VIII.] THE CORTEX AND BARK OF TREES. 201 



it is not difficult to see that, unless certain provisions 

 are made for keeping up the continuity of the cortical 

 tissues, they must give way under the pressure from 

 within. As we shall see, such a catastrophe is in part 

 prevented by a very peculiar and efficient process. 



Before we can understand this, however, we must 

 take a glance at the structural characters of the whole 

 of this jacket (Fig. 26). While the branch or stem is 

 still young, it may be conveniently considered as con- 

 sisting of three chief parts. 



(1) On the outside is a thin layer of flat, tabular 

 cork-cells (Fig. 26, Co\ which increase in number by 

 the activity of certain layers of cells along a plane 

 parallel to the surface of the stem or branch. These 

 CQ\\s(C.Ca) behave very much like the proper cambium, 

 but the cells divided off from them do not undergo 

 the profound changes suffered by those which are to 

 become elements of the wood and inner cortex. The 

 cells formed on the outside of the line C.Ca in fact 

 simply become cork-cells ; while those formed on the 

 inside of the line C.Ca become living cells (7) very like 

 those I am now going to describe. 



(2) Inside this cork-forming layer is a mass of soft, 

 thin-walled, "juicy" cells, pa, which are all living, and 

 most of which contain granules of chlorophyll, and 

 thus give the green colour to the young cortex a 



