specifically designed for human consumption have adverse effects on human health. In fact, some GM foods are potentially healthier than non-GM foods. Nevertheless, because of health concerns, GMO opponents lobby for the clear labeling of all foods containing products of GMOs. Many ecologists are also concerned that the growing of GM crops might have unforeseen effects on nontarget organisms. One laboratory study indicated that the larvae of monarch butterflies responded adversely and even died after eating milkweed leaves heavily dusted with pollen from transgenic Bt maize. Recent studies, however, have shown that the spraying of pesticides on the Bt maize is much more harmful to the nearby monarch population that the Bt in the maize population. Although the effects of Bt maize pollen on monarch butterfly larvae appear to be minor, the controversy has emphasized the need for accurate field testing of all GM crops and the importance of targeting gene expression to specific tissue to improve safety. However, the most serious concern raised about GM crops is the possibility of the introduced genes escaping from a transgenic crop into related weeds through crop-to-weed hybridization. The fear is that the spontaneous hybridization between a crop engineered for herbicide resistance and a wild relative might give rise to a “superweed” that would have selective advantage over other weeds in the wild and would be much more difficult to control. Prevention of transgene escape, however, have already been strategized, although the likelihood of creating a new destructive species of plant is possible.

In my honest opinion, I believe that the benefits of GMO crops outweigh the concerns. As listed above, GMO crops have many beneficial uses in reducing world hunger, increasing productivity, reducing fossil fuels, and improving the environment. However, I believe we should still take into consideration with the concerns surrounding the development of GM crops. We should be ready for potential threats that may or may not arise from GMOs. I also believe that just because GM crops dominate the markets, there is no reason to stop our society from embracing traditional breeding. Preservation of traditional values is a good thing, allowing us to explore methods that we have used before the introduction of biotechnology.

Entry Six: Flowering Plants, Pollination, and Coevolution

In recent evolutionary times, some flowering plants have formed relationships with an animal that not only disperses their seeds but also provides the plants with water and mineral nutrients and vigorously protects them from encroaching competitors, pathogens, and predators. In return for these favors, the animal gets to eat a fraction of the plants’ seeds and fruits. Take a guess at what the animal is. Come on, guess. After reading five entries of words and information and facts and more words, it’s okay. You were probably too bored to use your brain, so I will tell you that the plants involved in these mutually beneficial interactions are called crops and the animals are humans. Since the origins of crop domestication over ten thousand years ago, plant breeders have genetically manipulated the traits of a few hundred angiosperm species by artificial selection, transforming them into the crops we grow today. In this entry, we will be talking about angiosperm reproduction, or angiosperm sex.

The life cycles of plants are characterized by an alternation of generations, in which multicellular haploid (n) and diploid (2n) 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 multi cellular gametophytes, the male and female haploid plants that produce gametes (sperms and eggs).  Fertilization, the fusion of gametes, results in diploid zygotes, which divides by mitosis and form new sporophytes.

Flowers, the reproductive shoots of angiosperm sporophytes, are typically composed of four whorls of modified leaves called floral organs. Flowers are determinate shoots; they cease growing after flower and fruit are formed. Floral organs – sepals, petals, stamens, and carpels – are attached to a part of the 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 look more leaf-like in appearance than the other floral organs. Petals are typically more brightly colored 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 (pollen sacs) that produce 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. Complete flowers have all four basic floral organs. Some species have incomplete flowers, lacking sepals, petals, stamens, or carpels. Flowers also vary in size, shape, color, odor, organ arrangement, and time of opening.

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, the release of enormous quantities of smaller-sized pollen compensates for the randomness of dispersal by the wind. Some species of aquatic plants rely on water to disperse pollen. The majority of angiosperm species, however, depend on insects, birds, or other animal pollinators to transfer pollen directly from one flower to another. If pollination is successful, a pollen grain produces a pollen tube, which then grows down into the ovary via the style for fertilization.

