60 GENERAL BIOLOGY 



etc. In the ingenious traps possessed by such 

 plants unwary insects are caught and killed ; 

 digestive fluids there secreted dissolve the tissues 

 of the prey, and they are absorbed precisely as 

 they would be in the stomach of a carnivorous 

 animal. 



Foods in General. In the light of what has just 

 been said, it will be seen that we must modify our 

 notions of foods. It is not enough to classify foods 

 as the scientific cook books do, merely as carbo- 

 hydrates, fats, and proteins. Since the' sole purpose 

 of taking food is, as we have seen, the accumulation 

 of a store of energy, we might define a food to be 

 anything that contains potential energy. The CO 2 and 

 H 2 O are foods to a green plant only when combined 

 with the energy of sunlight. They are better called 

 food-materials. A welsh rarebit, the food value of 

 which is very high, which I may eat with impunity, 

 may be "the other man's poison." But a stick of 

 hickory that supplies the wood-boring beetle larva all 

 the nourishment it requires, is to me useless because, 

 for my purposes, the large amount of energy locked 

 up in it is not available. If, however, I reduce the 

 stick to sawdust and boil it with sulphuric acid, 

 thereby converting it into glucose, it becomes a very 

 good food. We must, therefore, modify our previous 

 definition by designating as a food anything that 

 contains available potential energy. 



Fate of the Foods in the Higher Animals. In 



