TISSUE. 



99 



Tier. 



tationis, either with or without an apical cell. The Fern 

 root furnishes an example of the first (Fig. 220), and 

 the root of any of the higher plants an example of the 

 second (Fig. 221). While the tender punetum veffetationis 

 of the stem is protected by the overarching of the young 

 leaves, that of the root is furnished with a peculiar shield- 

 ing structure, called 

 the root-cap (Fig. 

 221, E.c). This con- 

 sists of a mass of cells 

 developed from the 

 dermatogen. The lat- 

 ter divides into two 

 layers, the innermost 

 continuing as the der- 

 matogen, to be subse- 

 quently divided in the 

 same manner. The 

 outermost multiplies 

 its cells copiously, and 

 constitutes the root-cap. The outer cells of this are, of 

 course, continually abraded as the rootlets push their way 

 through the soil, but as constantly replenished from behind. 

 The root is further distinguished from the stem in having 

 its branches (side roots) developed as endogenous struc- 

 tures, that is, from cells a distance below the epidermis ; 

 having no stomates, no joints, no chlorophyll, and no leaves. 



Fig. 221. Diagram of longitudinal section through the root-tip, showing the 

 initial cells (;'./.}, and the root-cap (i?.^). 