The joint evolution of two interacting species each in response to selection imposed by the other is called coevolution. Many species of flowering plants have coevolved with specific pollinators. Natural selection favors individual plants or insects having slight deviations of structure that enhance the flower-pollination mutualism. An example of coevolution would be how the long floral tube of the Madagascar orchid has coevolved with the twenty eight centimeters long proboscis, a straw-like mouthpart, of the hawkmoth, its pollinator. Another example of coevolution would be how plant toxins with caterpillars. Caterpillars evolved to be able to eat more poisonous plants. As they eat more of these plants, they build better resistance to these toxins and even predators and are able to pass their traits toward their offspring. The plants are also evolving in which they would produce more toxins to ensure their survival.

Entry Five: Resource Acquisition and Transport in Plants

Land plants typically inhabit two worlds – above ground, where their shoot systems acquire sunlight and carbon dioxide, and below ground, where their root systems acquire water and minerals. Without adaptations that allow acquisition of these resources, plants could not have colonized land. As land plants evolved and increased in number, however, competition for light, water, and nutrients intensified. Natural selection favors plants capable of efficient long-distance transport of water, minerals, and products of photosynthesis. We shall examine in this entry how the basic architectural design of shoots and roots helps plants move water and other nutrients throughout the organism.

Shoot Architecture and Light Capture:

In shoot systems, stems serve as supporting structures for leaves and as conductors in the transport of water and nutrients. Stems also contribute in the amount of light capture needed for photosynthesis. The arrangement of leaves on a stem, known as phyllotaxy, and leaf orientation is an architectural feature of significant importance in light capture. The height of shoots and their branching patterns are two other architectural features affecting light capture. Branching generally enables plants to harvest sunlight for photosynthesis more effectively.

Root Architecture and Acquisition of Water and Minerals:

Soil contains resources mined by the root system. The evolution of root branching enabled land plants to more effectively acquire water and nutrients from the soil. Plants can rapidly adjust the architecture and physiology of their roots to exploit patches of available nutrients in the soil. The roots of many plants, for example, may respond to pockets of low nitrate availability in soils by extending straight through the pockets instead of branching within them.

The Plant Tissue and the Movement of Substances:

Plant tissues have two major compartments: the apoplast and symplast. The apoplast consists of everything external to the plasma membranes of living cells and include cell ways, 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 the cytosol. Substances must cross a plasma membrane before moving from cell to cell via the plasmodesmata. In the transmembrane route, water and solutes move out of one cell, across the cell wall, and into the neighboring cell. This route requires repeated crossings of plasma membranes.

Transpiration:

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 occurs there. 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. After passing into the vascular cylinder, the xylem sap, the water and dissolved minerals in the xylem, gets transported long distance by bulk flow (movement of liquid in response to a pressure gradient that is faster than diffusion or active transport) through the tracheids and vessel elements and into the veins that branch throughout each of the leaf. The process of transporting xylem sap, however, involves the loss of a significant amount of water by transpiration, the loss of water vapor from leaves and other aerial parts of plants. The question remains: Was the xylem sap pushed upwards from the roots, or is it mainly pulled upward? Well, water that flows in from the root cortex generates root pressure, a push of xylem gap; however, root pressure is a minor mechanism driving the ascent of xylem sap. According to the cohesion-tension hypothesis, transpiration provides the pull for the ascent of xylem sap, and the cohesion of water molecules transmits this pull along the entire length of the xylem from shoots to roots. Hence, xylem sap is normally under negative pressure.

Translocation: 

Transpiration cannot meet all the long-distance transport needs of the plant. The flow of water and minerals from the soil to roots to leaves is largely in a direction opposite to the direction necessary for transporting sugars from mature leaves to lower parts of the plants, such as root tips that require large amounts of sugars for energy and growth. The transport of the products of photosynthesis, or translocation, is carried out by the phloem. In angiosperms, the specialized cells, in the phloem, that conduct 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 from the xylem sap that is transported by tracheids and vessel elements. Phloem sap contains primarily sugar, as well as amino acids, hormones, and minerals. Unlike the transport of xylem sap from roots to leaves, phloem sap moves from sites of sugar production to sites of sugar use or storage. 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 of depository of sugar. Growing roots, buds, stems, and fruits are sugar sinks. Sinks usually receive sugar from the nearest sugar sources. So, what is the mechanism for translocation? Researchers have concluded that phloem sap moves through the sieve tubes of angiosperms by bulk flow driven by positive pressure, known as pressure flow.

