72 BOTANY 



As the stem grows we find additional vascular strands 

 continually being developed, a change directed especially 

 to the strengthening of the stem, as the new strands are 

 not directly connected with the leaves. The original 

 strands also are much enlarged and strengthened. 



All this work is done by the cambium layer. Part of 

 the original bundles, as we have seen, consists of this 

 tissue, which is hence called fascicular cambium. By 

 continual division of its cells, mainly in a direction 

 parallel with the outside of the stem, masses of cells are 

 produced between the wood and the bast. One layer of 

 these remains cambium those on the inside of it are 

 changed into wood, those on its outside into bast. 



Very soon after this process has been set up in the 

 bundle the cells which lie between the strands, known as 

 the medullary rays, are the seat of change. No doubt the 

 multiplication of the cells of the cambium sets up a 

 strain in the ray cells adjoining them, stretching them, 

 or dragging upon them. The stimulus of this strain 

 makes certain of these cells, extending across the ray, 

 begin to divide in their turn, and soon the rays are all 

 crossed by layers of meristematic cells, joining up into 

 a ring the isolated cambiums of the bundles. These 

 new portions, which complete the ring, are known as 

 inter fascicular cambium. The whole ring now behaves 

 as the original cambium of the bundles does, and soon a 

 ring of wood is formed in front of and a ring of bast behind 

 it. The parts of the ring formed by the interfascicular 

 cambium have no connection directly with the leaves. 



As new leaves arise at the apices of the twigs the 

 vascular strands belonging to them are connected with 

 this vascular cylinder as were the primary ones, for the 

 structure of the young twigs resembles in all points that 

 which we* have described for the seedling. 



At the end of each year, in our climate, the growth of 

 most trees ceases, owing to the fact that the leaves fall 



