THE ROOT 117 



Lastly, there are such plants as lichens, for instance, 

 which settle like froth and scum upon the inhospitable 

 surface of rocks and stones, and even, it is said, attach 

 themselves to the surface of polished glass, destroying 

 these bodies in drawing out of them the mineral food 

 they require. It is a remarkable fact that these plants 

 are distinguished by the abundance of acids in them, 

 especially oxalic acid. 



We have now to answer the last of the three questions 

 raised at the beginning of the lecture. Why is it that, 

 among the various substances with which the roots 

 come into contact in the soil, they attract especially 

 those that are necessary to the plant ? Let us study 

 the fact itself more closely before we answer this ques- 

 tion. If a plant is grown in a solution of two salts, 

 say saltpetre and common table-salt, it is soon evident 

 that the root entirely absorbs one of these salts, namely 

 the saltpetre, while it scarcely draws at all upon the 

 common salt, which it does not require. Such facts 

 were formerly disconcerting to scientists ; it looked as 

 if roots could discriminate between the different sub- 

 stances and choose their own food, accepting one sub- 

 stance and refusing another. How, indeed, can such 

 discrimination be explained ? Surely we cannot admit 

 that a root is endowed with will-power or instinct ? 

 The explanation is very simple, and we came across it 

 some time ago. You remember our artificial cell and 

 the behaviour of iron salt towards it (see chapter ii.). 

 Similarly, both saltpetre and common salt easily diffuse, 

 and therefore both will penetrate into the cells of the 

 root, and hence into the rest of the plant. But the 

 subsequent fate of the two salts inside the plant will 

 be totally different. The saltpetre will there be decom- 

 posed, and its nitrogen will serve to form albuminoids 



