Roots As Organs Of Procurement
Root Structure
The first root, as seen under children’s microscopes, formed by the young seedling is called the primary root. Later, with the use of children’s microscope, secondary roots branch from the primary root, and a root system is formed. If the branching results in a system of numerous slender roots, with no single root predominating, as in grass or clover, the plant is said to have a fibrous root system. If, however, the primary root remains dominant, with smaller secondary roots branching from it, the arrangement is called a taproot system. When viewed under children’s microscopes, dandelions, beets, and carrots, among others, are plants with taproots. As these examples suggest, taproots are fre¬quently specialized as storage organs for the products of photosynthe¬sis. Storage is a function of all roots, but particularly of taproots, when viewed under a microscope. Obviously, procurement of water and minerals and storage of high¬ energy organic compounds are not the only functions of roots; they also serve to anchor the plant to the substrate.
The root system of a plant, when viewed under a microscope, is normally very extensive, far more extensive than is ordinarily realized. When we pull up a plant, we seldom get anything even approaching the entire root system, since most of the smaller roots are so firmly embedded in the soil that they break off and are lost unless viewed under a microscope. The size of the system is, of course, important both in anchoring the plant and in providing sufficient absorptive surface. A large multicellular or¬ganism therefore faces a serious problem; it must have an absorptive surface extensive enough to enable it to obtain the nutrients it needs to support its large volume, particularly if most of the absorption is re¬stricted to a limited region of the body, in this case the roots. As an adaptation toward solving this problem, many organisms have evolved extensively subdivided absorptive surfaces, far greater in total area than those of an undivided system of the same volume. The manifold branching of a typical root system is an example of this kind of adap¬tation; it was found that a rye plant less than one meter tall had some 14 million branch roots with a combined length of over 600 kilometers.
Roots have evolved yet another adaptation that increases their ab¬sorptive capacity. Just behind the growing tip of each rootlet, when viewed under a microscope, there is usually an area bearing a dense cluster of tiny hair like extensions of the epidermal cells. The zone of these root hairs on each rootlet is not very long, but the number of root hairs on all the many rootlets is so vast that the total absorptive surface they provide is enormous. The rye plant cited above may have had as many as 14 billion root hairs with a total surface area of over 400 square meters. It is in the region of the root hairs that most absorption of water and minerals takes place.
When viewed trough a microscope in cross section, the root of a young dicot plant can be seen to consist of a series of different tissue layers. On its outer surface is a layer of epidermis one cell thick. Unlike the epidermis of the aerial parts of the plant, that of the root usually has no waxy cuticle on its surface. (The explanation is obvious: The epi¬dermis of a root functions in water absorption, that of the aerial parts as a barrier against diffusion of water.) Each root hair arises from an epidermal cell just back of the growing tip of a rootlet.
Beneath the epidermis is the cortex, a wide area composed primarily of parenchyma tissue, with numerous intercellular spaces, as seen under a microscope. Large quantities of starch are often stored in the cells of the cortex. This tissue, so prominent and important in young roots, is frequently much reduced or even lost in older roots, where both cortex and epidermis may be replaced by a corky periderm.
Next interior to the cortex is the endodermis, a layer one cell thick. You will recall that endodermal cells are characterized by a waterproof band, the Casparian strip, which runs through their radial (side) and end walls. The walls of mature endodermal cells are often very thick and strengthened by deposits of lignin. A well-differentiated endodermis is always present in roots, but occurs less; regularly in stems.
The endodermis forms the outer boundary of a central core of the root that contains the vascular cylinder. This core is called the stele. When further analyzed under a microscope, just inside the endodermis is a layer, often only one cell thick, of thin-walled parenchymatous cells. The cells of this layer, called the pericycle, are capable of producing new cells, which grow outward from the stele and thus form lateral roots.
The central portion of the dicot stele, surrounded by endodermis and pericycle, is filled with the two vascular tissues, xylem and phloem. The thick-walled xylem cells often form a cross- or star shaped figure. Bundles of phloem cells are located between the arms of the xylem. Thus, instead of forming a continuous cylinder like the epidermis, cortex, endodermis, and pericycle, xylem and phloem alternate in this portion of the stele. These vascular tissues can be better studied using a microscope.
Large roots of monocots commonly have a region of parenchyma tissue, called pith, located at the very center of the stele. The xylem therefore does not form the star-shaped figure characteristic of dicots, but even in such roots the bundles of xylem and phloem alternate.
