TIIE WALLFLOWER 87 



the piliferous layer, is remarkable for the great growth 

 in length of those of its cells which form the root-hairs, 

 organs of which we have already learnt the high 

 physiological importance. The development of the 

 cortex calls for no remark. 



The plerome here, as in the stem, gives rise to 

 procambial strands, which in the root occupy much 

 the greater part of the young vascular cylinder, as 

 there is only a little conjunctive parenchyma between 

 them, and no pith is present. In some plants, how- 

 ever, the root possesses a pith. The external part 

 of the conjunctive parenchyma forms a single-layered 

 pericycle, which extends all round the cylinder. In 

 the root each procanibial strand gives rise to xylem 

 or to phloem only. 



The branching of the root is very characteristic of 

 that organ, and quite different from that of the stem. 

 All the branches arise endogenously that is to say, 

 they are formed from deep-seated tissues, and have 

 to make their way through the external structures 

 in order to reach the surface. In the Wallflower, 

 and indeed in all Flowering Plants, the pericycle is the 

 layer from which the rootlets are developed. 



The position in which the branch-roots or rootlets 

 are formed has a definite relation to the xylem. 

 In the Wallflower and its nearest allies the rootlets 

 usually form four rows, which are so placed that one 

 lies a little to the right, and another a little to the 

 left, oi each of the two protoxylem strands of the 

 parent root. Sometimes, in place of the four rows, 

 there are only two, which are then situated exactly 

 opposite the two protoxylem strands. The develop- 



