86 



STRUCTURAL BOTANY 



the stem, gives rise to the vascular cylinder. The 

 next outer layer is the periblem, which develops into 

 the cortex ; the pericycle is formed from the outer- 

 most layer of the plerome, and the endodermis from 

 the innermost layer of the periblem. So far there 

 is no difference from the meristem of the shoot. 



The external part of the meristem gives rise at 



once to the piliferous 

 layer and to the root-cap. 

 We may call it the 

 calyptrogen, or cap-form- 

 ing layer. Its cells divide 

 not only by walls at right 

 angles to the surface, but 

 also in the direction 

 parallel to it. The cells 

 which are thus cut off 

 towards the outside of 

 the calyptrogen go to 

 form the root-cap, those 



cut off towards the inside form the piliferous layer, 

 which always remains one cell thick. 



As regards the differentiation of the various tissues 

 from the meristem, there are a few points to be 

 noticed, though, on the whole, what has been said 

 of the stem applies here also. The cells of the 

 root-cap remain living for a time, and grow larger. 

 The oldest of them, however, which lie farthest 

 towards the outside, gradually die off, and are replaced 

 by new cells formed from the calyptrogen. The 

 other tissue formed from the calyptrogen, namely, 



FIG. 33. Young rootlet of 

 Sinapis (allied to Wallflower) in 

 longitudinal section, c, calyptro- 

 gen ; p, periblem ; pi, plerome. 

 (After Van Tieghem.) Magnified 

 about 250. 



