Inter-Relations of Living Creatures 653 



worm called Convoluta, and makes the worm a sort of plant- 

 animal, and a very succssful association it is. 



The Double Life of Lichens 



It was the great botanist De Bary who first applied the term 

 symbiosis to the partnership illustrated by those strange encrust- 

 ing plants called lichens which are so familiar on trees and rocks. 

 They are even stranger than they look, for they are double-plants, 

 as we have seen (see p. 610) . 



It is impossible not to be interested in lichens, pioneers in soil- 

 making, sheltering and feeding those animals that are the outposts 

 of life's ceaseless campaign, but is not their supreme interest 

 that they represent a very distinctive adventure in evolution the 

 adventure of symbiosis? 



The Seamy Side of Heather 



Everyone knows that heather grows well on poor and un- 

 promising soil where relatively few other plants will thrive. The 

 water of the moorland is apt to be in such an acid state that the 

 roots of plants cannot use it. The nitrogenous supplies in the soil 

 are unavailable because bacteria do not flourish in peaty environ- 

 ment. The same is true of earthworms, which make soil elsewhere. 

 What, then, is the heather's secret, for it certainly thrives on 

 mountain and moorland? It has a partner-fungus that sends its 

 threads or hypha? not only into the cells of the root, but through 

 and through the stem and leaves, and even into the seed-box. The 

 fungus acts as the intermediary between the heather and the soil ; 

 it absorbs water and organic material; it is perhaps able in some 

 measure to fix atmospheric nitrogen. In any case, the heather 

 has been able to effect a compromise with what was probably, to 

 start with, a predatory intruder ; indeed, the compromise has gone 

 so far that the heather cannot thrive without its partner. As Dr. 

 Rayner says, the heathers "have solved the problem of growth on 

 poor and unpromising soils, but have solved it at the price of their 



