Fine Structure of a Root.[9]--When we examine a delicate root in thin longitudinal section under the compound microscope, we find the entire root to be made up of cells, the walls of which are uniformly rather thin. Over the lower end of the root is found a collection of cells, most of which are dead, loosely arranged so as to form a cap over the growing tip. This is evidently an adaptation which protects the young and actively growing cells just under the root cap. In the body of the root a central cylinder can easily be distinguished from the surrounding cells. In a longitudinal section a series of tubelike structures may be found within the central cylinder. These structures are cells which have grown together at the small end, the long axis of the cells running the length of the main root. In their development the cells mentioned have grown together in such a manner as to lose their small ends, and now form continuous hollow tubes with rather strong walls. Other cells have come to develop greatly thickened walls; these cells give mechanical support to the tubelike cells. Collections of such tubes and supporting woody cells together make up what are known as _fibrovascular bundles_. Footnote 9: Sections of tradescantia roots are excellent for demonstration of these structures. [Illustration: Cross section of a young taproot; _a_, _a_, root hairs; _b_, outer layer of bark; _c_, inner layer of bark; _d_, wood or central cylinder.] Root Hairs.--Careful examination of the root of one of the seedlings of mustard, radish, or barley grown in the pocket germinator shows a covering of tiny fuzzy structures. These structures are very minute, at most 3 to 4 millimeters in length. They vary in length according to their position on the root, the most and the longest root hairs being found near the point marked _R. H._ in the figure. These structures are outgrowths of the outer layer of the root (the _epidermis_), and are of very great importance to the living plant. [Illustration: Young embryo of corn, showing root hairs (_R. H._) and growing stem (_P._).]