104 THE PHYSIOLOGY OF PLANTS 



cortex, too, and the medullary ray cells can take part in the 

 nutrition of the cambium, as they are temporary storage tissues 

 for reserve material (chiefly in the form of starch) with which 

 they can supply the cambium in case of need. 



This advantageous position of the cambium explains its 

 continuous activity, which enables it to form on the inside 

 new layers of wood cells {splint-wood), and on the outside new 

 layers of bast, which push the older ones farther towards the 

 periphery. 



From this continuous activity of the cambium results the 

 growth in thickness (the secondary thicliening) of the stem. 



The wood increases by additions to the outside, the bast by 

 additions to its inner layers. Both regions of growth lie 

 close together, and are only separated by the narrow band of 

 cambium cells which are in process of division. 



The new layers which are formed by the activity of the 

 cambium, as they become older, grow and expand, and need 

 therefore more room ; consequently they press the cortex 

 which lies externally to them towards the outside. But this 

 is not done without resistance, for the cortex is surrounded 

 by a belt of cork, the elasticity of which tends to retain the 

 cortex in its original position. This tendency of the cork 

 takes the form of a pressure, which makes itself felt as far as 

 the cambium layer itself, and is one of the causes in determin- 

 ing the form of the new wood cells. That this is the case 

 may be gathered from the fact, that immediately this pressure 

 is relieved by cutting thi'ough the cork, the form of the wood 

 cells is changed. 



The increasing thickness of the stem finally ruptures the 

 cork in various places ; then new layers of rectangular cork 

 cells will be formed within the green cortical tissues. The 

 layers of cells outside these new cork layers are cut off from 

 their sources of nutrition, they dry up and peel off in the form 

 of scales. This is the mode of origin of bark scales. 



Though the cork of a stem is very useful to the plant as a 

 protection for the green cortex (for it only lets the smallest 

 traces of water or gases pass through it), it is often very trouble- 

 some to gardeners. Trees grown on wet soil possess a very 

 tough cork layer, which retards the formation of bark and the 



