Botany Notes: 007 Chapter 4

Chapter 4. Tissues Plants are divided by botanists into two categories: vascular and non-vascular plants. Vascular plants (such as ferns, conifers, and flowering plants) have vascular tissues specialized to transport water and dissolved substances throughout the plant. Non-vascular plants such as mosses and liverworts lack vascular tissues. Of the more than 275,000 species of plants, more than 245,000 species are vascular. These plants can be divided into two groups: gymnosperms and angiosperms. Gymnosperms, around 10,000 species, are plants whose pollen goes directly to ovules (unfertilized seeds) and whose seeds are naked (i.e., are not enclosed in fruits). Thus by definition, they are all fruitless seed plants. On the other hand, angiosperms, which make up about 235,000 species, are seed-producing plants. Since angiosperms make up the bulk of the plant kingdom, this type of plants will be the focus in our discussions. Plants > 275,000 sp Vascular > 245,000 sp Non-vascular < 30,000 sp Gymnosperms ~10,000 sp

Angiosperms ~235,000 sp

Monocots

Dicots

*Vascular plants – have well developed conducting tissues through which water and solutes are transported to different body regions Non-vascular plants – have no internal transport systems (or have very simple ones) Gymnosperms – seed-producing, vascular, non-flowering plants Angiosperms – seed-producing, vascular, flowering plants

Primary Growth The primary body of plants consists of four tissues: meristems, ground tissue, dermal tissue, and vascular tissue. These tissues consist of distinctive types of cells and usually have more than one function. Meristems Meristems are localized regions of cellular division that form a plant’s cells. Not all parts of a plant grow at the same time. Rather, plant growth is restricted to perpetually embryonic regions called meristems. Plants have four types of meristems: apical meristems, axillary meristems, lateral meristems, and intercalary meristems. Apical Meristems. Apical meristems occur near the tips of roots and shoots, and produce primary tissues. These meristems account for primary growth, which is elongation of roots and shoots. Primary growth is important because it enables a plant to explore new environments for light, water, and nutrients. Shoot Apical Meristems. A shoot apex consists of two zones – a tunica and a corpus. The tunica is the outermost layer(s) of the shoot apex. Most angiosperms have one or two tunica layers, the outermost of which becomes the protoderm. The corpus includes cells of the shoot tip below the tunica. Root Apical Meristems. The root tip is covered by a slimy root cap that protects the root apical meristem and helps the root move through the soil.

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Axillary Meristems. Axillary buds are reiterations of the apical meristem left behind by primary growth. Axillary buds occur in the axil of a leaf and typically undergo a dormant period. Axillary buds are important because they are a shoot’s insurance policy: they are inactive cells that can form a branch or a flower. Intercalary Meristems. Intercalary meristems occur between mature tissues. These meristems are most common in grasses, where they occur at the bases of nodes. Intercalary meristems are important because they help regenerate parts removed by grazing herbivores (or lawn mowers). Lateral Meristems. Lateral meristems are cylindrical meristems that form in sub-apical regions of the roots and shoots of woody plants. Lateral meristems produce secondary growth, which increases the girth of the plant. Secondary growth is important because it makes a plant sturdier. This, in turn, enables the plant to grow taller and intercept light. There are two kinds of lateral meristems: the vascular cambium and phellogen (cork cambium). These meristems form a woody secondary body consisting of wood and bark. The vascular cambium forms wood (secondary xylem) and secondary phloem. Phloem forms the periderm. The periderm forms a layer of dead, suberized cells that protects the inner tissues of the secondary plant body. Functions of the Apical Meristems Roots and shoots enlarge by primary growth from apical meristems located near their tips. The cells found in apical meristems usually have thin walls, prominent nuclei, and small vacuoles. Although they often appear simple and referred to as undifferentiated, there are distinct types of meristematic cells with unique structures even though they may be in the same plant. Apical meristems have two primary functions: establishing patterns and producing new, genetically healthy cells. Establishing Patterns. Apical meristems establish many patterns in plants (e.g. the arrangement of leaves). When an apical meristem is damaged, the remaining tissues continue to grow. However, only those tissues whose patterns were already set by the apical meristem develop normally. Continued patterned development is delayed until a meristem is regenerated. Providing New, Genetically Healthy Cells. Cells continue to divide after they are pushed out of the meristem. These cells, the derivatives of meristems, rather than the cells of the meristem itself, continue to divide for several weeks or months and form the various tissues of the plant body. This is important because it allows meristems to function as a reserve of mitotically young and genetically healthy cells. Recent studies show that intact meristems recognize and inhibit growth of mutant cells. Derivatives of Meristems Most cell divisions in tips of roots and shoots occur in derivatives of meristems. These derivatives of meristems form regions of cellular division and specialization called transitional meristems. There are three transitional meristems in plants: the protoderm, the procambium, and the ground meristem. Protoderm. The protoderm is the transitional meristem that forms epidermis. Protodermal cells usually divide perpendicular to the surface and form a sheet of new epidermal cells that covers the growing root or shoot. The protoderm is irreplaceable; if damaged, no other cells can assume its functions. Procambium. The procambium is the transitional meristem that produces vascular tissues. Procambium differentiates as strands connected to the plant’s mature vascular tissue. 25

