108 



PLANT RELATIONS. 



cesses of the lower groups that it is regarded as the only 

 true root. It is quite uniform in structure, consisting of a 

 tough and fibrous central axis surrounded by a region of 

 more spongy structure. The tough axis is mostly made 

 up of vessels, so called because they 

 conduct material, and is called the 

 vascular axis. The outer more spongy 

 region is the cortex, which covers 

 the vascular axis like a thick skin 

 (see Fig. 105). 



One of the peculiarities of the 

 root, in which it differs from the 

 stem, is tha^ the branches come from 

 the vascular axis and burrow through 

 the cortex, so that when the latter 

 is peeled off the branches are left 

 attached to the axis, and the cortex 

 shows the holes through which they 

 passed. It is evident that when such 

 a root is absorbing, the absorbed ma- 

 terial (water with various materials 

 in solution) is received into the 



FIG. 105. A longitudinal 

 section through the root 

 tip of shepherd's purse, 

 showing the central vas- 

 cular axis (//), surrounded 

 by the cortex (p), outside 



of the cortex the epi- , , . ., 



dermis (> which disap- cortex, through which it must pass 

 to the vascular axis to be conducted 

 to the stem. 



pears in the older parts of 

 the root, and the promi- 

 nent root-cap (c). 



Another peculiarity of the root 

 is that it elongates only by growth at the tip, while the 

 stem usually continues to elongate some distance behind 

 its growing tip. In the soil this delicate growing tip is 

 protected by a* little cap of cells, known as the root-cap 

 (see Fig. 105). 



