134 VEGETABLE PHYSIOLOGY 



constituents which are within the reach of the roots of two 

 plants growing almost side by side may naturally be 

 materially different in their proportions. This considera- 

 tion 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 applica- 

 tion, 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 composition 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 previously 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 processes already described as 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. 



To make a destructive analysis of the plant, it must be 



