THE ROOTS OF PLANTS. 161 



simple trichomes the root-hairs. The fibro-vascular bun- 

 dles are, especially in the higher plants, of a much lower 

 type than those in the stems and leaves. The fundamental 

 system is also poorly developed, and has not that variety of 

 tissues found in other portions of the plant. 



207. Another remarkable peculiarity of roots is that they 

 differ much less from one another in structure than do their 

 steins. The young roots of Monocotyledons have very nearly 

 the same structure that those of Dicotyledons have, and those 

 of Pteridophytes do not differ much from either. The older 

 roots of Monocotyledons and Dicotyledons differ considerably, 

 on account of changes in their structure which take place 

 later, and then each root bears a closer resemblance to the 

 stem from which it grows, or to which it belongs. 



208. The general structure of the root-cap may be easily 

 understood from the accompanying figure (Fig. 137). It is 

 a cap-like mass of parenchymatous cells which surrounds 

 the end of the root ; its outer cells are loose, and in some 

 cases are more or less changed into a mucilaginous mass; 

 in any event they gradually lose their protoplasm and become 

 detached and destroyed. The inner layers (i, s, Fig. 137) are 

 constantly developing from a deep-lying tissue, the Dermato- 

 gen* (not shown in the figure), so that as the cap is destroyed 

 on the outside it is renewed from the interior. By its lat- 

 eral growth it in some cases ensheathes the terminal part of 

 the root for a considerable distance. 



209. Back of the root-cap lies the primary meristem of 

 the root, composed, in Phanerogams, of a mass of small and 

 actively dividing cells. In this meristem there is as yet no 

 differentiation, but as it is prolonged by rapid cell-multipli- 

 cation the cells become modified in its posterior portion. 

 There is thus a constantly advancing formation of meristem, 

 followed at a little distance by as constant a modification 

 into other tissues. The usual course of this differentiation 

 is first into a central cylindrical mass, the Plerome\ (Fig. 



* From the Greek dipfia, SspnaroS, skin, and yswdu, to bring forth or 

 generate. 



f So named by Hanstein (" Scheitelzellegruppe im Vegetationspunkt 

 der Phanerogamen," 1868), from the Greek 7rA?/pu/ia, a filling up. 



