THE WHITE LILY 



169 



ternal layer of the apex is the dermatogen, from 



which the epidermis is formed. The periblem and 



plerome probably 



have a common 



initial group. From 



the plerome, which 



is the inner part of 



the meristem, the 



vascular cylinder, 



including all its 



bundles, is developed. 



The development of 



the vascular bundles 



begins at the interior 



of the cylinder and 



advances outwards. 



When the outermost 



FIG. 67. Longitudinal median section of 

 apex of young root of a Monocotyledon 

 (Amaryllis), s (shaded), "digestive 

 sac," formed from endodermis of parent- 

 root ; c, calyptrogen, giving rise to root- 



been 



cap only ; p, periblem (the outermost 

 layer formed from this bears the root- 

 hairs); pi, plerome. Magnified about 

 150. (After Van Tieghem and Douliot. > 



bundles have 

 formed, the peri- 

 cycle thickens its 

 cell-walls, and all further development ceases. 



The leaves here, as usual, develop from below upwards, 

 so that the youngest are always nearest the apex. 

 Only the outer layers of the growing- point take any 

 part in the formation of the leaves and branches of 

 the stem (see Fig. 66, Q. The growth of each leaf 

 after its first origin goes on chiefly at its base. 



The apex of the root is on the whole similar to 

 that of a Dicotyledon, and we find the same three 

 layers of meristem, giving rise to the root-cap, cortex, 

 and vascular cylinder respectively. The superficial 



