40 BOTANY 



that the pericycle at these points becomes several cells 

 thick. The innermost of these cells, lying in contact 

 with the protoxylem, become cambium, and soon extend 

 to unite the two bands of cambium approaching them 

 from the two bast strands between which the bundle of 

 wood is lying, so that a complete ring of cambium is 

 formed. At first it is necessarily sinuous or wavy, but 

 as more and more wood is formed inside the bast masses 

 it is pushed further and further outwards there, till the 

 waviness of the ring disappears. This cambium ring 

 then continues to add more and more wood in the same 

 way to the secondary wood already formed. Behind 

 the protoxylem groups, which form the outer edge of 

 the primary wood bundles, no secondary wood is formed, 

 but only rows of thin-walled cells; consequently the 

 secondary wood is divided into separate masses by 

 these rows of cells, which are known as medullary rays. 

 They are formed with a view to the transport of food 

 substances from the bast into the interior of the wood. 



The cambium produces a little secondary bast out- 

 side the ring in the same way as it forms wood inside it, 

 but the quantity of bast is much less than that of wood. 

 This is natural, as the bast has only to provide a path 

 of transit for the actual food of the root cells, while the 

 wood has to furnish a continually increasing amount of 

 water-transporting tissue. 



This woody formation in the centre of the root is dis- 

 posed very advantageously for maintaining its stability. 

 A structure with a hard central core is the most suitable 

 to resist such a vertical pull as would cause uprooting. 

 This vertical pull is continually being made by the 

 movement of the storm-tossed upper region of such a 

 structure as a tree. 



The young root as it increases in thickness in the soil 

 encounters two dangers, one internal, the other external. 

 The process of thickening compresses very severely 



