THE ANGIOSPERMAE : STEMS 879 



appear. The various bark patterns on old tree trunks, which are characteristic 

 of different species, are thus traceable to variation in the arrangement of the 

 phellogen layers. 





:i- 



A B 



Fig. 865. — Types of bark. A,Castanea satha (Sweet 

 Chestnut). B, Fagus sylvatica (Beech). 



An abnormal formation of cork is commonly associated with injuries, 

 when it is known as wound cork (Fig. 866). Wounding or almost any sort 

 of lesion of the integrity of the plant body may call it forth, and any living 

 tissue may produce it. The first reaction to the wound is the suberization 

 of the outer surfaces of exposed but intact cells. Suberization then spreads 

 inwards for several cell layers. Below this barrier cell division starts and a 

 cambium is organized which produces a few layers of regular cork cells, 

 forming an impermeable barrier against desiccation, parasites and other 

 inimical influences. 



5. The Endodermis. The innermost layer of the cortex, surrounding the 

 stele, is called, as in roots, the endodermis (see Chapter XIII). A typical 

 endodermis, with Casparian bands, recognizably the same as that in roots, is 

 not common in stems. The chief exceptions are among water plants, such 

 as Potamogeton and Hippuris, in which the vascular tissues are concentrated 

 into a relatively slender axial strand, as is normal in roots. Many other 

 exceptions occur which are not, however, related to special environmental 



