128 VEGETABLE PHYSIOLOGY 



from uniform, and the constituents which are within 

 the reach of the roots of two plants growing almost side 

 by side naturally may be materially different in their 

 proportions. This consideration makes it almost or quite 

 impossible to ascertain, by observation of the soil and the 

 plant growing in it, what are the substances which are 

 entering its roots. 



The other method, which is of much more general 

 application, consists in making an analysis of the whole 

 body of the plant after its removal from the soil, and so 

 ascertaining what chemical elements it contains. A plant 

 gives off no solid excreta, and consequently whatever it 

 absorbs remains in its substance. The ultimate composi- 

 tion of the true nutritive matters, proteins, carbohydrates, 

 fats, &c., is known. Such an analysis having shown what 

 elements enter into the composition of a plant, and of the 

 food which it has stored in its tissues, it becomes possible 

 to inquire into the manner in which each is supplied to the 

 plant under examination, and into the work which is done 

 upon them in its cells. 



As already noticed, the structure of the plant demands 

 that all the materials of a solid character shall be in such 

 a solution that they can enter its substance by means of 

 the physical process of osmosis taking place through the 

 cell-wall. Similar considerations apply to gases, of which 

 there is considerable absorption by all plants, whatever 

 may be the nature of their habitat. 



The details of absorption vary to some extent, however, 

 according to the environment of the plant. Aquatic plants 

 can absorb water, and whatever is dissolved in it, whether 

 of gaseous or solid character, by all parts of their surface. 

 Those which grow with their roots embedded in soil, and 

 their shoots exposed to the air, show a certain division of 

 labour in this respect. The mineral constituents obtained 

 from the soil are taken in by the root-hairs with the stream 

 of water ; those of a gaseous nature mainly find entry 

 through the leaves and other green parts. 



