Chapter 36: Plant Form
Overview of Meristems
Meristems are clusters of small cells with dense cytoplasm and proportionately large nuclei that act as stem cells do in animals. That is, one cell divides to give rise to two cells, of which one remains meristematic, while the other undergoes differentiation and contributes to the plant body (figure 36.3). In this way, the population of meristem cells is continually renewed.
External leaf structure reflects vascular morphology
Most eudicot leaves have a flattened blade and a slender stalk, the petiole, while grasses and other monocots usually lack a petiole. Veins (a term used for the vascular bundles in leaves) consist of both xylem and phloem and are distributed throughout the leaf blades. The main veins are parallel in most monocot leaves; the veins of eudicots, on the other hand, form an often intricate network (figure 36.30).
Learning Outcomes Review 36.4
Shoots grow from apical and lateral meristems. Axillary buds may develop into branches, flowers, or leaves. In monocots, vascular tissue is evenly spaced throughout the stem ground tissue; in eudicots, vascular tissue is arranged in a ring with inner and outer ground tissues. Some plants produce modified stems for support, vegetative reproduction, or nutrient storage.
Root Hairs
Root hairs, which are tubular extensions of individual epidermal cells, occur in a zone just behind the tips of young, growing roots (figure 36.10). Because a root hair is simply an extension of an epidermal cell and not a separate cell, no cross-wall isolates the hair from the rest of the cell. Root hairs keep the root in intimate contact with the surrounding soil particles and greatly increase the root's surface area and efficiency of absorption.
Lateral Meristems
Although secondary growth increases girth in many nonwoody plants, its effects are most dramatic in woody plants, which have two lateral meristems. Tissues formed from lateral meristems, which comprise most of the trunk, branches, and older roots of trees and shrubs, are known as secondary tissues and are collectively called the secondary plant body.
Learning Outcomes Review 36.2
Dermal tissue protects a plant from its environment and contains specialized cells such as guard cells, trichomes, and root hairs. Ground tissue serves several functions, including storage (parenchyma cells), photosynthesis (specialized parenchyma called chlorenchyma), and structural support (collenchyma and sclerenchyma). Vascular tissue carries water through the xylem (primarily vessels) and nutrients through the phloem (primarily sieve-tube members).
Apical Meristems
The three primary meristems are the protoderm, which forms the epidermis; the procambium, which produces primary vascular tissues (primary xylem for water transport and primary phloem for nutrient transport); and the ground meristem, which differentiates further into ground tissue. In some plants, such as horsetails and corn, intercalary meristems arise in stem internodes (spaces between leaf attachments), adding to the internode lengths.
New Growth Occurs at Meristems
Although embryo cells can undergo division and differentiation to form many cell types, the fate of most adult cells is more restricted. Further development of the plant body depends on the activities of meristems found in shoot and root apices, as well as other parts of the plant. Meristem cells are undifferentiated cells that can divide indefinitely and give rise to many types of differentiated cells.
Collenchyma
Flexible collenchyma cells provide support for plant organs, allowing them to bend without breaking.
Roots are adapted for growing underground and absorbing water and solutes
Four regions are commonly recognized in developing roots: the root cap, the zone of cell division, the zone of elongation, and the zone of maturation (figure 36.14). In these last three zones, the boundaries are not clearly defined.
Ground tissue cells perform many functions, including storage, photosynthesis, and support
Ground tissue consists primarily of thin-walled parenchyma cells that function in storage, photosynthesis, and secretion. Other ground tissue, composed of collenchyma cells and sclerenchyma cells, provides support and protection.
Guard Cells
Guard cells are paired, sausage-shaped cells flanking a stoma (plural, stomata), a mouth-shaped epidermal opening. Stomata serve a dual function. Exchange of oxygen and carbon dioxide gases through the stomata is essential for photosynthesis. In addition, diffusion of water in vapor form takes place almost exclusively through the stomata, enabling transport of water and minerals from the roots to the leaves.
