Plant Tissue Systems: Dermal, Ground, and Vascular
The epidermis is the dermal tissue of young plants undergoing primary growth (see textbook Figure 1.3). It is generally composed of specialized, flattened polygonal cells that occur on all plant surfaces. Shoot surfaces are usually coated with a waxy cuticle to prevent water loss and are often covered with hairs, or trichomes, which are epidermal cell extensions.
Pairs of specialized epidermal cells, the guard cells, are found surrounding microscopic pores in all leaves (see textbook Figure 1.3A). The guard cells and pores are called stomata (singular stoma), and they permit gas exchange (water loss, CO2 uptake, and O2 release or uptake) between the atmosphere and the interior of the leaf.
The root epidermis is adapted for absorption of water and minerals, and its outer wall surface typically does not have a waxy cuticle. Extensions from the root epidermal cells, the root hairs , increase the surface area over which absorption can take place (see textbook Figure 1.3C).
Ground tissue. Making up the bulk of the plant are cells termed the ground tissue. There are three types of ground tissue: parenchyma, collenchyma, and sclerenchyma.
Parenchyma, the most abundant ground tissue, consists of thin-walled, metabolically active cells that carry out a variety of functions in the plant, including photosynthesis and storage (see textbook Figure 1.3B)
Collenchyma tissue is composed of narrow, elongated cells with thick primary walls (see textbook Figure 1.3C). Collenchyma cells provide structural support to the growing plant body, particularly shoots, and their thickened walls are nonlignified, so they can stretch as the organ elongates. Collenchyma cells are typically arranged in bundles or layers near the periphery of stems or leaf petioles.
Sclerenchyma consists of two types of cells, sclereids and fibers (see textbook Figure 1.3D) Both have thick secondary walls and are frequently dead at maturity. Sclereids occur in a variety of shapes, ranging from roughly spherical to branched, and are widely distributed throughout the plant. In contrast, fibers are narrow, elongated cells that are commonly associated with vascular tissues. The main function of sclerenchyma is to provide mechanical support, particularly to parts of the plant that are no longer elongating.
In the stem, the pith and the cortex make up the ground tissue (see textbook Figure 1.1B). The pith is located within the cylinder of vascular tissue, where it often exhibits a spongy texture because of the presence of large intercellular air spaces. If the growth of the pith fails to keep up with that of the surrounding tissues, the pith may degenerate, producing a hollow stem. In general, roots lack piths, although there are exceptions to this rule. In contrast, the cortex , which is located between the epidermis and the vascular cylinder, is present in both stems and roots (see textbook Figure 1.1B and C).
At the boundary between the ground tissue and the vascular tissue in roots, and occasionally in stems, is a specialized layer of co rtex known as the endodermis (see textbook Figure 1.1C). This single layer of cells originates from cortical tissue at the innermost layer of the root cortex and forms a cylinder that surrounds the central vascular tissue, or stele. Early in root development, a narrow band composed of the waxy substance suberin is formed in the cell walls circumscribing each endodermal cell (see textbook Figure 1.1). These suberin deposits, called Casparian strips, form a barrier in the endodermal walls to the intercellular movement of water, ions, and other water-soluble solutes to the vascular cells.
Leaves have two interior layers of ground tissue that are collectively known as the mesophyll (see textbook Figure 1.1A). The palisade parenchyma consists of closely spaced, columnar cells located beneath the upper epidermis. There is usually one layer of palisade parenchyma in the leaf. Palisade parenchyma cells are rich in chloroplasts and are a primary site of photosynthesis in the leaf. Below the palisade parenchyma are i rregularly shaped, widely spaced spongy mesophyll cells. The spongy mesophyll cells are also photosynthetic, and the large spaces between these cells allow diffusion of carbon dioxide. The spongy mesophyll also contributes to leaf flexibility in the wind, and this flexibility facilitates the movement of gases within the leaf.
Vascular tissues: xylem and phloem. The vascular tissue is composed of two major conducting systems: the xylem and the phloem. The xylem transports water and mineral ions from the root to the rest of the plant. The phloem distributes the products of photosynthesis and a variety of other solutes throughout the plant (see textbook Figure 1.1B and C).
The tracheids and vessel elements are the conducting cells of the xylem (see textbook Figure 1.3E). Both of these cell types have elaborate secondary-wall thickenings and lose their cytoplasm at maturity; that is, they are dead when functional. Tracheids overlap each other, whereas vessel elements have open end walls and are arranged end to end to form a larger unit called a vessel. Other cell types present in the xylem include parenchyma cells, which are important for the storage of energy-rich molecules and phenolic compounds, and sclerenchyma fibers.
The sieve elements and sieve cells are responsible for sugar translocation in the phloem (see textbook Figure 1.3E). The former are found in angiosperms; the latter perform the same function in gymnosperms. Like vessel elements, sieve elements are often stacked in vertical rows, forming larger units called sieve tubes, whereas sieve cells form overlapping arrays. Both types of conducting cells are living when functional, but they lack nuclei and central vacuoles and have relatively few cytoplasmic organelles.
Substances are translocated from sieve cell to sieve cell laterally through circular or oval zones containing enlarged pores, called sieve areas. In contrast, sieve tubes translocate substances through large pores in the end walls of the sieve elements, called sieve plates. Sugar movement through sieve tubes is more efficient and rapid than through sieve cells and represents a more evolutionarily advanced mechanism.
Sieve elements are associated with, and depend on, densely cytoplasmic parenchyma cells called companion cells. The analogous cells adjacent to the sieve cells of gymnosperms are called albuminous cells. Companion cells provide proteins and metabolites necessary for the functions of the sieve tube elements. In addition, the phloem frequently contains storage parenchyma and fibers that provide mechanical support.