38 BOTANY 



of water through it, so that the water of the cortex can 

 pass directly to the wood strands, but cannot pass from 

 one endodermal cell to another, being prevented by 

 bands of a cuticularised substance that pass round the 

 radial walls (Fig. 15). By their interlocking together 

 they make the endodermis separate the intercellular 

 passages of the cortex from those of the cylinder, so that 

 air cannot penetrate directly to the latter. 



As the root grows older and larger and the upper part 

 or shoot system of the plant develops to a corresponding 

 extent, this primary structure becomes insufficient for 



its requirements. They call 

 for a greater amount of 

 conducting tissue as the 

 branches and leaves of the 

 shoot multiply, for all the 

 latter need a supply of 

 water. The stability of 

 the whole structure needs 



pericycie. strengthening, in view of the 



greater size being acquired 



above ground. There is, as we have seen, a great growth 

 in thickness of the root and the development of a 

 system of branches, each behaving like the parent root. 

 In the stage we have examined the young root shows 

 no provision for this increase of thickness. It can take 

 place only by the formation of new cells, and such forma- 

 tion is not going on except at the apical meristem. A 

 new departure has accordingly to be made (Fig. 16). 

 It begins by a curved band of cells of the supporting 

 tissue lying in front of each strand of bast becoming 

 meristematic, beginning to divide by walls which are 

 parallel in direction with the circumference of the root. 

 These tangential divisions cause the formation of several 

 rows of cells, one of which, the nearest to the bast, 

 retains the power of division and is called cambium. 