The Zone of Elongation
In the zone of elongation, roots lengthen because the cells produced by the primary meristems elongate. The small vacuoles merge and grow until they occupy 90% or more of the volume of each cell.
Learning Outcomes Review 36.5
Leaves come in a range of forms. A simple leaf is undivided, whereas a compound leaf has a number of separate leaflets. Monocots typically produce leaves with parallel veins, whereas those of eudicots are netted. Mesophyll cells carry out photosynthesis; in monocots, mesophyll is undifferentiated, whereas in eudicots it is divided into palisade and spongy mesophyll. Leaves may be modified for reproduction, protection, water conservation, uptake of nutrients, and even as traps for insects.
Leaves: Photosynthetic Organs
Leaves, which are initiated as primordia by the apical meristems (see figure 36.20), are vital to life as we know it. They are the principal sites of photosynthesis on land, providing the base of the food chain. Because leaves are crucial to a plant, features such as their arrangement, form, size, and internal structure are highly significant and can differ greatly. Different patterns have adaptive value in different environments.
Modified roots accomplish specialized functions
Most plants produce either a taproot system, characterized by a single large root with smaller branch roots, or a fibrous root system, composed of many smaller roots of similar diameter.
Parenchyma
Parenchyma cells are the most common type of plant cell. They have large vacuoles, thin walls, and are initially (but briefly) more or less spherical. These cells, which have living protoplasts, push up against each other shortly after they are produced, however, and assume other shapes, often ending up with 11 to 17 sides. Parenchyma cells may live for many years; in some plants (for example, cacti), they may live to be over 100 years old. They function in the storage of food and water, photosynthesis, and secretion. They are the most abundant cells of primary tissues and may also occur, to a much lesser extent, in secondary tissues (figure 36.11a).
Phloem
Phloem, which is located toward the outer part of roots and stems, is the principal food-conducting tissue in vascular plants.
Roots and shoots are composed of three types of tissues
Plant cell types can be distinguished by the size of their vacuoles, whether they are living or not at maturity, and by the thickness of their cellulose cell walls, a distinguishing feature of plant cells (see chapter 4 to review cell structure). Dermal tissue, primarily epidermis, is one cell layer thick in most plants, and it forms an outer protective covering for the plant. Ground tissue cells function in storage, photosynthesis, and secretion, in addition to forming fibers that support and protect plants. Vascular tissue conducts fluids and dissolved substances throughout the plant body. Each of these tissues and their many functions are described in more detail in later sections.
The Zone of Cell Division
The apical meristem is located in the center of the root tip in the area protected by the root cap. Most of the activity in this zone of cell division takes place toward the edges of the meristem, where the cells divide every 12 to 37 hours, often coordinately, reaching a peak of division once or twice a day. A group of cells in the center of the root apical meristem, termed the quiescent center, divides only very infrequently.
The Zone of Maturation
The cells that have elongated in the zone of elongation become differentiated into specific cell types in the zone of maturation (figure 36.17, see also figure 36.14). Parenchyma cells are produced by the ground meristem just inside the epidermis. This tissue, called the cortex, may be many cell layers wide and functions in food storage. In monocot (and a few eudicot) roots, the primary xylem is in discrete vascular bundles arranged in a ring, which surrounds parenchyma cells, called pith, at the very center of the root (figure 36.17).
Organization of the Plant Body: An Overview
The earliest vascular plants, many of which are extinct, did not have a clear differentiation of the plant body into specialized organs such as roots and leaves. Among modern vascular plants, the presence of these organs reflects increasing specialization, particularly in relation to the demands of a terrestrial existence. Obtaining water, for example, is a major challenge, and roots are adapted for water absorption from the soil. Leaves, roots, branches, and flowers all exhibit variations in size and number from plant to plant. Development of the form and structure of these parts may be precisely controlled, but some aspects of leaf, stem, and root development are quite flexible.
