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CHAPTER VI. 



THE ABSORPTION OF FOOD MATERIALS BY A GREEN PLANT. 



We have seen that the materials which protoplasm is eventually 

 able to assimilate, and which, therefore, constitute its food, are of 

 a similar nature to those deposited in seeds and other storehouses 

 of nutriment which are variously disposed in its substance. We 

 know, further, that these are not the materials w4iich an ordinary 

 green plant takes into itself from the environment in which it 

 lives. On the contrary, we know that its structure prevents its 

 taking in anything solid, but that it is continually absorbing 

 liquid by means of its roots. Between the raw materials that 

 can be so absorbed and the complex products which are needful 

 for the nutrition of its substance there is a great difference, and 

 the manufacture of these latter fi'om the raw materials taken in 

 constitutes a very important part of the metabolic processes. 



To ascertain what this work of construction consists of, we 

 must find out what elements the substance of the plant and the 

 true nutritive matter contain, how these are supplied to the plant, 

 and what work is done upon tliem 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 form of 

 solution that they can enter its substance by means of the 

 physical process of osmosis, modified as w'e have described. 

 This is equally true of gases, of which there is considerable 

 absorption b^' all plants, whatever 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, b}' all parts of their surface. Those which 

 grow with their roots embedded in soil and their shoots exposed 

 to the air sliow a certain division of labour in this respect. The 

 mineral constituents obtained from the soil are taken in by the 

 roots with the stream of water ; those of a gaseous nature mainly 

 find entry through the leaves and other green parts. 



If we examine the food stuffs described as bemg essential, we 

 find that proteids contain carbon, hydrogen, oxygen, nitrogen, 

 sulphur, and perhaps phosphorus. Carbohydrates and fats con- 

 tain only the first three of these elements. \ destructive analysis 

 of the plant, made by burning it, show^s that it contains others 

 also ; for after all the volatile products of combustion have been 



K F 2 



