HOW PLANTS DRINK. 65 



forms a jointed snare for catching insects. It is 

 hinged at the middle ; and when a fly lights upon 

 it, the two edges bend over upon him, and the 

 bristles on the margin interlock firmly. As long 

 as the insect struggles they remain tightly closed; 

 when he ceases to move, and is quite dead, they 

 open once more, and set their trap afresh for an- 

 other insect. A great many such carnivorous and 

 insectivorous plants are now known : and in al- 

 most every case they inhabit places where the 

 marshy and waterlogged soil is markedly wanting 

 in nitrogen compounds. Insect-eating leaves are 

 thus a device to supply the plant with nitrogen 

 by means of its foliage, in circumstances where 

 the roots prove powerless for that purpose. 



Simpler forms of the same sort of habit may 

 be seen in many other familiar plants. Thus our 

 English catchflies and several other of our com- 

 mon weeds have sticky glandular stems, which 

 exude a viscid secretion, by whose aid they catch 

 and digest flies. This is the beginning of the in- 

 sect-eating habit, more fully evolved by natural 

 selection in marsh-plants like sundew, and espe- 

 cially in larger subtropical types like the Venus's 

 fly-trap. If you collect English wild-flowers you 

 will soon perceive that a great many of them have 

 sticky glands on the summit of the stem, near the 

 flowering heads ; and this is useful to them, be- 

 cause the flowers and seeds are particularly in 

 want of nitrogenous matter for the pollen and 

 ovules and the development of the seed. In short, 

 though plants get their nitrogen mainly by means 

 of the roots, they often lay in a supplementary 

 store by their stems and their foliage. 



Our common English teasel shows us the be- 

 ginnings of another form of insect-eating, which 

 5 