Learning Outcomes Review 36.3
The root cap protects the root apical meristem and helps to sense gravity. New cells formed in the zone of cell division grow in length in the zone of elongation. Cells differentiate in the zone of maturation, and root hairs appear here. Root hairs greatly increase the absorptive surface area of roots. Modified roots allow plants to carry out many additional functions, including bracing, aeration, and storage of nutrients and water.
Learning Outcomes Review 36.1
The root system anchors plants and absorbs water and nutrients, whereas the shoot system, consisting of stems, leaves, and flowers, carries out photosynthesis and sexual reproduction. The three general types of tissue in both roots and shoots are dermal, ground, and vascular tissue. Primary growth is produced by apical meristems at the tips of roots and shoots; secondary growth is produced by lateral meristems that are peripheral and increase girth.
External Stem Structure
The shoot apical meristem produces stem tissue and intermittently produces bulges (primordia) that are capable of developing into leaves, other shoots, or even flowers (figure 36.20). Leaves may be arranged in a spiral around the stem, or they may be in pairs opposite or alternate to one another; they also may occur in whorls (circles) of three or more (figure 36.21). In plants, the pattern of leaf arrangement, called phyllotaxy, may optimize the exposure of leaves to the Sun. The point of leaf attachment to the stem is called a node; the portion of stem between two nodes is called an internode.
Internal leaf structure regulates gas exchange and evaporation
The tissue between the upper and lower epidermis is called mesophyll. Mesophyll is interspersed with veins of various sizes.
Trichomes
Trichomes are cellular or multicellular hairlike outgrowths of the epidermis (figure 36.8).
Vascular tissue conducts water and nutrients throughout the plant
Vascular tissue, as mentioned in section 36.2, includes two kinds of conducting tissues: (1) xylem, which conducts water and dissolved minerals, and (2) phloem, which conducts a solution of carbohydrates—mainly sucrose—used by plants for food. The phloem also transports hormones, amino acids, and other substances that are necessary for plant growth.
Xylem
Xylem, the principal water-conducting tissue of plants, usually contains a combination of vessels, which are continuous tubes formed from dead, hollow, cylindrical cells arranged end-to-end, and tracheids, which are dead cells that taper at the ends and overlap one another (figure 36.12). In some plants, such as gymnosperms (but not flowering plants), tracheids are the only water-conducting cells present; water passes in an unbroken stream through the xylem from the roots up through the shoot and into the leaves. When the water reaches the leaves, much of it diffuses as water vapor into the intercellular spaces and out of the leaves into the surrounding air, mainly through the stomata. This diffusion of water vapor from a plant is known as transpiration (see chapter 37). In addition to conducting water, dissolved minerals, and inorganic ions such as nitrates and phosphates throughout the plant, xylem supports the plant body. Vessel members tend to be shorter and wider than tracheids. When viewed with a microscope, they resemble beverage cans with both ends removed. A vessel is a stack of vessel members. Both vessel members and tracheids have thick, lignified secondary walls and no living protoplasts at maturity. Lignin is produced by the cell and secreted to strengthen the cellulose cell walls before the protoplast dies, leaving only the cell wall. Page 741 Tracheids contain pits, which are small areas where no secondary wall material has been deposited. Primary wall is present, but it is water permeable. The pits of adjacent cells occur opposite one another; the continuous stream of water flows through these pits from tracheid to tracheid. In contrast, vessel members, which are joined end to end, may have open ends or may have bars or strips of wall material across the open ends (figure 36.12). Vessels conduct water more efficiently than do the overlapping strands of tracheids. We know this partly because vessel members have evolved from tracheids independently in several groups of plants, suggesting that they are favored by natural selection. In addition to the conducting cells, xylem typically includes fibers and parenchyma cells (ground tissue cells). Some types of fibers likely evolved from tracheids, becoming specialized for strengthening rather than conducting.