ABSORPTION OF WATER 



151 



leaves, but more especially in the tubers. This water reserve is 

 spent very slowly during the drier periods. To facilitate the ab- 

 sorption of atmospheric moisture, such plants possess special adap- 

 tations. Many orchids have long aerial roots, hanging down 

 (Fig. 61) or trailing over the bark of trees. These roots are cov- 

 ered by several layers of 

 empty cells with perforated 

 walls, which absorb water 

 like a sponge. The water is 

 then transmitted to the inner 

 living tissues of the root. 

 This spongy tissue is termed 

 velamen (Fig. 62). In their 

 structure, the cells of this 

 tissue remind one of the 

 empty cells in the leaves of 

 sphagnum moss, which also 

 serve to retain water. In 



Iff! 



Fig. 62. — Cross-section through the root 

 Fig. 61. — A tropical orchid with of an orchid. From epidermis to e-velamen 

 aerial roots. (after N athansohn) . 



Bromeliaceae, on the leaves are found valvelike scales, which, being 

 appressed to the epidermis during dry weather, prevent the escape 

 of water. During rains, the cells forming the stipe swell, the scales 

 are raised and allow the entrance of water. In addition, the 

 leaves of these plants are arranged in such a way that in their 

 clasping bases the rainwater is continually and gradually absorbed. 