Entry Four: Evolution of Plant and Photosynthesis

Researchers have identified green algae called charophytes as the closest relatives of land plants. But what is the evidence? Well, many key traits of land plants also appear in some protists, primarily algae. Both are multicellular, eukaryotic, and photosynthetic autotrophs that possess cell walls. However, the charophytes are the only algae that share the following four distinctive traits with land plants, suggesting their ancestry:

Both possess rings of cellulose-synthesizing proteins. The cells of both land plants and charophytes have distinctive circular rings of proteins in the plasma membrane that synthesize the cellulose microfibrils of the cell wall.

Both contain peroxisome enzymes. The perixosomes found in the cells of both land plants and charophytes contain enzymes that help minimize the loss of organic products resulting from photorespiration.

Both have structure of flagellated sperm. In species of land plants that have flagellated sperm, the structure of the sperm closely resembles that of charophyte sperm.

Both are involved in the formation of a phragmoplast. In  land plants and certain charophytes, a group of microtubules known as the phragmoplast forms between the daughter nuclei of a dividing cells during cell division.

Many species of charophyte algae inhabit shallow waters around the edges of ponds and lakes, where they are subject to occasional drying. In such environments, natural selection favors those algae that can survive periods when they are not submerged in water. In charophytes, a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out.  A similar chemical adaptation is found in the tough sporopollenin walls that encase the spores of plants. The accumulation of such traits by at least one population of charophyte ancestor most like enabled their descendents – the first land plants – to live permanently above water. These evolutionary novelties opened a new frontier: a terrestrial habitat that offered enormous benefits.

Whatever the precise age of the first land plants, those ancestral species gave rise to the vast diversity of living plants. One way to distinguish groups of plants is whether or not they have an extensive system of vascular tissue, cells joined into tubes that transport water and nutrients throughout the plant body. Plants with a complex vascular tissue system are called vascular plants. Plants that do not have an extensive transport system – liverworts, mosses, hornworts – are called nonvascular plants, or bryophytes. 

As we have all know, the chloroplasts of plants has the extraordinary power to capture light energy that has traveled one hundred and fifty million kilometers from the sun and convert it to chemical energy that is stored in sugar and other organic molecules through a process called photosynthesis. Photosynthesis involves two processes, each with multiple steps. These two stages are known as light reactions and the Calvin cycle.

The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. During the reaction, water is split, providing a source of electrons and protons and giving off oxygen as a by-product. Light absorbed by chlorophyll powers the transfer of the electrons and hydrogen ions from water to an acceptor called NADP (nicotinamide adenine dinucleotide phosphate) where they are temporarily stored. The light reactions then use solar power to reduce NADP to NADPH by adding a pair of electrons along with a hydrogen atom. The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Overall, light energy is converted into chemical energy in the form of two compounds, NADPH and ATP.

The Calvin cycle begins by incorporating carbon dioxide from the air into organic molecules already present in the chloroplast, a process known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH from the light reactions. To convert carbon dioxide into carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, also already provided by the light reactions. Thus, the Calvin cycle that makes sugar can only work with the help of the NADPH and ATP produced by the light reactions.

C4 plants are plants in which the Calvin cycle is preceded by reactions that incorporate carbon dioxide into a four-carbon compound, the end product o which supplies carbon dioxide for the Calvin cycle. CAM plants are plants that uses crassulacean acid metabolism, an adaptation for photosynthesis in arid conditions. In this process, carbon dioxide entering open stomata during the night is converted to organic acids, which release carbon dioxide for the Calvin cycle during the day, when stomata are closed. Both C4 plants and CAM plants, as you can see, are plants that use an alternate mode of carbon fixation to gather carbon dioxide during photosynthesis. Using these alternative methods allow these plants to extract more carbon dioxide from a given amount of air, helping them prevent water loss and adapt in arid climate.

