THE ANGIOSPERMAE : STEMS 



877 



contains diffusible fatty substances the materials for the formation of suberin 

 in the cork layers are also accumulated at the level of the phellogen. 



The suberization of the walls of phellem cells consists in the deposit of a 

 layer of suberin, unmixed with cellulose, between the middle lamella and the 

 inner wall of pure cellulose. This layer varies greatly in thickness in different 

 cases, and in the massive " cork wings " (Fig. 864) on the branches of species 

 of some trees, such as Vhmis and Acer, it may be practically absent, which 

 has led to the application of the term phelloid to such unsuberized cork 

 tissues. The suberin itself is apparently a mixture of the anhydrides of fatty 



Fig. 864. — Ulmus glabra xzr. hollandica. Transverse 

 section of a small branch showing cork wings. 



acids, the chief of which is phellonic acid, C22H43O3, and it is thus closely 

 similar to cutin. Both substances seem to be dependent on contact with air 

 for their development. 



There are no pits through the suberin layer, and as it is highly impermeable 

 the suberized cell soon dies. Tannin and related substances often accumulate 

 in the cells before suberization, and these give the cork its dark colour and 

 its value for tanning leather. 



Haberlandt showed that the protective value of two-year-old cork layers 

 against evaporation is about the same as that of the primary epidermis. That 

 it is not greater is probably due to the cracking, caused by growth pressure, 

 which fissures the outer layers of the cork, and would destroy its protective 

 value if fresh layers were not constantly added from within by the phellogen. 

 The heavy cork layers of old stems are highly protective. 



The formation of a cortical or other superficial phellogen begins below 

 the stomata, and is preceded by cutinization of the cells lying beneath the 



