Conceptual Overview

The plant body consists of integrated structures called organs, such as roots, stems, leaves, petals etc. which in turn are made of tissues. Tissues are cell complexes which are similar in origin and structure, and are designed to carry out specific functions. Tissues may be simple (consisting of one cell type only) as parenchyma, collenchyma or sclerenchyma, or complex (consisting of two or more cell types) as in the case of epidermis, xylem and phloem.

Parenchyma proper, or ground tissue, is usually considered as a tissue lacking in a high degree of functional and structural specialization, and which consists of highly vacuolated thin-walled cells with a minimal amount of cytoplasm. Examples of such tissue are cortical parenchyma of stems and roots as well as the pith of stems. However, parenchyma cells may be highly specialized when they are parts of a complex tissue like xylem parenchyma. Other examples of highly specialized types of parenchyma include assimilation parenchyma of leaves (mesophyll), aerenchyma and storage parenchyma of tubers and seeds.

Parenchyma is often called fundamental, or ground tissue, since it appears as a ground substance in which other tissues, primarily vascular, are embedded. It is also the foundation of the plant inasmuch as the apical meristems and the reproductive cells are parenchymatous in nature. It is also noteworthy to point out that more primitive multicellular non-tracheophytes tend to consist of parenchyma only. Some of the most important activities of the plant, such as photosynthesis, assimilation, respiration, storage, and secretion are primarily based in parenchymatous tissues. While parenchyma cells are simple, primitive, or unspecialized, they also possess the highest degree of developmental plasticity that enables them in specialized circumstances to become transformed into other cell types (e.g. sclerenchyma).

While parenchyma cells are generally thought of as being thin-walled, they sometimes have very thick primary walls as in the case of endosperm cells of date palm, persimmon, Asparagus and Coffea arabica. Mature parenchyma tissues are either close-packed, or may be permeated by a large intercellular space system. Storage parenchyma of fleshy fruits generally has abundant intercellular spaces. In contrast, the endosperm of most seeds contains no (or small) intercellular spaces. The intercellular (or air) spaces reach their highest development in the aquatic angiosperms, both in individual size and in combined volume. In these plants, the air in the spaces serves not only for aeration, but (when they occur in floating leaves or stems) also to give the plants buoyancy and support. The air spaces form an elaborate system that appears to be continuous from the leaf to the root, thereby allowing oxygen to diffuse from its point of highest concentration in the leaves to places of scarcity in the tissues that are lacking chlorophyll.

Parenchyma cells have been a matter of considerable study as far as three-dimensional shape is concerned. Most studies have shown that parenchyma cells consist of a variety of polyhedra that have an average of 14 faces. A geometrically perfect, 14-sided polyhedron with 8 hexagonal and 6 quadrilateral faces has been named an orthic tetrakaidecahedron. This ideal figure is compromised in plants which may have a range of facets, but usually averaging 14. Pressure and surface tension both appear to play a role in determining the number of facets for a particular parenchyma cell.

Collenchyma, together with sclerenchyma, belongs to a group of plant tissues often designated as supporting or mechanical. Collenchyma is characteristically found in leaves and elongating stems. In leaves, it appears as strands, often located above and below major veins, as well as in petioles and sometimes leaf blade margins. In stems, it appears as a hollow cylinder around vascular tissues, or as peripheral longitudinal strands. Collenchyma cells have unequally thickened primary walls, especially when observed in cross-sectional view. The different thickness patterns of the wall is a characteristic feature formed during elongation. There are four primary types of collenchyma: angular, annular, lamellar (or plate), and lacunar.

Collenchyma is a living tissue composed of elongated cells with thick non-lignified primary walls. Such cells are most closely aligned physiologically with parenchyma cells. Where collenchyma and parenchyma cells are found adjacent to each other, they frequently intergrade through transitional cells. The resemblance to parenchyma is further stressed by the common occurrence of chloroplasts in collenchyma and by the ability of this tissue to undergo reversible changes in wall thickness, and to engage in meristematic activities. Thus, it is entirely appropriate to consider these two cell types in the same unit of study.

It is the cell walls of collenchyma that are their most distinctive feature. Their primary walls are thickened unevenly, and in a variety of patterns. The primary patterns are listed above. Most reports of collenchyma are based on observations from cross-sectional views. However, in longitudinal sections, collenchyma shows thin and thick wall portions, depending on the direction of the cut with reference to the thickenings. The nearly transverse end walls (or septa) are usually thin, whereas pointed ends may appear to be solid because of the accumulation of wall material. Primary pit fields occur in collenchyma cells, both in the thin and thick parts of the walls. The walls consist mainly of cellulose and hemicelluloses, and contain much water (as much as 67% based on fresh weight). Thickening of the walls occurs during elongation growth of the cells, with successive layers of wall material formed around the entire cell, but they are wider in the places of thickenings. Cellulose microfibrils have a helicoidal texture in collenchyma cell walls. In some cases it has been reported that the degree of wall thickening in collenchyma is increased if, during development, the plants are exposed to motion by wind or other mechanical forces. On the other hand, wall thickenings may be removed in response to injuries and wound-healing reactions. Collenchyma that differentiates early in a given organ becomes more highly specialized, whereas that formed later is often more like parenchyma.