98 ABSORPTION OF WATER AND MINERALS 



and both are thickly beset with overlapping scales under which 

 rain and dew gather and find entrance by osmosis into the cell 

 cavities. Here the scales, like the velamen, serve both for the 

 absorption of water and protection against its loss. The scales 

 when dry are shrunken and lie close against the stem or leaf; 

 but when wet their thicker outer wall swells and bulges outward, 



FIG. 47. Cross section through a water-absorbing scale of Tillandsia usneoides; a, a, water- 

 absorbing cells partially filled with water. (After Schimper.) 



water is drawn into the cell cavities, and by their turgidity the 

 scales rise and make room for more water beneath them (Fig. 47). 

 Practically the whole plant body is thus enabled to imbibe water, 

 and solutes that have come in the form of dust. 



Other Methods of Absorbing Water. Some desert plants 

 have devices for absorbing water into the leaves. Diplotaxis 

 Harra, for example, a cruciferous plant of the Egyptian and 

 Arabian deserts, has its foliage beset with stiff hairs which, acting 

 as points for the radiation of heat, after sunset gather dew. The 

 hair is practically waterproof excepting at its base, where the 

 dew, running down from above, forms a film over the wall and 

 is quickly absorbed. 



There are some interesting anatomical details in these hairs of 

 Diplotaxis (Fig. 48). Cellulose additions to the wall fill the 

 cell cavity down to the spreading base, where the cavity enlarges 

 and is lined with an unusually thick protoplastic layer. The 

 wall separating the hair from the body of the leaf has many pits 

 through which the imbibed water can pass into a water reservoir 

 tissue beneath, whence it is distributed directly to the mesophyll 

 cells. The entire leaf is so thoroughly waterproofed that only 



