[Chap. XXVIH TISSUES AND PROCESSES OF STEMS 281 



organ ( Fig. 103 ) . Cork cells are short-lived; but the cell walls, which are 

 composed primarily of layers of suberin and lignified cellulose lamellae, 

 last indefinitely because very few kinds of non-green plants can digest 

 them. On stems that increase in diameter by cambial growth, however, 

 the cortex is ruptured and sloughed off. 



CORK 



CORTEX 



Fig. 103. Photomicrograph of a portion of a cross section of geranium stem 

 in which the cork and cork cambium (CC) are evident. P. F., pericycle fibers; 

 V. C, vascular cambium. 



In herbaceous plants the cortex may live as long as the stem does, but 

 in many woody stems it may die and be sloughed off while the stem is 

 comparatively young. New cork cambiums periodically develop in 

 deeper-lying tissues. When cork develops in the pericycle, it prevents 

 the diffusion of water from the xylem to the cortex, which soon dies. In 

 beech the cortex lives and grows as long as the tree is alive. It is also 

 long-lived in birch and certain other trees. Chlorenchyma is present in 

 the cortex of most young aerial stems. 



Sclerenchyma and collenchyma are frequently present in the cortex. 

 In some species of plants there may be many other kinds of cells in the 

 cortex, such as isolated sclerenchyma cells (stone cells or grit cells), 

 groups of stone cells surrounded by a cambium, and isolated cells or 

 groups of cells containing anthocyanins, essential oils, tannins, or cal- 



