432 MANUAL OF BOTANY 



Even in adult plants of the most considerable complexity we 

 find instances of the same thing ; the living substance is nou- 

 rished by materials which have been constructed by it and stored 

 at various places in its substance till their consumption has been 

 called for. 



What, then, are these substances which, in the strict sense, 

 constitute the food of plants ? We can ascertain what are 

 necessary by inquiring what are the materials which are de- 

 posited in the seed for the nutrition of the embrj'o when the seed 

 germinates. We find there examples of great classes of food 

 stuffs which are similar to those on which animal protoplasm is 

 supported, and are led again and again to the idea that vegetable 

 protoplasm is essentially identical with the latter. Proteids, carbo- 

 hydrates, fats or oils are the materials which, in varied forms, 

 are niet with. If we study the protoplasm of a living active 

 vegetable cell and treat it with appropriate solvents, we can 

 extract representatives of these, or of some of them, from its 

 substance, the meshes of which are the places in which it is 

 deposited. The nutrition of the protoplasm can only take place 

 when these substances are brought into the most intimate rela- 

 tions with it ; from them, no doubt, in ways not yet discovered, 

 it builds itself up, and by its own decomposition it reproduces 

 them. The interchange of matter between the living substance 

 and its food, the way in which the latter is transformed into the 

 former, are points about which yet almost everything essential 

 remains to be discovered. 



But while we recognise that the ultimate nutrition of proto- 

 plasm is dependent upon a supply to it of such materials, we 

 are face to face with the fact that such materials are not furnished 

 from the environment to the ordinary green plant. On the 

 other hand we find the latter taking in by ordinary ph\'sical 

 processes CO^ from the air and a variety of salts from the soil, and 

 we ascertain that if these are supplied under suitable conditions, 

 the plant can flourish and develop. In one sense, then, these 

 substances constitute its food, but we must carefully consider 

 what we mean by the term. In the nutrition of such plants 

 there are clearly two very different processes combined, which 

 should be kept carefully distinct. We have the absorption of 

 food materials rather than of food in the true sense, and we 

 have, following such absorption, the expenditure of a consider- 

 able amount of energy upon these food materials, with the 

 result that they are worked up into the complex compounds 

 which we find protoplasm can assimilate, and which are those 