Ground Meristem. Ground meristem produces ground tissues and consists of two other meristems: the flank meristem and the rib meristem. The flank meristem forms the cortex, and the rib meristem forms the pith. The procambium and ground meristem form in response to developmental signals received from nearby organs and tissues. For example, young leaves control the differentiation of procambium: removing leaves from a shoot apex stops procambium differentiation and grafting leaves to a mass of plant tissue induces formation of procambium and vascular tissue. Dermal Tissue Dermal tissues cover the plant body. The dermal tissue that covers the primary body of plants is the epidermis. The epidermis has several functions including the absorption of water and minerals, secretion of cuticle, protection against herbivores, and control of gas exchange. Most epidermal cells are flat, tile-like cells packed together like bricks in a brick wall. These cells typically lack chloroplasts and are transparent. However, the vacuoles of epidermal cells occasionally contain pigments. The epidermis differentiates from protoderm and is usually only one cell thick. Stomata. The only intercellular spaces in the epidermis are stomata (sing. stoma). Each stoma is surrounded by two guard cells. Stomata occur on stems, leaves, flowers, and fruits but tend to be most abundant on the undersides of leaves. They regulate the exchange of gases. Stomata open and close in response to environmentally induced changes in the turgor pressure of guard cells. Bulliform cells. Other specialized cells found in the upper epidermis of leaves are large, vacuolated cells called bulliform cells. When it is hot and dry, these cells rapidly lose water from their vacuoles and shrink, causing the leaf to roll into a protected cylinder. Leaf rolling reduces the surface area that is exposed to the environment, thereby helping the leaves conserve water. Cuticle. The outer walls of the epidermal cells are covered with a waterproof cuticle made of a fatty material called cutin. The cuticle protects the plant from dessication by helping to maintain a watery, aquatic environment inside the plant. It also protects the plant from microbes because it resists microbial infection and degradation. The cuticle is often covered by epicuticular wax. Trichomes. Trichomes are single-celled or mult-icellular outgrowths of epidermal cells. The most economically important trichomes are cotton fibers. They serve to protect the plant from insects and are used by man for clothing. Other trichomes also function in the protection of plants by forming stinging nettles and releasing irritating chemicals. Others impale predators by hook-shaped trichomes or by releasing sticky substances. Other trichomes contain substances that are commercially useful for man. Other trichomes have roles in the nutrition of plants and in the absorption of substances from the environment. Carnivorous plants have trichomes that release enzymes that help liquefy their prey. Root hairs help increase the surface area of roots thereby enabling the plant to extract water and dissolved minerals more effectively. Vascular Tissue Vascular tissues are specialized for long-distance transport of water and dissolved solutes. They are easily seen as veins in leaves. Xylem and phloem are the two kinds of vascular tissues in plants. 26

Xylem. Xylem transports water and dissolved nutrients in an unbroken stream from the roots to all parts of a plant. Primary xylem differentiates from procambium in the apical meristem and occurs throughout the primary body of a plant. Water moves through xylary elements called tracheids and vessel elements, which are dead, hollow cells having thick secondary walls. Phloem. Phloem transports dissolved organic material (especially sucrose) in all directions throughout the plant. Primary phloem differentiates from the procambium and extends throughout the primary body. Solutes move through sieve elements called sieve cells and sieve tube members, which are living at maturity. Ground Tissue Ground tissue differentiates from the ground meristem and constitutes most of the primary body of the plant. Ground tissue occurs throughout the plant, including other tissues such as vascular tissues. The cortex and the pith of stems and roots consist almost entirely of ground tissue. It has several functions including storage, basic metabolism, and support. These functions are performed by three kinds of cells: parenchyma, collenchyma, and schlerenchyma. Parenchyma. Parenchyma cells are the most abundant and versatile cells in plants. These cells have a few distinctive structural characteristics. They function as storage cells for starch and are also the primary sites of the metabolic functions including photosynthesis, respiration, and protein synthesis. Parenchyma cells can change activities and become more specialized. This is important because it is a primary way that plants develop and adapt to various influences such as wounding and changing environments. Parenchyma cells are made by all of a plant’s meristems and occur throughout the plant body. They comprise the photosynthetic tissue of a leaf, the flesh of fruit, and the storage tissue of roots and seeds. Chlorenchyma cells are chloroplast-containing parenchyma cells. Aerenchyma cells contain prominent intercellular spaces usually promoting gas exchange. Transfer cells are parenchyma cells specialized for short-distance transport of solutes. Collenchyma. Collenchyma cells are elongate cells having unevenly thickened primary cell walls adapted for support. They support growing regions of shoots and are therefore common in expanding leaves, petioles, and elongating stems. They often differentiate in strands or as cylinders beneath the epidermis. For example, they form the resilient strings in petioles of celery. Schlerenchyma. Schlerenchyma cells are rigid and have thick, non-stretchable secondary cell walls. They support and strengthen non-extending regions of plants such as mature stems are usually dead at maturity. They occur in all mature parts of plants, including leaves, stems, roots, and bark. Some shclerenchyma cells (sclereids) are the structural element of seed coats and nut shells; others (fibers) are long, tapered cells. Fibers have been used by man for more than 10,000 years as: flax to make linen; ramie for textile; jute to make bags and sacks; and hemp to make ropes, cords, and twines. Secretory Structures Plants secrete a variety of substances from structures called secretory structures. They are seldom classified as a separate type of plant tissue because they often intergrade with other tissues. 27

