188 HOW CROPS GROW. 



experiment winch is conclusive on this point. He gathered 

 a number of peppermint plants, and in some determined 

 the amount of dry-matter, which was 40.3 per cent. The 

 roots of others were then immersed in pure water, and the 

 plants were allowed to vegetate 2J months in a place ex* 

 posed to air and light, but sheltered from rain. 



At the termination of the experiment, the plants, which 

 originally weighed 100, had increased to 216 parts, and 

 the dry matter of these plants, which at first was 40.3, had 

 "become 62 parts. The plants could have acquired from 

 the glass vessels and pure water no considerable quantity of 

 mineral matters. It is plain, then, that the ash-ingredients 

 which were contained in two parts of the peppermint were 

 sufficient for the production and existence of three parts. 

 We may assume, therefore, that at least one-third of the 

 ash of the original plants was in excess, and accidental. 



The fact of excessive absorption of essential ash-in- 

 gredients is also demonstrated by the precise experiments 

 of Wolff on buckwheat, already described, (see p. 164,) 

 where the point in question is incidentally alluded to, and 

 the difficulties of deciding how much excess may occur, 

 are brought to notice. (See also pp. 176 and 179 in regard 

 to potash and oxide of iron.) 



As a further striking instance of the influence of the 

 nourishing medium on the quantity of ash-ingredients in 

 the plant, the following is adduced, which may serve to 

 put in still stronger light the fact that a plant does not 

 always require what it contains. 



Nobbe & Siegert have made a comparative study of 

 the composition of buckwheat, grown on the one hand in 

 garden soil, and on the other in an aqueous solution of 

 saline matters. (The solution contained sulphate of mag- 

 nesia, chloride of calcium, phosphate and nitrate of potash, 

 with phosphate of iron, which together constituted 0.316 | 

 of the liquid.) The ash-percentage was much higher in 



