EPIPHYTES AND PARASITES 



with tiny rootlets. In a state of nature these rootlets carry the fungi 

 along with them. Such a mutual dependence of two different 

 organisms is known as symbiosis. There are many other woodland 

 plants whose roots need the assistance of filamentous fungi ; and the 

 Lichens always consist of two different co-operating organisms, 

 namely, of fungi and algae, which are plants of an inferior order. 



When an Orchid that perches on a bough begins to grow, the first 

 thing that it has to do is to anchor itself, and this it does with its 

 roots, which penetrate the chinks of the bark in all directions. In 

 other Orchids the roots are ligulate or strap-like, holding them so 

 firmly that when the Orchid is torn from the tree portions of the 

 bark come with it. But the Orchids do not put forth holding roots 

 only. In the Brazilian tribe of Oncidium a long mane of white, 

 hair-like roots hangs from the bough on which the plant is seated. 

 The whiteness of these roots is due to their dead and porous outer 

 stratum, which, like blotting-paper, sucks up every drop of liquid, 

 and even absorbs the moisture of the atmosphere. Such Orchids are 

 consequently able to grow upon the smooth trunks of palm-trees, 

 from which the rain runs off so quickly that other plants have no 

 time to absorb it in sufficient quantities. But the roots of these 

 Orchids suck it up immediately, and the inner, living portion of the 

 root is able to convey it to the plant at its leisure. In a protracted 

 drought these porous, air-filled coverings protect the inner portion 

 of the root from desiccation. In a sense, therefore, the sheath of the 

 root fulfils the function of the soil. 



The aerial roots of some Orchids do not hang downwards, but 

 thrust themselves outwards or upwards into the air. In the hill 

 country of Ceylon I found the Dendrobium, whose shimmering 

 blossoms lifted themselves from a great besom of bristling aerial 

 roots. Such roots may even serve the purpose of aeration ; there are 

 actually Orchids in which the roots turn green and help to build up 

 the tissues of the plant, work which is properly that of the leaves. 

 But these Orchids have lost their leaves, and the flowers grow directly 

 from the roots. 



Many of the Orchids have developed their leaves into reservoirs. 

 In such cases the tissue of the leaf stores water, and lays up a pro- 

 vision of nutritive material, so that the leaf becomes fleshy and 

 swollen. The work of building up the body of the plant is performed 

 only by its green outer surface. Orchids with such leaves remind 

 one of Cacti, and the significance of such formations is the same in 

 both groups of plants ; in either case a hard outer skin preserves the 



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