88 STRUCTURAL BOTANY 



ment of the rootlets begins fairly early, but not so 

 close to the apex as is the case with the appendages 

 of the stem. It begins, however, before the central 

 vessels of the xylem-plate have become lignified. A 

 group of pericyclic cells, situated (in the more usual case) 

 a little on one side of one of the protoxylem strands, 

 begins to elongate radially, the middle cell elongating 

 most. All these cells then divide by tangential walls. 

 The inner cells thus cut off go to form the plerome of 

 the rootlet (see Fig. 34,.^). The outer layer divides 

 again (with the exception of the cells lying at the 

 extreme edge of the group), and of the two layers 

 thus formed the more internal gives rise to the 

 periblem (p), while the outermost becomes the calyp- 

 trogen (c), producing the piliferous layer and root-cap. 

 We thus have the three meristematic layers marked 

 off from the first. They continue to grow and divide 

 in the usual manner. The young rootlet grows out 

 through the cortex of the parent root, the cells of 

 which are not mechanically pushed aside, but are 

 actually digested by the rootlet, and their substance 

 absorbed by it as food (see Fig. 34, d). Eventually 

 the piliferous layer is reached and absorbed, and then 

 the tip of the rootlet becomes free and penetrates 

 into the ground. 



We see that the origin of the branches of the root 

 is just the converse of that of the leaves and branches 

 of the stem. In the latter all the tissues of the leaf 

 or branch are formed from the dermatogen and 

 periblem, the plerome taking no part in the process ; 

 in the root it is the plerome alone which forms the 



