202 



render the latter more or less worthless for construction. It 

 is probable that this plant transfers some of its carbonaceous 

 food materials to its host, but it is not known whether it absorbs 

 any organic food substances from the host in addition to the 

 supplies of v, T ater and mineral salts. The chief practical 

 method oi combating this pest consists in cutting off and de- 

 stroying the attacked branches. 



177. Among parasitic phane- 

 rogams which are furnished with an abundance of chlo- 

 rophyll the well-known sandal tree (Santalum album,) re- 

 quires brief notice. It is now known that this tree is 

 dependent to a very large extent on the roots of the plants 

 near which it grows for its supplies of water and mineral salts, 

 the Sandal roots developing numerous haustoria which 

 penetrate the root tissues of the host-plants and draw from 

 them the necessary supplies of food materials. No definite 

 information is at present available as to the extent to 

 which the host-plants are damaged by this parasitism, and it 

 appears probable that they may benefit to some extent by a 

 supply of carbonaceous food which has been manufactured in 

 the green sandal leaves, in which case the partnership would 

 be more or less a reciprocally-symbiotic one. Numerous costly 

 attempts have been made to artificially propagate sandal in pure 

 plantations which, owing to the absence of other trees and shrubs, 

 were of course failures, and although this tree is of such great 

 economic importance it is a remarkable fact that its para- 

 sitism was until quite recently regarded as uncertain. It 

 is, therefore, not a matter for surprise that the artificial pro- 

 pagation of this tree was frequently a failure, or that the 

 notorious " spike " disease from which it suffered had baffled 

 all attempts at explanation and prevention. Until we know 

 the principal conditions essential for the healthy development 

 of our important species, it is obvious that we -cannot expect 

 to propagate them successfully or to discover the cause of, 

 or remedy for, any disease which may attack them. Hence 

 we must recognize the paramount importance of studying 

 the life-histories of our forest plants and the relations which 

 exist not only between them and their non-living environ- 

 ment but also between them and other living organisms. 



Leaving parasites we now pass on to : 



Lichens. 



example of 



(c) Symbionts. 



178. Perhaps the most typical 

 symbiosis among plants is afforded by 



