256 EXPERIMENTS WITH PLANTS 



the Squash or Pumpkin. The usual arrangement is 

 a single mass of bast lying outside the wood, instead 

 of both outside arid inside, as in the Squash. 



If we examine a branch of a tree (see Figs. 

 135 and 138) we find the bast lying just outside the 

 cambium. In the stems illustrated the sieve -plates 

 occur on the side -walls as well as on the end -walls; 

 this is quite common in trees; it permits a more rapid 

 transfer of proteids from cell to cell. The outer part 

 of the bast soon dies and then becomes crushed by the 

 pressure of the surrounding cells: this is shown in 

 Fig. 138 (ob). As the branch grows older, thin layers 

 of cork are formed here and there in the rind, cutting 

 off small portions of it from the interior; these por- 

 tions die and eventually fall away; the cork -formation 

 finally encroaches on the bast. The result is bark, 

 which has an inner portion consisting of living cells 

 and an outer portion consisting of cells which have 

 become dry and dead; these cells, even though dead, 

 render valuable service to the plant, since they pro- 

 tect it against insects, fungi, gnawing animals, fire 

 and many other foes. Notice how quickly the cam- 

 bium and other tissues dry up and die whenever the 

 bark is removed. (Protection of the stem against ani- 

 mals is, in many cases, afforded by hairs and spines; 

 see page 221.) 



What happens if we ring the tree so as to prevent the 

 proteids from passing downward in the soft bast? To 



