68 ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



It is clear that by the action of nitrifying bacteria and tubercle- 

 fungi the nitrogen of the air is steadily diminished, but here again 

 we find a means of compensation. For there are certain denitrify- 

 ing bacteria, which, in the absence of oxygen, act upon decaying 

 organic matter in such a way that free nitrogen is liberated. 



RELATIONS BETWEEN THE NUTRITION OF PLANTS AND ANIMALS. 

 A little reflection will show that animals are entirely dependent 

 upon plants in the matter of food. 'Chis is obviously so as 

 regards purely vegetarian animals, while carnivorous forms are 

 indirectly dependent upon the vegetable world. Many flesh- 

 eaters feed entirely upon vegetarians, but if they prey upon other 

 flesh-eaters, and these again upon still other carnivorous creatures, 

 and so on, we get to plants in the end. 



Plants, considered as food for animals, have been concerned 

 in the evolution of burrowing, climbing, parachuting, and flying 

 forms, especially the last three. (For details, see vol. iii, pp. 

 231, 281, 292.) 



On the other hand, animals contribute to the store of plant 

 food. For, as we have already seen (pp. 65-67), they breathe 

 out carbon dioxide, which green plants take up, while their nitro- 

 genous excreta and dead bodies are partly converted into nitrates, 

 which the same plants are able to use for the purposes of nutrition. 



There are also a number of CARNIVOROUS PLANTS, which do 

 not altogether rely upon simple substances as food, but are 

 provided with " traps " for the capture and digestion of insects or 

 other small creatures. One of Darwin's most interesting books 

 (Insectivorous Plants] is devoted to these forms, some of which 

 are native to our own country, while several others may be seen 

 in botanic gardens. One of the simplest kinds of arrangement is 

 seen in the Butterwort (Pinguicula), that is often to be found 

 growing in damp places among our mountains. The pale-green 

 slippery leaves are arranged in a rosette, from the centre of 

 which violet flowers grow up. Small flies alighting on the leaves 

 are held fast by a sticky fluid, secreted by a multitude of little 

 knobbed hairs which project from the surface. The edges of the 

 leaves then curl over the prey, and there is an increased exudation 

 of the fluid in question, which acts very much like gastric juice, 

 converting the flesh of the booty into a soluble form that is 

 then absorbed as food. The widely distributed members of the 

 Sundew family (Droseracece} exhibit greater specialization in 