Entry Three: Plant Growth

Growth occurs throughout a plant’s life, a process known as indeterminate growth. Most plants grow continuously, however, some plants organs, such as leaves, thorns, and flowers, undergo determinate growth – that is, they stop growing after reaching a certain size. Plants are capable of indeterminate growth because they have perpetually undifferentiated tissues called meristems that divide when conditions permit, leading to elongation. There are two 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. Growth in thickness, however, is known as secondary growth and is caused by lateral meristems called the vascular cambium and cork cambium. These cylinders of dividing cells extend along the lengths of roots and stems. The vascular cambium adds layers of vascular tissue called secondary xylem, or wood, and secondary phloem. The cork cambium adds secondary dermal tissue, replacing the epidermis with the thicker, tougher periderm. Although plants grow throughout their lives, they die like any other living things. Based on the length of their life cycle, flowering plants can be categorized as annuals, biennials, or perennials. Annuals complete their life cycle in a single year or less. Biennials require two growing seasons to complete their life cycle. Perennials live for many years.

Primary growth lengthens roots and shoots. Although the elongation of both roots and shoots arises from cells derived from apical meristems, the primary growth of roots and primary growth of shoots differ in many ways.

Let’s talk about the primary growth in roots first. 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 soil during primary growth. Growth occurs behind the cap in three overlapping zones: the zones of 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 zone, including cells of the root cap. The zone of elongation lies a few millimeter behind the tip of the root. In this zone, root cells elongate – sometimes to more than times their original length. As it elongates, the root apical meristem keeps adding cells to the younger end of the zone of elongation. In the zone of differentiation, cells complete their differentiation and become distinct cell types. The primary growth of a root produces its epidermis, ground tissue, and vascular tissue. Lateral roots arise from the pericycle, the outermost cell layer in the vascular tissue.

Now let’s talk about the primary growth in shoots. The shoot apical meristem is a dome-shaped mass of dividing cells at the shoot tip. Shoot elongation is due to the lengthening of internode cells below the shoot tip. Branching, another part of primary growth, arises from the activation of axillary buds, which also contains a shoot apical meristem. Leaves develop from leaf primordia, finger-like projections along the sides of the apical meristem.

Secondary growth increases the diameter of stems and roots in woody plants. As mentioned before, secondary growth consists of the tissues produced by the vascular cambium and cork cambium. The vascular cambium adds secondary xylem (wood) and secondary phloem, thereby increasing vascular flow and support for the shoots. 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.

Let’s look at the vascular cambium and secondary vascular tissue in details. Specifically, the vascular cambium is a cylinder of meristematic cells, often only one cell thick. It increases in circumference and also adds layers of secondary xylem to its interior and secondary phloem to its exterior. In this way, the vascular cambium thickens roots and stems. Viewed in cross section, the vascular cambium appears as a ring of stem cells, or initials, and marks the annual growth of woody plants. Some initials are elongated and are orientated with their long axis parallel to the axis of the stem or root. They produce the water-conducting cells in xylem and the sugar-conducting cells in the phloem. Other initials 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 nutrients between the secondary xylem and phloem, store carbohydrates, and aid in wound repair.

Now let’s look at the cork cambium and the production of periderm in details. During the early stages of secondary 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 dividing cells that arises in the outer cortex of stems and in the outer layer of the pericycle in roots. One tissue, called phelloderm, is a thin layer of parenchyma cells that forms to the interior of the cork cambium. The other tissue consists of cork cells that accumulate to the exterior of the cork cambium. Each cork cambium and the tissues it produces comprise a layer of periderm. The thickening of a stem or root often splits the first cork cambium, which loses its meristemic activity and differentiates into cork cells. A new cork cambium forms to the inside, resulting in another layer of periderm.

