LIFE HISTORIES OF FAMILIAR PLANTS 



means of a microscope (Fig. 50, Plate 33), you will 

 find that it is protected by a thimble-like mass of 

 loose tissue, or a ^^ root-cap/' as the botanist terms 

 it. Within this is the true growing tip of the root, 

 but it is the sensitive root-cap which guides the 

 root-tip to suitable quarters, where it can set to 

 work its army of delicate root-hairs, which it 

 carries at its rear (see Fig. 50). It is by means 

 of these root-hairs that the plant is supphed 

 with water and mineral food, for the root-tip does 

 not itself feed, nor does the sensitive root-cap. It 

 is plain, therefore, that sensibility in plant struc- 

 ture manifests itself at a very early period. 



This root sensitiveness is highly developed in 

 some orchids, an example of which is shown in 

 Fig. 51 (Plate 34). The roots of the orchid shown 

 are seen to have left the pot which holds the plant 

 and suspended themselves in the air ; in their 

 natural environment these orchids grow on, and 

 adhere to, the bark of trees, but do not feed upon 

 their hosts, simply using their branches as a sup- 

 port. Attaching themselves with a few root fibres, 

 they then throw out into the atmosphere their 

 feeding roots. The aerial roots are clothed with a 

 paper-like covering, and when moisture is 

 absorbed this membranous covering prevents it 

 from evaporating from the root tissues ; thus the 

 orchids stand the long periods of drought ; indeed, 

 they become veritable vegetable camels, storing 

 their water like the " ships of the desert " for a 

 time of need. Yet so sensitive are these roots to 

 their natural environment, the atmosphere, that if 



68 



