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 and 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 ffuite 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 {oh) . 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 5arA;, 

 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 



