238 



BOTANY 



PART I 



plants and different parts of the same plant. The difference brought 

 out by the table in the proportions of the more important phos- 

 phoric acid and of the less essential silica and lime contained in 

 Rye and Pea seeds, as compared with the amounts of the same 

 substances in the straw, is worthy of notice. The Potato contains 

 much K 2 and little CaO, while the wood of Spruce shows the 

 opposite condition. 



In the preceding table the figures do not express absolutely constant proportions, 

 as the percentage of the constituents of the ash of plants varies according to the 

 character of the soil. 



The mineral substances which form the ash were at first regarded 

 as accidental impurities of the organic substance of the plant. But 

 every attempt to obtain a plant free from mineral substances shows 

 that they form essential constituents. 



It was first asserted by BERTHOLLET (1803), and afterwards 

 emphasised by KARL SPRENGEL (1832), and later by LIEBIG, that the 

 mineral salts contained in plants were essential constituents of plant 

 food. Conclusive proof of this important fact was first obtained in 

 1842 by the investigations of WIEGMANN and POLSTORFF. 



This conclusion can be reached by two methods, which at the 

 same time show whether all or only certain of the substances in 

 the ash are necessary. The first method is to cultivate the plant 

 in an artificial soil composed of insoluble substances such as platinum, 

 pure carbon, pure quartz, with which the substances to be investi- 

 gated can be mixed. The second method, that of WATER CULTURE, 

 is more convenient. Many plants are able to develop their root-system 

 in water instead of in the earth. It is thus possible to add to the 

 water the elements found in the ash in various combinations, and so 

 to ascertain which elements are necessary and which superfluous. As 

 Fig. 245, /, shows, the plant (Buckwheat) succeeds well in such a 

 food solution if of suitable composition ; it can form roots, shoots, 

 flowers, and fruits, and increase its dry weight a hundredfold or a 

 thousandfold, just as if it were growing in the soil. In distilled 



