NOURISHMENT OF PLANTS. 23 



disputable facts, and in such a form as to be availa- 

 ble for the purposes of practical experiment. 



That plants, like animals, must obtain nourish- 

 ment in order to live and grow, is indubitable. To 

 ascertain, however, the kind of nourishment which 

 they do procure is an inquiry far more difficult than 

 that which makes us acquainted with the food of 

 animals ; inasmuch as by our senses we are unable 

 to perceive v/hat vegetables take as food, or how ^ 

 they take it; nor can we in this way discover any (' 

 thing beyond the fact that they absorb water, and \ 

 again exhale it. Thus much, indeed, is generally j 

 known, that soil, moisture, air, warmth, and light 

 are necessary to the growth of plants; but this 

 amounts to very little, for very dissimilar elements 

 are contained in the soil, the water, and the air. 

 The essential point is to ascertain which of these 

 separate elements must be regarded as nourishment 

 to the plant, and which do not subserve this end. In 

 earlier times men lived in the belief that the knowl- 

 edge of these individual constituents was in no re- 

 spect important, because vegetables possessed the 

 power of converting one body into another ; for ex- 

 ample, lime into silica, or silica into lime, just as one 

 or the other might be needed. This belief has been 

 shown, however, to be erroneous. It is now known 

 with entire certainty, that plants have not this 

 power ; it is further known, that they can grow vig- 

 orously and reach complete development only when 

 all the constituent elements requisite to their organic 



