108 



PLANT STUDIES 



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 surround- 

 ed by a spongy region 

 (Fig. 105). The 

 tough axis is most- 

 ly made up of ves- 

 sels, so called be- 

 cause 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. 



One of the pecu- 

 liarities of the root is 

 that the branches 

 come from the vascu- 

 lar axis and burrow 

 through the cortex, 

 so that when the lat- 

 ter is peeled off the 

 branches are left at- 

 tached to the axis, 

 and the cortex shows 

 the holes through 

 which they passed. 



Another pecu- 

 liarity of the root is 

 that it elongates only by growth at the tip, and in the soil 

 this delicate growing tip is protected by a little cap of cells, 

 known as the roof-cap. 



Fig. 105. A longitudinal section through the root 

 tip of spiderwort, showing the central vascular 

 axis (2)1), surrounded by the cortex (p), outside 

 of the cortex the epidermis (e) which disappears 

 in the older parts of the root, and the promi- 

 nent root-cap (c). 



