188 HOW CROPS GROW. 
experiment which 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 24 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 
