LIFE OF THE WHOLE PLAINT. 795 



The consideration of the above facts shows how important it 

 is for the agriculturist to have some acquaintance with vegetable 

 physiology and chemistry. He should know the composition 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 so be able to adapt the parti- 

 cular plants to the soils proper for them. If such soils do 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 agriculture are thus seen 

 to be most important, and the great practical improvements 

 which have of late years taken place are mainly due to the in- 

 creased interest taken in such matters, and the many admirable 

 researches to which it has led. However interesting in an agri- 

 cultural 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, or 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 consider- 

 ation of these matters involves a notice of the functions of 

 vegetation ; namely, of Absorption, Circulation, Eespiration, 

 Assimilation, Development, and Secretion. 



The more important facts connected with these have, how- 

 ever, already been referred to in treating of the Special Physi- 

 ology of the Elementary Tissues, and of the Eoot, Stem, and 

 Leaf; so that it now remains only for us in this place to give 

 a general recapitulation of the functions of the plant, and to 

 consider them as working together for the common benefit of 

 the whole organism. 



1. Absorption. — The root, as already noticed, is the main 

 organ by which food is taken up in a state of solution, for 

 the uses of the plant. No matter can be absorbed in an 

 undissolved condition ; and this absorptive power is owing to 

 the superior density of the contents of the cells of the young 

 extremities of the roots over the fluid matters surrounding them 

 in the soil leading to the production of endosmotic action through 

 the cell-walls (see p. 736, and/^. 1106). 



That the roots do thus absorb fluid matters may be proved 



'"■ Eeference may be made to Professors Church and Dyer's 

 edition of Johnson's Hoiv Crops Grow (1869) for the results of 

 recent analyses of agricultural plants, and the chemistry of 

 vegetation generally. 



