THE WALLFLOWER 75 



(1) The alternating arrangement of the xylem and 

 phloem groups. 

 ^ (2) The centripetal development of the xylem. 



(3) The greater concentration of the bundles, and 

 consequent smaller diameter of the central cylinder, 

 which contains but little parenchyma between the 

 bundles, and often has no pith. 



The vascular cylinder is surrounded by a layer of 

 thin-walled cells, the pericyde, which here, as in the 

 stem, forms the outermost layer of the cylinder. In 

 the root this layer is of special importance, for from 

 it all the branches of the root arise. 



The pericyde, again, is surrounded by another layer, 

 the endodermis, which here has the peculiarity that its 

 cell- walls, in so far as they border on other endodermal 

 cells, are cuticularised. This is not the case with the 

 inner and outer walls, which abut on the pericycle and 

 cortex respectively. We can recognise the endodermis 

 in the root in transverse sections by the dark appear- 

 ance of its cuticularised radial walls (see Fig. 28, e)\ 

 we know that cuticularised walls resist the passage of 

 water much more than cellulose walls do, so the effect 

 of this arrangement is that water has free passage 

 through the endodermis from the cortex into the 

 vascular cylinder, but cannot turn aside to pass from 

 one endodermal cell into another. The cuticularised 

 walls of the endodermal cells fit closely together, so that 

 the intercellular spaces of the cortex are completely 

 shut off from those of the cylinder. This structure of 

 the endodermis is sometimes found in the stem also, 

 but not so constantly as in roots. The endodermis ia 



