280 PHOSPHATES, ETC. [BOOK n. 



that the roots of plants containing alumina, either exert a 

 peculiar catalytic action upon the aluminous compounds with 

 which they are in contact, or excrete an acid ; for were the 

 aluminous constituents of the soil soluble per se in the fluids 

 of the soil, they would also be absorbed by other plants. 

 (Chemical Gazette, 1847. p. 307.) 



Phosphates. All plants contain phosphoric acid. It is 

 from plants that animals obtain it. It does not, however, 

 exist in a free state in nature, but in the condition of phos- 

 phates, in which form it is found in plants. It is one of the 

 most essential constituents of soils; probably gives much of 

 their true value to manures ; and its absence would be 

 accompanied by sterility. It is, however, of more importance 

 to some plants, as Crucifers, than to others, as Cereals, as will 

 be apparent from the preceding table. 



Sulphates. It would seem that sulphuric acid in a state of 

 combination is also universally present in plants, varying like 

 phosphoric acid in its proportional quantity. Crucifers 

 abound in it, while very little is found in Oak leaves or in 

 Ash timber. Hence the sulphates of soda, lime, and magnesia, 

 and substances evolving sulphuretted hydrogen, are all 

 important as manures. 



Oxides of iron, manganese, tyc., are very often present in 

 plants. It is, however, doubtful whether they usually exer- 

 cise any influence in the functions of vegetation. Professor 

 Johnson supposes them to enter the roots, "either in the 

 state of soluble sulphate or of carbonate dissolved in carbonic 

 acid, or of some other of those numerous soluble compounds 

 with organic acids, which may be expected to be occasionally 

 present in the soil." 



In addition to the food thus supplied to plants from with- 

 out, a very abundant source of nutrition to the young parts 

 is derived from their own secretions, the most important of 

 which is starch. The purpose which nature intends this 

 almost universally diffused substance to answer, in the system 

 of vegetation, is essentially nutritive. It is formed in plants 

 soon after their parts become organised, and it collects there 

 till in some instances, such as albumen, tubers, rhizomes, 



