234 now CROPS PKEt>. 



and 100 grams of the solution left on evajjoration 13S 

 mgrnis. of residue. The same amount of humus extract, 

 that had been kept in a contiguous vessel containing no 

 plant, left a residue of 136 mgrms. The disappearance of 

 humus from the solution is thus mostly accounted for by- 

 its oxidation. 



De Saussure considers that his experiments demonstrate 

 that humic acid and (in his third exp,) tlie matters ex- 

 tracted from peat by water (crenic and apocrenic acids) 

 are absorbed by plants. Wiegmann and Polstorf attrib- 

 ute any apparent absorption in their trials to the una- 

 voidable errors of experiment. Tlie quantities that may 

 have been absorbed were indeed small, but in our judg- 

 ment not smaller than ought to be estimated with certainty. 



Other experiments by Soubeiran, Malaguti, and Mulder, 

 are on record, mostly agreeing in this, viz., that agricul- 

 tural plants (beans, oats, cresses, peas, barley) grow well 

 when their roots are immersed in, or watered by, solutions 

 of humates, ulmates, crenates, and apocrenates of ammo- 

 nia and potash. These experiments are, however, all un- 

 adapted to demonstrate that humus is absorbed by plants, 

 and the trials of De Saussure and of Wiegmann, and Pol- 

 storf, are the only ones that have been made under condi- 

 tions at all satisfactory to a just criticism. These do not, 

 perhaps, conclusively demonstrate the nutritive function 

 of liumus. It is to be observed, however, that w^hat evi- 

 dence they do furnish is in its favor. They prove eifec- 

 tually that humus is not injurious to plants, though Liebig 

 and Wolff have strenuously insisted that it is poisonous* 



Let us now turn to the probabilities bearing on the 

 question. 



In the first place there are plants — those living in bogs 

 and flourishing in dung-heap liquor — which throughout 

 the whole period of tiieir growth must tolerate^ if not ab- 

 Borb, somewhat strong solutions of humus. 



Again, the cultivated soil invariably yields some humus 



