Stetjctuke and Growth. 27 



greatlj in different species. The wheat (the entire plant, in- 

 cluding the seed) in 1,000 lbs. gave 11| lbs. of ashes, composed 

 as follows : 



Potash 2.25 [ Silica 4.00 



Soda 2.40 Sulphuric acid 50 



Lime 96 Phosphoric acid 40 



Magnesia 90 Chlorine 10 



Alumina 26 | Iron, a trace. 



An analysis of perfected plants of the same species, although 

 growing in very different soils, will give the same proportion ; 

 while different species, although growing in the same soil, will 

 give very different proportions, showing that plants require 

 definite quantities of the inorganic elements in order to perfect 

 growth, and that in soil that does not yield these elements an 

 imperfect growth only can be obtained. 



These facts lie at the foundation of rotation of crops and 

 manuring. When any given plant has exhausted the soil of 

 the soluble elements requisite for its growth, another plant 

 requiring different elements, or the same in different propor- 

 tions, may grow luxuriantly and in perfection in the same soil. 

 Generally the grasses, such as wheat, rye, oats, etc., require 

 large quantities of silica ; peas, clover, and tobacco, much lime ; 

 turnips, beets, and sweet potatoes, potash and soda. The stalk 

 and fruit often require different elements in different proportions. 

 Both of course must be supplied. These elements might exist 

 in the soil, but not in a soluble condition, and of course yield 

 no benefit to the plant.* 



VII.— CONDITIONS ESSENTIAL TO GPvOWTH. 



All the conditions essential to growth have already been 

 either specially explained or incidentally mentioned ; but it may 

 be useful briefly to recapitulate : 



1. Warmth. — Without this, fts we have seen, the latent 

 powers of vegetable life can not be called into action. It is 



* See Appendix, B, and also " The Farm," for more on this point. 



