Primary roots have not been the subject of as many or such full studies as has been the case with stems or leaves. They do, however, show a wide range of variation which is influenced both by environment, in terms of ecological adaptation, as well as by the genotype. Compared with stems and leaves, root fragments can be difficult to identify in the primary state. Roots of monocotyledons show some variation, but generally not enough to provide reliable data for identification of unknown samples. This is not entirely because they are relatively undescribed or poorly represented in reference microscope slide collections, but partly because there is, overall, less variation.
We noted that the root is an organ which has to take strains or pulling forces. It rarely has to bend or flex, since it is usually found in a more or less solid medium. As a consequence of this, the main strengthen-ing tissues are positioned in the central region of the root and function like a rope.
The typical primary root is bounded by an epidermis. Inside this is the cortex, which may be several- to many-layered, and this is bounded to the inner side by an endodermis. Next follows the pericycle, and then the vas-cular system is in the middle. Each of these parts is described in detail below, and the terminology is defined.
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