Parenchyma tissue is composed of parenchyma cells, which are the most abundant of the cell types and are found in almost all major parts of higher plants. They are more or less spherical in shape when they are first produced, but when all the parenchyma cells push up against one another, their thin, pliable walls are flattened at the points of contact. As a result, parenchyma cells assume various shapes and sizes, with the majority having 14 sides. They tend to have large vacuoles and may contain starch grains, oils, tannins (tanning or dyeing substances), crystals, and various other secretions.
More often than not, parenchyma cells have spaces between them; in fact, in water lilies and other aquatic plants, the intercellular spaces are quite extensive and form a network throughout the entire plant. This type of parenchyma tissue—with extensive connected air spaces—is referred to as aerenchyma.
Parenchyma cells containing numerous chloroplasts (as found in leaves) are collectively referred to as chlorenchyma tissue. Chlorenchyma tissues function mainly in photosynthesis, while parenchyma tissues without chloroplasts function mostly in food or water storage. For example, the soft, edible parts of most fruits and vegetables consist largely of parenchyma. Some parenchyma cells develop irregular extensions of the inner wall that greatly increase the surface area of the plasma membrane. Such cells, called transfer cells, are found in nectaries of flowers and in carnivorous plants, where they apparently play a role in transferring dissolved substances between adjacent cells. Many parenchyma cells live a long time; in some cacti, for example, they may live to be over 100 years old.
Mature parenchyma cells can divide long after they were produced by a meristem. In fact, when a cutting (segment of stem) is induced to grow, it is parenchyma cells that start dividing and give rise to new roots. When a plant is damaged or wounded, the capacity of parenchyma cells to multiply is especially important in repair of tissues.
Collenchyma cells, like parenchyma cells, have living cytoplasm and may remain alive a long time. Their walls generally are thicker and more uneven in thickness than those of parenchyma cells. The unevenness is due to extra primary wall in the corners. Collenchyma cells often occur just beneath the epidermis; typically, they are longer than they are wide, and their walls are pliable as well as strong. They provide flexible support for both growing organs and mature organs, such as leaves and floral parts. The “strings” of celery that get stuck in our teeth, for example, are composed of collenchyma cells.
Sclerenchyma tissue consists of cells that have thick, tough, secondary walls, normally impregnated with lignin. Most sclerenchyma cells are dead at maturity and function in support. Two forms of sclerenchyma occur: sclereids and fibers. Sclereids may be randomly distributed in other tissues. For example, the slightly gritty texture of pears is due to the presence of groups of sclereids, or stone cells, as they are sometimes called. The hardness of nut shells and the pits of peaches and other stone fruits is due to sclereids.
Sclereids tend to be about as long as they are wide and sometimes occur in specific zones (e.g., the margins of camellia leaves) rather than being scattered within other tissues. Fibers may be found in association with a number of different tissues in roots, stems, leaves, and fruits. They are usually much longer than they are wide and have a proportionately tiny cavity, or lumen, in the center of the cell. At present, fibers from more than 40 different families of plants are in commercial use in the manufacture of textile goods, ropes, string, canvas, and similar products. Archaeological evidence indicates that humans have been using plant fibers for at least 10,000 years.