8i8 



A TEXTBOOK OF THEORETICAL BOTANY 



is highly thickened, there are unthickened " passage cells " produced at 

 intervals, just as in the endodermis. As the velamen is external to the 

 exodermis it must be a derivative of the dermatogen and is therefore analogous 

 to the multiple epidermis often found on shoots and leaves. 



The velamen is colourless and papery, so that from the outside the root 

 looks dead, but the dead layer is also translucent and the cortex within is full 

 of active chloroplasts to which light can penetrate. The obvious suitability 

 of velamen to the conditions of epiphytic life has often caused it to be described 

 as a choice example of " adaptation." Unfortunately for this idea, the tissue 

 is not restricted to epiphytes. It is found also in many soil-rooting Orchids 

 and in other Monocotyledons, e.g., Asparagus, where no epiphytism exists. 

 Needless to say, this does not detract from the usefulness of the velamen in 

 the epiphytes which possess it, but it does point to another and probably 

 more correct view than the adaptational one, namely, that the velamen is an 

 anatomical peculiarity which has made possible the adoption of the epiphytic 

 habit by plants which possessed it, though it has not obliged all of them to 

 adopt epiphytism. 



(/) Pneumatophores 



Analogous to the epiphytic roots which hang in the air are those which 

 grow up into the air from beneath the slime of tropical swamps, especially 



Fig. 8 1 6. — Sonneratia alba. Pneumatophores springing from under- 

 ground roots around the trunk of an old tree. {From Schmidt, in 

 " Vegitatiofisbilder.") 



in the tidal swamp forests called Mangrove. These are known as pneumato- 

 phores or "breathing roots" (Fig. 8i6). They arise from long horizontal 

 roots, and as they are negatively geotropic they grow vertically upwards, 

 projecting eventually several inches into the air. They are usually only a 

 few inches apart, so that very large numbers may surround a single plant, 

 strongly recalling in appearance a crop of young Asparagus shoots. There 



