100 STRUCTURAL BOTANY 



the roots of other plants of the same class there 

 are often more than two groups of xylem, and a pith 

 may even be present ; still, if we can once make out 

 the position of the protoxylem elements, we can 

 always be certain whether it is a root or a stem with 

 which we have to do. 



Only very young parts of the root retain their root- 

 hairs, and absorb food from the soil. The older parts 

 soon lose their hairs, cease to be absorptive, and now 

 serve only to conduct the food, which the younger 

 part of the root has taken up, towards the stem. 

 These older parts of the root acquire a covering of 

 periderm. The pericycle divides up to form a 

 phellogen, which produces phelloderm internally and 

 corky layers externally, just as in the stem. 1 It is 

 from the outer layer of the pericycle (where it is 

 double) that the phellogen arises. We now see 

 that the pericycle in these roots is an extremely 

 important layer, being the seat of three distinct new 

 formations 



(1) Lateral roots. 



(2) Cambium (in part). 



(3) Phellogen. 



In consequence of the formation of cork by the 

 phellogen, all the external tissues, from the endoderinis 

 outwards, wither away, so that the older roots consist 

 entirely of the central cylinder surrounded by peri- 

 derm. A curious effect of this throwing off of the 

 cortex, which takes place very early in roots, is that 



1 In the case of roots the origin of the phellogen from the pericycle 

 is the rule, while in stems it is exceptional. 



