32 SYMBIOSIS 



connection, and what is now more specially to be insisted upon 

 is, that the success of the lichen as a symbiotist is essentially 

 connected with cross-feeding. 



It is owing to the power of disintegrating by both mechanical and 

 chemical means (says the Encyclopedia Britannica), the rocks upon 

 which they are growing, that lichens play such an important part in soil 

 production. 



The lichen thus draws its food pre-eminently from the 

 inorganic world, which I claim to be vitally important. It is 

 what I call " cross-feeding " for the plant, and is parallel to the 

 symbiotic draft by the animal on the vegetable world. 



The instance of the lichen as a successful " cross-feeder," of 

 course, is not unique ; but the same connection, or sequence, 

 holds universally amongst plants, as we shall presently see in 

 greater detail. The case of clover may serve as a first example, 

 illustrating the pronounced good effects of cross-feeding. Says 

 Prof. James Long : 



There is nothing in romance or ancient story more thrilling than the 

 fact that by the employment of certain mineral fertilisers (cross-feeding !) 

 the clovers and superior grasses, almost unknown before, appear and grow 

 with luxuriance ; while the inferior grasses and weeds disappear, unable 

 to contend against those species of plants, which fed by man (cross- 

 fed !) obtain the mastery of the situation Clover is a deep- 

 rooted plant and a nitrogen gatherer ; while it revels in particular minerals. 

 Sometimes one alone, although sometimes two or three are required. Thus, 

 when those foods are supplied, clover responds with its beautiful foliage, 

 its roots simultaneously piercing the soil to great depths in search of water, 

 and at the same time appropriating foods, which they find down below and 

 which they bring near the surface for the benefit of neighbouring shallow- 

 rooted plants. (Italics mine.) 



What strikes one as at least curious is that a case of ordinary 

 legitimate feeding, although indeed combined with pioneer-work 

 in organic civilisation, is to be set down as almost belonging to 

 the sphere of " romance," when it in reality concerns the most 

 universal work-a-day life of plants, on which all organic 

 existence fundamentally depends. 



It is now, however, a well established fact that plants generally 

 thrive on mineral food, i.e., on what I call " cross-feeding," and 

 my thesis that Nature prefers " cross-feeding " in the interest 

 of organic civilisation, may be seen to receive considerable 

 corroboration from the following facts and considerations. In 

 a valuable contribution to The Principles of Crop Production, 



