8 Feb., 1907.] J-llc/iiciits of Aiiinuil Plivsiol 



those plants which are exposed to light and have the exposed parts coloured 

 (generally green). Starting from carbon dioxide and water, these plants 

 can build up complex sugars and starches. But we have seen in a former 

 chapter that it is from the break-down of such complex bodies that living 

 things get their energy. How do the plants at once build up and break 

 down? If breaking down gives off energy, then building up requires 

 enerrfv, and where do the plants obtain this? The answer is from sunlight. 

 This process is taking place all around us wherever there is a green leaf or 

 blade ; light is being absorbed, and the energy in the light acting on the 

 carbon dioxide and water is welding them together into the large molecules 

 of sugar or the still larger molecules of starch. Then, from the breaking 

 down of these into the two substances from which they were originally 

 built, energv can be liberated by the plant in different parts of its struc- 

 ture, both to build up the simple nitrogen bodies absorbed by the roots, into- 

 the complex nitrogen compounds which we shall shortly study, and to main- 

 tain its other manifold activities, such as its upward growth and the raising 

 of its sap. 



With animals the case is different ; they have no power of building up — 

 thev can only break down — and the complex substances which they require 

 have all to be obtained from the vegetable world. Though animals are 

 unquestionably on a higher plane of evolution, they are but poor chemists 

 compared with plants. Plants can link the elements at their disposal, 

 especially carbon, hydrogen, nitrogen, and oxygen, in many different ways, 

 and produce an almost endless series of compounds. Animals, on the other 

 hand, can do very little with the simpler bodies, and have to be content 

 with the linkings which plants have produced. Some few chemical trans- 

 formations they can bring about, but these are of a comparatively elemen- 

 tary nature. In fact, it pays the animal to get its complex bodies read\ 

 made, to be used in growth and in the repair of its bioplasm, and^ by their 

 break-down, to furnish the energy which it requires in a much higher de- 

 gree than the plant. We thus see that animals are parasites on the 

 vegetable world, for, e\-en though some of them get their complex carbon 

 and nitrogen compounds from the bodies of other animals, these, in their 

 turn, live on vegetables or on other animals which do so. But the plant 

 kingdom is not a complete loser by this parasitism, for every animal gives 

 off carbon dioxide, and, therefore, adds to the store of this substance with- 

 out which light, acting on the green leaf, could produce nothing; again, 

 the excreta of animals, as well as their dead bodies, contain nitrogenous 

 compounds which the plant can absorb by its roots and use in the manu- 

 facture of its bioplasm. 



In dealing with the special chemistry of animals, it will be as well to 

 include also certain compounds not found in the animal body, but present 

 in the vegetable substances taken as food. The compounds to be studied 

 will be considered under the following groups : — Water, metallic compounds, 

 carbohydrates, fats and lipoid, proteins. 



Water. 



The importance of water will be at once admitted when we consider 

 that it is the vehicle which brings nutriment to the tissues and carries waste 

 matter from them ; it is the chief ingredient of every cell, and is never 

 completely absent from any part of the body. Deprivation of water kills 

 quicker than stoppage of any other supply except oxygen. 



