96 



ABSORPTION OF WATER AND MINERALS 



vides carbon dioxide and oxygen just as it does for other green 

 plants, and brings soil substances in the form of dust. These 

 plants are not therefore living on the air alone, as their name 

 might imply, but they have essentially the same raw materials 



for building their food as have 

 plants rooted in the soil. 



To illustrate the essential 

 mode of their obtaining water 

 and solutes two types of 

 aerophytes will be discussed, 

 namely, the type where true 

 roots are sent forth into the 

 air, and the type which, pro- 

 ducing no roots, has its stems 

 and leaves equipped for ab- 

 sorption. In the first in- 

 stance, some tropical orchids, 

 and other aerophytes lodging 

 on the branches of trees, send 

 forth roots into the air that 

 have an external covering of 

 dead tissue known as the 

 velamen (Fig. 45). The cells 

 of this tissue have many very 

 minute openings through their 

 exterior and interior walls 

 through which water passes 

 when the root is wet with rain 

 or dew. The velamen is 

 formed by tangential division 

 of the protoderm, beginning a short distance from the apex and 

 giving rise to layers of cells varying from one to eighteen or 

 more according to the species. The cell-walls of the velamen 

 sometimes remain thin, but usually they are thickened, either 

 uniformly, or in the form of a network or spiral bands. After 

 the. cells have reached maturity the protoplasts soon die, and 



Fig. 45. — ^Portion of a cross section through 

 an aerial root of Stanhopea oculata. h, The 

 velamen; i, exodermis; j, cortex; k, endo- 

 dermis. (After Haberlandt.) 



