SOME PLANTS WHICH ENTRAP INSECTS. 429 



without getting a meal it will open within twenty-four hours and will 

 he at once ready for a capture. The number of times that a trap will 

 close and digest nitrogenous fond is comparatively few; its delicate 

 organization is soon worn out and ants may crawl over its face with 

 impunity. 



There is a wealth of interesting variety among other species and 

 genera of the insectivorous plants, but the few deserihed may well 

 stand as examples of the whole numher, for none depart very far from 

 sonic one of these in the build and working of their pitfalls. While 

 some of the traps are simple in structure and slow in action, and others 

 are complicated and swift, yet there is a compensation in the fact that 

 the more complicated devices are the most short lived. 



The plants of insectivorous habit are not all memhers of a single 

 family, and indeed are not all closely related — the group is a purely 

 physiological one and owes its coherence to the fact that in all its 

 members the meaning of the capture of insects is the same — the sup- 

 plying of nitrogenous food. The very natural and pertinent question 

 here arises : Why do these plants need such a remarkable means of 

 getting nitrogenous food when other plants get on so well without this 

 means? The insectivorous plants are obviously not able to subsist on 

 insects alone, and — at least in the beginning of the habit — they were 

 not different from other plants in their needs for nitrogenous food. 

 So far as concerns the manufacture of starch and other non-nitro- 

 genous foods the insectivorous plants are like all other green plants, 

 deriving their carbon from the atmosphere, their hydrogen and oxygen 

 from the water of the soil. It is in the elaboration of its more com- 

 plex foods and the building up of new living substances in the process 

 of growth, and particularly in the maturing of seed that a plant meets 

 its need for nitrogen. In the ordinary plant this supply of nitrogen 

 is derived from the nitrates which, together with several elements 

 needed in small quantities, are brought up by the roots from the ma- 

 terials dissolved in the water of the soil. 



Xow, insectivorous plants in all parts of the world are found grow- 

 ing only in peat bogs, swamps, undrained pools and ditches — places in 

 which it has been found that the water about their roots, from which 

 they would be expected normally to draw their supply of nitrogen, is 

 exceedingly poor in nitrates. Higher compounds of nitrogen exist in 

 such places in plant and animal remains, but fail to be reduced to 

 nitrates because of the absence of certain kinds of bacteria which earn- 

 on this work in all ordinary soils but are absent here because of the 

 lack of drainage and consequent non-aeration of the water. It is this 

 deficiency in available nitrogen, then, that is made up by a diet of 

 insects, which are of course rich in nitrogenous substances. 



How have such complex structures as those of pitcher-plant and 



