778 PHYSIOLOGY. 



Cereal Crop, requires more especially for its proper growth a full 

 supply of silica and phosphates ; hence it will only flourish in 

 a soil containing the necessary amount of such substances. As 

 growth proceeds, these constituents are absorbed in a state of 

 solution by the roots, and are applied to the requirements of the 

 plants. When the grain is ripe, it is removed as well as the 

 straw, and hence the silica and phosphates obtained from the soil 

 will have been also removed with them : the result of this is 

 necessarily, except in fertile virgin soil, that these ingredients 

 will not be then contained in the soil in sufficient quantities for 

 the immediate vigorous growth of the same class of plants ; but 

 by growing in a soil thus exhausted by Wheat another crop of a 

 different class of plants, such as Clover, Peas, Beans, &c. &c., 

 which require either altogether different substances, or a dif- 

 ferent amount or distinct combinations of the same substances, 

 we may obtain a profitable crop, while at the same time certain 

 chemical changes will go on in the soil, and other ingredients 

 will be taken up from the atmosphere, &c., by which the land 

 will be again adapted for the growth of Wheat, &c. 



The consideration of the above facts shows how important it 

 is for the agriculturist to have some acquaintance with vege- 

 table physiology and chemistry. He should know the compo- 

 sition of the various soils, and the plants which he cultivates, as 

 well as the nature of the compounds required by them, and the 

 modes in which they are taken up, and hence he would be able 

 to adapt the particular plants to the soils proper for them, and if 

 such soils did not contain the substances necessary for their life 

 and vigour, he must supply them in the form of manures. The 

 applications of chemistry and vegetable physiology to agricul- 

 ture are thus seen to be most important, and the great improve- 

 ments which have of late years taken place in agriculture are 

 mainly due to the increased interest taken in such matters, and 

 the many admirable researches to which it has led. However 

 interesting in an agricultural point of view this connection may 

 be, our necessary limits will not allow us to dwell upon it 

 further. 



Section 2. Life of the whole Plant, ob the Plant in 

 Action. 



The various substances required by plants as food having now 

 been considered, we have, in the next place, briefly to show, how 

 that food is taken up by them, distributed through their tissues, 

 and altered and adapted for their requirements. The considera- 

 tion of tliese matters involves a notice of the functions of 

 vegetation, namely, of Absorption, Circulation, Kespiration, 

 Assimilation, Development, and Secretion. 



The more imi)ortant facts connected with these have, how- 



