ORGANIC PARTNERSHIPS OR SYMBIOSIS 175 



even if they have fallen on good ground, can only develop into 

 a new lichen if chance bring to them the proper partner alga. 



Obviously there must be, in the formation of the soredia, great 

 advantage for the species, or rather 'for the two species,' for the 

 fungi as well as the algge benefit by the arrangement, which ensures 

 the continuance of the partnership. It was not without reason, 

 however, that the dual organism was so long regarded as a simple 

 species in the natural history sense, for that is what it really is, 

 although it has arisen in a manner quite different from the usual 

 origin of species. As we know species which consist only of single 

 cells, and others which consist of many cells, differentiated in different 

 ways, and forming a cell-community or 'person,' and, finally, others 

 which consist of a community of diversely differentiated personse, 

 making up a ' stock ' ; so in the lichens we see that even different 

 species may combine to form a new physiological whole, a vital unit, 

 an individual of the highest order. When, at the outset of these 

 lectures, I said that the theory of evolution was now no longer 

 a mere hypothesis, and that its general truth could no longer be 

 doubted by any one acquainted with the facts available, I had in my 

 mind, among other facts, especially that of symbiosis, and above all 

 the case of the lichens. 



There are many other interesting cases of symbiosis between 

 two different kinds of plants, and one side of the partnership is 

 represented by fungi in a relatively large number of instances. The 

 reason is not far to seek : fungi must always be dependent on other 

 plants for their food; they must be parasitic, because they cannot 

 themselves produce the organic substances they require. They must 

 therefore associate themselves in some way with other organisms, 

 living or dead, and as a general rule they simply prey upon their 

 associate, sucking up its juices and killing it. But in not a few cases 

 they can render services in return, and, as we have seen in the case of 

 the lichens, symbiosis may then occur. Fungi in general have the 

 power of discovering and absorbing the least trace of water in 

 the soil, and with it they absorb the salts necessary to the plant, and 

 in this, apparently, consists the service which they are able to render 

 even to large plants fixed deep in the earth, such as shrubs and trees. 

 The roots of many of our forest trees, e.g. beech, oak, fir, silver 

 poplar, and bushes like broom, heaths, and rhododendrons, are thickly 

 wrapped round with a network of fungoid threads, and the mutual 

 relations just indicated exist between these and the plants in question 

 (Fig. 39, A and B). The plants give to the fungi some contribution 

 from the superfluity of their food-stuffs, and receive in return water 



