152 PLANT LIFE 



to bring out the more salient features of this 

 remarkable structure, which is generally known 

 as the velamen of the orchid root (Fig. 19). 



The cell walls of the velamen are strength- 

 ened by bars of thickening, which gives them 

 a spiral or netted appearance under the 

 microscope. The function of the velamen 

 as a whole is to act as a sort of sponge which 

 soaks up liquid falling on it with extreme 

 rapidity. Thus the plant is able quickly to 

 replenish its supplies of water during a shower. 

 In many orchids the bases of the stems and 

 sometimes the leafy joints are swollen with 

 the so-called " pseudobulbs," which form 

 additional storehouses for the water thus 

 obtained. During the periods which inter- 

 vene between the rains, the plant often 

 throws off its leaves, its flowers drawing on 

 the water supplies stored in the pseudobulb. 

 The surface of the latter becomes more and 

 more wrinkled as its storage cells become 

 depleted of their water contents. 



The roots, although specialised in the way 

 described above, have retained, in many in- 

 stances at any rate, the power of growing like 

 ordinary ones if they should happen to pene- 

 trate the substratum. This sometimes occurs 

 when there is sufficient vegetable detritus 

 caught in the orchid clump, or when the root 

 penetrates a piece of damp rotten wood. 

 Root- hairs are then produced, and the velamen 

 may be scarcely produced at all. 



It is a remarkable fact that some of the 



