and the Exfoliation of the Cellular Coats of their Extremities. 185 



the cortical, and the woody or central region; the former of 

 these is altogether parenchymatous ; the latter consists for 

 the most part of woody tissue in natural roots, but contains 

 abundance of parenchyma in plants where the roots become 

 fleshy. 



'^ The cortical region is continuous with the rind-structure of 

 the stem, and in young roots consists of a thin layer of squarish 

 parenchymatous cells, more or less densely filled with mucilagi- 

 nous contents, but completely covered in on the outer surface 

 by a layer of cells firmly connected side by side, forming a kind 

 of skin, called the epidermis. This skin is distinguished from 

 that clothing the leaves and young shoots, in accordance with 

 the difference of function, by the absence of the peculiar breath- 

 ing pores or stomata, by which the internal structures of the 

 leaves, &c., are placed in direct communication with the atmo- 

 sphere. There are no openings of any kind through the skin 

 covering the surface of roots; and the notion formerly enter- 

 tained of the existence of sponge-like regions at the extremities 

 of roots was an error arising out of imperfect observation, as 

 will appear presently. The cortical region exhibits some striking 

 differences in its subsequent history in different plants. In 

 most cases, especially in the roots of Dicotyledons, and in the 

 branching roots of Monocotyledons, many of the epidermal cells, 

 at a little distance from the growing point of the root or rootlet, 

 grow out into filaments or hair-like processes, constituting the 

 'fibrils^ of roots. These are mostly invisible to the naked eye; 

 and their presence is chiefly betrayed by the adhesion of the 

 soil to them. When young roots are carefully washed and placed 

 under a magnifying glass, these fibrils are seen very clearly; 

 and on such roots as those of barley, for instance, they exist in 

 enormous numbers. 



" At the growing points of roots, the epidermis passes insensi- 

 bly into the mass of nascent or cambial tissue ; but the growing 

 point of a root is not at its absolute extremity, which is covered 

 by a cap-shaped or hood-like portion of epidermis of its own, 

 continuous likewise behind with the cambial structure. This 

 cap-like sheath of the point of the root may be compared with 

 the head of an arrow, forming a firm body, which can be pushed 

 forward by the growing force behind, to penetrate through the 

 resisting soil. This cap is subject to destruction and decom- 

 position by external agencies, and is less distinctly seen in roots 

 growing in earth than in those of aquatic plants. In all cases 

 it is constantly undergoing renewal by cell-development at the 

 back part; and when it remains undissolved, as in many water- 

 plants, it becomes very large ; when it undergoes decomposition 

 in proportion as it is renewed behind, it presents an irregular. 



