72 THE VEGETATIVE ORGANS 



is being made for a new protective covering. There arises 

 in the cortex another kind of cambium, known as the cork- 

 cambium, because its cells by repeated division form, not 

 wood and bast, but cork. Fig. 38 shows this cork-cambium 

 (c.c) arising just below the epidermis (e) in the stem of the 

 Elder, and its cells have divided in such a way as to form 

 somewhat regular rows of cells. These enlarge, lose their 

 living contents, and their walls become transformed into 

 cork (ck). The epidermis gives way under this extra strain, 

 producing the cracks which may be easily seen on the 

 surface of a twig. Thus the stem becomes covered by 

 a layer of cork, which is a dead impervious layer, well 

 adapted as a protective coat. As the stem thickens from 

 year to year, the outer cork layers split, new layers are 

 formed beneath and the bark thickens, and in time takes 

 on the ruggedness characteristic of the species. Often an 

 irregular group of cork cells is formed beneath a stoma, the 

 cells being so arranged as to permit air to enter or leave the 

 stem. These structures, which take the place of stomata, 

 are called lenticels (Fig. 42, /) and are formed on most 

 trees, their shape being peculiar to the species. 



Other stems should be examined and compared. In the 

 Laburnum the cork-cambium is formed in the middle of the 

 cortex, while in the Black Currant it arises deeper still, 

 near the ring of vascular bundles (Fig. 40 c.c). As the cork 

 develops, the cortex to the outside of it dies and is eventually 

 thrown off. 



Callus and separation-layer. The formation of cork is 

 useful to plants in many ways, and especially as a means of 

 healing wounds. Examine the trees in a wood, look for 

 examples of pruning, and note the change taking place 

 around the cut surface of a branch. You will be able to 

 lind all stages of cork-formation from a narrow ring just 

 outside the wood to others broader and broader, gradually 

 encroaching and growing over the surface of the wound 



