208 THE CONDITIONS OF LIFE IN THE SEA [PART III 



those which make use of both animal and vegetable food. But 

 whether or not an animal feeds upon other animals or upon plants 

 it is the case that its mode of nutrition is such as to compel it 

 to make use of the substances which have been elaborated by 

 other living organisms. The " proximate food-stuffs " upon which 

 an animal lives are the proteids, carbohydrates and fats, which 

 have been built up Avithin the living cells of other animals or 

 plants. Proteids, such as the albumens and allied substances 

 which make up the " flesh " of animals, or the nitrogenous parts 

 of the tissues of plants, are among the most complex of compounds 

 known to modern chemical science. Each molecule is composed 

 of thousands of atoms which are grouped together to form the 

 "building-stones" which by their farther combinations form the pro- 

 teid molecule-complex. These proteid "building-stones" are them- 

 selves complex molecules — amido-compounds — which are united 

 together by acidic or basic affinities, are " labile " and are probably 

 in a continual state of chemical dissociation while in the condition 

 of living protoplasm. Carbohydrates, such as starches, sugars or 

 gums are much simpler chemical compounds, and differ further 

 from the proteids in containing no nitrogen. Fats are again 

 relatively simple bodies, and differ from both carbohydrates and 

 proteids in containing much more carbon than either of the above 

 substances — probably the numbers of atoms contained in the 

 molecules of these fats and carbohydrates may be counted by the 

 dozen. Now it is essential for the nutrition of the animals that 

 they should obtain their food from these proximate food-stuffs. 



The mode of nutrition of the typical terrestrial plant is 

 fundamentally different from that of the typical animal. While 

 the latter demands organised, and therefore chemically complex, 

 food-stuffs, the former may build up its living substance, and 

 carry on its life processes, by making use of the simplest compounds 

 known to chemistry. Salts of ammonia and nitric acid, carbon 

 dioxide, and water are composed of molecules which contain less 

 than a dozen atoms, nevertheless the plant organism can obtain 

 its nitrogen, carbon, hj^drogen and oxygen from such simple 

 compounds ; and can build them up into its living tissues. A 

 typical plant obtains its ammonia and nitric acid salts from the 

 ground water by means of its roots, and its carbonic acid and 