Below is a diagram of a leaf structure. The epidermis is interrupted by pores called stomata, which allow the release of evaporative water and the exchange of carbon dioxide and oxygen between the surrounding air and the photosynthetic cells inside the leaf. Two guard cells regulate the opening and closing of the pores. The ground tissue of a leaf, a region called the mesophyll, is sandwiched between the upper and lower epidermal layers. Mesophyll consists mainly of parenchyma cells specialized for photosynthesis. They have two distinct layers: palisade mesophyll and spongy mesophyll. Palisade mesophyll consists of one or more layers on the upper part of the leaf. The spongy mesophyll rests below the palisade mesophyll. 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.

Entry Two: Plant Structures and Different Types of Plant Cells

Plants, like most animals, have a hierarchical organization consisting of organs, tissues, and cells. The basic morphology of vascular plants reflects their evolutionary history as terrestrial organisms that inhabit and draw resources from two different environments – below the ground and above the ground. The ability to acquire these resources efficiently traces back to the evolution of three basic organs: roots, stems, and leaves.

A root is an organ that anchors a vascular plant in the soil, absorbs mineral and water, and often stores carbohydrates. Most plants have a taproot system, consisting of one main vertical root, the taproot, which develops from the embryonic root and lateral roots, also known as branch roots. Taproot systems generally penetrate deeply and are well adapted to deep soils, where groundwater is not close to the surface. Some roots also form the fibrous root system, a mat of generally thin roots spreading out below the soil surface.  Fibrous root systems consist of adventitious roots, roots that arise from unusual location instead of the embryonic roots, and lateral roots branching from each of the small roots. They do not usually penetrate deeply and are best adapted to shallow soils or regions where rainfall is light and does not moisten the soil much below the surface layers. Although the root system helps anchor a plant, absorption of water and minerals in most plants occur primarily near the tips of roots, where vast numbers of root hairs emerge and increase the surface area of the root enormously. Although not an organ, root hairs contribute little to anchorage and its primary function is absorption.

A stem is an organ that raises or separates leaves, exposing them to sunlight, and reproductive structures, facilitating dispersal of pollen and fruit. Each stem consists of an alternating system of nodes, the points at which leaves are attached, and internodes, the stem segments between nodes. In the upper angle formed by each leaf and stem is an axillary bud, a structure that can form a lateral shoot, or a branch. Most of the growth of a young shoot is concentrated near the shoot tip, which consists of an apical bud, that is composed of developing leaves and a compact series of nodes and internodes. Removal of the apical bud stimulates growth of axillary buds, resulting in more branches.

A leaf is the main photosynthetic organ in most vascular plants. Leaves vary extensively in form but generally consist of a flattened blade and a stalk, the petioles, which joins the leaf to the stem at a node. Leaves also differ in the arrangement of veins, the vascular tissue of leaves. Some have parallel major veins that run the length of the blade, while others have a branched network of major veins. In identifying plants according to structure, taxonomists rely on variations in leaf morphology, such as leaf shape, the branching pattern of veins, and the spatial arrangement of leaves.

Each plant organ has dermal, vascular and ground tissues, each a part of the plant tissue system and have different functions. The dermal tissue system is the plant’s outer protective covering. It forms the first line of defense against physical damage and pathogens. 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 wood plants, protective tissues called periderm replace the epidermis in older regions of stems and roots.  In addition to protecting the plant against water loss and disease, some plants have epidermis that contains trichomes, hairlike outgrowths that provide defense against insects by forming a barrier or by secreting sticky fluids or toxic compounds. The vascular tissue system carries out long-distance transport of materials between the roots and shoot systems. The two types of vascular tissues are xylem and phloem. Xylem conducts water and dissolved minerals upward from roots into the shoots. Phloem transports sugars, the product of photosynthesis, from where they are produced to where they are needed.  The vascular tissue of a root and stem is collectively called the stele and its arrangement varies, depending on the species and organs. Tissues that are neither dermal nor vascular are part of the ground tissue system. It is responsible for most of the plant’s metabolic functions and is located between the dermal tissue and vascular tissue in each organ.

