NATURE OF THE PLASTIC POWER. 471 



principle which accomplishes in a plant the grouping of its parts as if it 

 were an agent, the foregoing illustrations show us that all the facts of the 

 case are equally well satisfied on the supposition that it is the continua- 

 tion of an operation. A multitude of parallel instances present them- 

 selves. In the making of leavened bread, all the phenomena would seem 

 to be accounted for either upon the hypothesis that there resides in the 

 leaven or ferment an agent, whose quality it is to determine a specific 

 change in the flour,, or that there is an operation which, because of the 

 chemical conditions existing, is gradually spreading, and which will not 

 cease until all the material submitted to it has been affected, and this no 

 matter whether it be in the same mass or in successive portions. Of 

 such hypotheses, the first is merely an elementary idea, the latter in- 

 volves a philosophical conception. 



In this way, therefore, the so-called plastic power of a cell or the germ 

 of a seed may be regarded as the continued manifestation Nature O f the 

 of an antecedent impression long ago made, and which, un- plastic power. 

 der the existing conditions, has no liability to wear out or die away; 

 and that impression may have been purely physical in its nature. 



Viewed in this attitude, the life of plants is a physical phenomenon. 

 The parts of which they are composed are furnished to them The life of 

 by influences of a mechanical kind : their carbon is taken by p h an s ^ai & 

 a true chemical decomposition from the carbonic acid of the phenomenon. 

 air ; their nitrogen comes from ammonia or from the atmosphere. Wa- 

 ter is drawn by capillary attraction from the ground. In virtue of its 

 chemical qualities, it carries into the growing system the various saline 

 bodies present in the soil, and which are needful for the economy. The 

 sunlight, heat, rain, winds, are the supplying and nurturing powers, and 

 the grouping agencies residing in the plant are of the same mechanical 

 derivation or order. 



The germination of a seed and the growth of a plant, as thus consid- 

 ered, show us to what an extent physical forces are concerned in vege- 

 table organization. The conclusion thus indicated is enforced in no 

 common manner when we direct our attention to the series instead of to 

 a single plant. This is what I propose to do in the following chapter. 



