184 HOW CEOPS GROW. 



In similar experiments, Nobbe obtained buckwheat 

 plants, six to seven feet high, bearing three hundred 

 plump and perfect seeds, and barley stools with twenty 

 grain-bearing stalks. (Vs. St., VII, p. 72.) 



In water-culture the composition of the solution is suf- 

 fering continual alteration, from the fact that the plant 

 makes, to a certain extent, a selection of the matters pre- 

 sented to it, and does not necessarily absorb them in the 

 proportions in which they originally existed. In this 

 way, disturbances arise which impede or become fatal to 

 growth. In the early experiments of Sachs and Knop, 

 in 1860, they frequently observed that their solutions 

 suddenly acquired the odor of hydrogen sulphide, and 

 black iron sulphide formed upon the roots, in consequence 

 of which they were shortly destroyed. This reduction of 

 a sulphate to a sulphide takes place only in an alkaline 

 liquid, and Stohmann was the first to notice that an acid 

 liquid might be made alkaline by the action of living 

 roots. The plant, in fact, has the power to decompose 

 salts, and by appropriating the acids more abundantly 

 than the bases, the latter accumulate in the solution in 

 the free state, or as carbonates with alkaline properties. 



To prevent the reduction of sulphates, the solution 

 must be kept slightly acid, if needful, by addition of a 

 very little free nitric acid, and, if the roots blacken, they 

 must be washed with a dilute acid, and, after rinsing with 

 water, must be transferred to a fresh solution. 



On the other hand, Kiihn has shown that when am 

 monium chloride is employed to supply maize with nitro- 

 gen, this salt is decomposed, its ammonia assimilated, and 

 its chlorine, which the plant cannot use, accumulates in 

 the solution in the form of hydrochloric acid to such an 

 extent as to prove fatal to the plant (Henneberg' s Journal, 

 1864, pp. 116 and 135). Such disturbances are avoided by 

 employing large volumes of solution, and by frequently 

 renewing them, 