There are many different types of plant cells: parenchyma cells, collenchyma cells, sclerenchyma cells, water-conducting cells of the xylem, and sugar-conducting cells of the phloem. Parenchyma cells have primary walls that are relatively thin and flexible, but lack secondary walls. They perform most of the metabolic functions of the plant, synthesizing and storing various organic products. Most parenchyma cells retain the ability to divide and differentiate into other types of plant cells under particular conditions, such as wound repairs. Grouped in strands, collenchyma cells are generally elongated cells that have thicker, though more uneven, primary walls than parenchyma cells. They provide flexible support without restraining growth and elongate with the stems and leaves they support as they reach maturity. Sclerenchyma cells also function as supporting elements in plants. They are more rigid than collenchyma cells and its secondary wall contain large amount of lignin, indigestible polymer that accounts for more than a quarter of the dry mass of wood. Mature sclerenchyma cells are dead at functional maturity, but their rigid walls remain as “skeletons” that can support the plant for hundreds of years. Two types of sclerenchyma cells are specialized entirely support and strengthening: sclereids and fibers.  Sclereids, which are more irregular in shape, have very thick, lignified walls, while fibers, which are usually grouped in strands, are long, slender, and tapered. The xylem has two types of water-conducting cells: tracheids and vessel elements. Both are tubular, elongated cells that are dead at functional maturity and have pits where water can migrate laterally between neighboring cells. The phloem has long, narrow sugar-conducting cells called sieve cells that are alive at functional maturity. The sieve tubes have sieve plates and pores that facilitate the flow of fluid from cell to cell.

Penny is a typical flowering tree plant. She has all three organs necessary for its function. With a functioning taproot system, she can easily absorb minerals and water and store carbohydrates that can will transported throughout the plant. She has a stem to hold her structure up and leaves to conduct photosynthesis. Because she is very sturdy and well-adapt to her environment, Penny must have a tough dermal tissue system and vascular  tissue system.

Entry One: Introduction and Overview

Entry One:
My name is Dan Luu. The picture to the left is an image of my plant, Penny. Since I do not possess a smart phone, I was unable to take a pictorial image of Penny during school, so I had to take a picture of her clone at home, where she is under better care than her twin at school.
Penny is a Kamatafuji peony tree. The Kamatafuji peony tree is a special type of tree, with its origin tracing back to ancient China. This type of tree can only be grown by the Chinese Emperor himself, which is why peony trees are typically called "The Emperor's flower." The Kamatafuji peony tree eventually spread internationally after the 8th century, when it reached Japan.
Penny is a flowering tree. Although its blooming/flowering period varies from year to year, the Kamatafuji peony tree usually flowers from late April to early May. A mature plant can reach a height of four to five feet tall and have in excess a hundred that spans more than ten inches wide. The flowers range in color from maroon, crimson, scarlet, various shades of pink, to pure white. For it to properly grow, the tree requires frequent watering and full sun exposure. It can adapt to any type of soil, but moisture must be well drained. The Kamatafuji peony tree is very sturdy under extreme heat and cold temperature and, if nurtured properly, can bloom for a lifetime.
My assignment in this Green Thumb Project is to be responsible for taking care of my plant. I must help it to develop into well-nurtured plant, while acquiring knowledge about my plant through botany. Additionally, I will consider how evolutionary processes have shaped my plant and the process in which it absorb nutrients throughout it structure. I am hoping to learn how plants transport its nutrients throughout its internal structure, photosynthesize, and reproduce. I am also interested in how GMO, pesticides, and other chemicals affect its growth.
I would consider myself a green thumb. Despite the fact that Penny is sturdy under extreme conditions, I have plenty of other plants grown in my backyard, such as tomatoes, pumpkins, and chili peppers, that did not wither and die under my care.