216 HOW CROPS GROW. 



inent existing in the seed,* which evidently was enough 

 for the necessities of growth until the stem was forty 

 inches high". Prom that point on the plants almost 

 ceased to grow, and gradually died from want of food 

 and inability to assimilate. 



We have already seen that, according to Hoppe-Seyler, 

 magnesium is a constant and presumably an essential in- 

 gredient of chlorophyllan, a crystallized derivative of 

 chlorophyl. This makes evident that m agnesia m is di- 

 rectly concerned in and needful to the fqrmatiop of the 

 chlorophyl granules which, so far as observation as yet 

 has gone, are the seat of those operations which first 

 construct organic substance from inorganic matter. 



Magnesium and calcium occur in the aleuroue of seeds 

 and, according to Griibler, form soluble, crystallizable 

 compounds with certain albuminoids, so that these ele- 

 ments, like potassium, may be concerned in the transport 

 of protein-bodies. 



Silica. — Humphrey Davy was the first to suggest that 

 the function of silica might be, in case of the grasses, 

 sedges, and equisetums, to give rigidity to th^lender 

 stems of these plants, and enable them to sustain the 

 often heavy T?eight of the fruit. 



The results of the many experiments in water-culture 

 by Sachs, Knop, Wolff, and others (see p. 200), in which 

 the supply of silica has been reduced to an extremely 

 small amount, without detriment to the development of 

 plants, commonly rich in this substance, prove in the 

 most conclusive manner, however, that silica does not 

 essentially contribute to the sbifEness of the stem. 



WolfE distinctly informs us that the maize and oat 

 plants produced by him, in solutions nearly free from 

 silica, were as firm in stalk, and as little inclined to 

 lodge or "lay," as those which grew in the field. 



* Common beans contain about one-fourth of one per cent of mag- 

 nesia. 



