8i6 



A TEXTBOOK OF THEORETICAL BOTANY 



of very rapid growth (4 to 6 in. per day). Roots of this kind are formed by 

 many tropical Aroids and they hang down in large numbers, like cords, from 

 the branches of the jungle trees, forming a remarkable feature of the forests 

 (Fig. 813). When they reach the ground they penetrate the soil and produce 



Fig. 813. — Aerial roots of epiphytic Aroids hanging from high 

 trees in the tropical jungle. Alto da Serra, Sao Paulo, Brazil. 



a normal, branching root-system. Among epiphytic Orchids the aerial roots 

 are, however, short and wholly aerial and hang loosely in the damp air of the 

 jungle (Fig. 814). They can absorb rain and condensed moisture directly 

 from the air. Such supplies of water are naturally intermittent, and the 

 ordinary equipment of delicate root hairs would be quite unsuitable for an 

 environment which is liable to periods of drought. Instead of a piliferous 

 layer these roots are clad with a special tissue called velamen, which may 

 be many cells thick (Fig. 815). The cells are dead and empty. Their walls 

 are perforated with round apertures and are striated with ribs of cuticle. 

 In fact the tissue is functionally a sponge, like the cortex of Sphagnum. On 

 its inner side the velamen is bounded by a layer of cells with thickened walls, 

 corresponding to the e'xodermis. When, as frequently happens, this layer 



