70 THE STOBY OP THE PLANTS. 



they remain tightly closed ; when he ceases to 

 move, and is quite dead, they open once more, 

 and set their trap afresh for another insect. A 

 great many such carnivorous and insectivorous 

 plants are now known : and in almost 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 

 common weeds have sticky glandular stems, 

 which exude a viscid secretion, by whose aid 

 they catch and digest flies. This is the begin- 

 ning of the insect-eating habit, more fully 

 evolved by natural selection in marsh-plants 

 . like sundew, and especially 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, because the flowers 

 and seeds are particularly in want of nitrogenous 

 matter for the pollen and ovules and the de- 

 velopment 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 

 beginnings of another form of insect-eating, 

 which is highly developed in certain American 



