120 THE LIFE OF THE PLANT 



of nutrition. It is owing to this marked difference that 

 these processes form the most characteristic and 

 essential feature of vegetable life. 



We already know in part what substances are absorbed 

 as food by the leaf. They must evidently be those 

 substances which enter into the composition of the 

 plant, but are not absorbed by the root. 



We have seen that among the eleven elements 

 enumerated in our last chapter (the twelfth, silicon, was 

 proved to be useless to the plant), the seven elements 

 of the ash, phosphorus, sulphur, chlorine, potassium, 

 calcium, magnesium, and iron, together with nitrogen, 

 enter by way of the root ; and that water also, con- 

 sisting of hydrogen and oxygen, enters the plant by 

 the same path. There still remains carbon — the foun- 

 dation of all organic substance. So far we have not 

 troubled about it in our artificial cultures of plants, 

 though these contain a thousand times, even many 

 thousand times, as much carbon as the seeds we took 

 for our experiment. Quantitatively carbon forms the 

 most important constituent of the plant (something 

 like 45 per cent.), yet we did not supply it to the root, 

 but even systematically removed it from the surrounding 

 soil. This means that a plant can live without absorb- 

 ing carbon by its root. 



These experiments are not, however, sufficient to 

 tell us the way the plant obtains carbon in the natural 

 conditions of life. To say that a plant can live with- 

 out absorbing carbon through its roots is obviously 

 different from saying that a plant is unable to absorb 

 carbon by its roots, although this mistake is often made. 

 It has not yet been proved that a plant is unable to 

 obtain its carbon from the organic substance of the soil. 

 The discussion of this point would, however, carry us too 

 far afield ; moreover, it is of little interest, because it is 

 easy to show that if carbon obtained in this way takes 

 any part at all in the life of the plant, that part is in- 



