THE NUTRITION OF THE ROOT 45 



substances and carbo-hydrates is therefore apparent from 

 these instances. ' 



After the discovery of these facts, the next question to be 

 solved was whether any of our cultivated plants which do 

 not absolutely require humus for their growth, might not be 

 benefited by it. This problem led to some very interesting 

 culture experiments, which consisted in watering one set of 

 plants with an extract of humus, while another lot of the 

 same plants received the ash of the same quantity of humus. 

 Now, if only the mineral constituents of the humus were con- 

 cerned in the nutrition of the plants, both series of cultures 

 should have yielded the sanie result. But this was not the 

 case. The plants which had received the humus extract 

 yielded a crop twice as large as those which received the 

 salts only. 



This experiment, therefore, must be taken as a proof that 

 plants in the open reap a distinct advantage if they receive 

 their food substances in part at least in an organic form, and 

 we have a scientific confirmation of a practical and popular 

 saying, that " The dung put into the ground accounts for nine- 

 tenths of the farmer's pound." 



Of course it is not ascertained as yet whether the greater 

 vigour of growth is due to the absorption and the more rapid 

 assimilation of the organic coriipounds, or whether it is due 

 to the fact that combination of the humus compounds with 

 inorganic substances causes them to absorb more of the very 

 important ammonium salts. At all events, we must hold that 

 the presence of humus in the soil greatly stimulates the growth 

 of the plants, and that the view held up to the present 

 with regard to the importance of the humus compounds must 

 be considerably extended. According to this older view of 

 Grandeau's, the function of the constantly changing humus 

 compounds was to keep a suflScient quantity of nutritive sub- 

 stances in solution in the soil and make them available for the 

 roots. Chemically these humus compounds are still very 

 insuflSciently determined. 



But in considering the efficacy of huinus, we must not only 

 take into account the substances from which it is formed, but 

 we must know at what stage of decomposition it has arrived. 