Despite their various origins in plants, secretory structures typically meet the requirement for classification as a plant tissue: they form a group of cells with a common function. External Secretory Structures Nectaries. Nectaries are structures that secrete nectar, a sugary exudates that attracts insects, birds and other animals. Floral nectarines, those associated with flowers, secrete nectar which is 10%-50% sugar and also contains amino acids. Nectar is typically pushed or diffuses through the walls of secretory cells but may also ooze from the stomata. Plants usually secrete small amounts of nectar, which forces foraging animals to visit several flowers before getting a full meal. Thus a single animal can pollinate tens or hundreds of plants. Extrafloral nectaries, those that occur on the vegetative parts of plants, attract animals that defend the plant. These, for example, may attract ants that protect the plant from leaf eating insects. Hydathodes. Hydathodes are tissues that secrete water through a process called guttation. They are loose arrangement of parenchyma cells usually located at vein endings along the edges of leaves. Guttation occurs through stomata that are always open. Their significance is unknown but they may help relieve pressure caused by the uptake of excessive water by roots at night. Digestive Glands of Carnivorous Plants. Carnivorous plants have trichomes that secrete enzymes and digest trapped prey. These enzymes help gather nitrogen from animal protein, thereby allowing the plants to survive in nitrogen-deficient soil. Salt Glands. Plants that grow in salty soil have secretory structures called salt glands. These glands often form a glistening crust of salt that covers the leaves of these plants. In effect, salt glands are dump sites for excess salt absorbed in water from the soil. Salt glands help plants adapt to life in saline environments. Internal Secretory Structures Secretory Cells. Secretory cells are large cells containing substances such as oils, tannins, resins, mucilage, and crystals. They often occur in groups and have several functions, including the storage and production of chemical deterrents to foraging animals. They are also a source of many oils used by humans. Canals, Ducts, and Cavities. Many plants secrete oils and resins into internal canals, ducts, and cavities. For example, myrrh, frankincense, Citrus oils, Eucalyptus oils, and pine resin are extracted from these structures. These oils deter grazing animals, and resin rapidly seals wounds. Laticifers. Laticifers are secretory structures that contain latex, which is cytoplasm containing a hodgepodge of carbohydrates, organic acids, alkaloids, terpenes, oils, resins, enzymes, and rubber. Latex oozes from wounded surfaces of plants. It may be white, colorless, or orangeyellow. Latex is important because it deters grazing animals and helps seal wounds. Humans have many uses for latex. They are economically important as a source of pharmaceutical products, gums, and rubber. Secondary Growth Woody plants are characterized by secondary growth. This type of growth results from lateral meristems that produce large amounts of secondary phloem and secondary xylem. These secondary tissues are protected by a suberized tissue called cork and increase the plant’s girth, thereby making the plant sturdier and able to grow taller. The increased height decreases the chances that the plant’s leaves will be shaded by other plants. Humans use secondary xylem (i.e. wood) to make a variety of products, most notably paper and lumber. 28

The Vascular Cambium. The vascular cambium is a thin band of cells arranged in an openended cylinder. Except for a few monocots such as Dracaena, only dicots and non-flowering seed plants have secondary growth. The vascular cambium differentiates from the latent procambium between xylem and phloem of both roots and shoots. It produces secondary xylem (wood) and secondary phloem and periderm (bark). Secondary Xylem: Wood. Secondary xylem, or wood, is the inner derivative of the vascular cambium and comprises about 90% of a typical tree. Wood is used for lumber, pulp and paper products, plywood and veneer, fuel, and other miscellaneous products such as fence posts, toothpicks, bowling pins, guitars, etc. Secondary Phloem and Periderm: Bark. All the tissues outside of the vascular cambium constitute bark which consists of two types of tissues: secondary phloem and periderm. Secondary Phloem. Secondary phloem is the outer derivative of the vascular cambium. It transports water and organic solutes between roots and leaves. Only the inner centimeter or so of secondary phloem contains functional sieve elements; sieve elements in the outer parts of secondary phloem are dead and non-functional, and help protect the inner tissues. Periderm. Radial expansion resulting from secondary growth eventually ruptures the epidermis of stems and roots. The ruptured epidermis is replaced by periderm, which protects the underlying tissues. Periderm consists of three tissues: Phellogen – cork cambium; is the meristem that produces the periderm. Phellem – the outer derivatives of the phellogen which are dead, suberized, and lack intercellular spaces. Gas exchange across the phellem occurs through lenticels, which are raised, localized areas of loosely packed cells. Phelloderm – the parenchymatous inner derivative of the phellogen. It is live, not suberized, and may be photosynthetic. Phelloderm also contains many intercellular spaces for gas exchange.

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