76 THE PHYSIOLOGY OF PLANTS 



thus perforated, there naturally exist no liquid cell contents 

 of the usual nature, but they are filled with air. It is the 

 presence of this air which gives to the roots their silvery 

 appearance. In consequence of the perforations of this root- 

 sheath (which must not be confounded with the root-cap, 

 although both are formed from the protoderm), it is able to 

 function like a sponge ; it sucks up in a moment as much 

 water as it can contain, and condenses water-vapour and 

 other gaseous matter which is contained in the atmosphere 

 like any porous body. This power of condensing vapouFS and 

 gases suggests the chief function of the aerial root under 

 ordinary circumstances ; it will use this power to nourish the 

 root-system and the whole plant, if it can pass the condensed 

 matter through the thickened layer of cells (the outer endo- 

 dermis) which separates the root-sheath from the cortical 

 tissues. The enormous power an aerial root possesses of con- 

 densing water-vapour is shown by an experiment with an 

 aerial root of Epidendrum, 1 2 inches long. In an atmosphere 

 saturated with moisture it absorbed in twenty-four hours an 

 amount of water equal to one-ninth of its total weight. 



The passage of water from the root-sheath to the succulent 

 cortex is made possible by the peculiar structure of the outer 

 endodermis. Speaking generally, this layer forms a protective 

 sheath, built up of long thick-walled cells, without pits or 

 canals ; nothing practically can pass through their thickened 

 walls. But a more careful examination reveals the presence 

 between the thick-walled elements of many small thin-walled 

 cells filled with protoplasm, and frequently arranged in rows. 

 These will attract the liquid which is condensed in the velamen, 

 and will easily pass it on into the cortex. The outer endodermis 

 of the aerial roots unites, therefore, just as we have seen in the 

 case of the endodermis of the ordinary roots, two very useful 

 functions. Its chief function is to protect the aerial roots 

 from drying up during periods of drought ; but at the same 

 time provision is made to conduct the substances which are 

 absorbed during periods favourable to the vegetative processes 

 into the more central tissues, whence they will be carried by 

 means of the vessels to the leaves. 



In some plants the root-sheath or velamen is thrown off 



