§ III. ADAPTATIONS TO ANIMALS. 



CHAPTER XXV. 

 ANIMALS AS FOOD, FOES, OR FRIENDS. 



1 . Carnivorous plants. 



372. Nitrogen supply. — The ordinary source from which 

 green plants obtain nitrogen for the making of their food is 

 the nitrogen compounds dissolved in the soil water. Plants 

 which live where the soil water contains little or no nitroge- 

 nous material are forced to resort to another source of supply. 

 Some plants solve the problem by entrapping animals, deriv- 

 ing from their, bodies the desired nitrogen compounds. Such 

 plants are called carnivorous plants, or, since the bulk of 

 their catch consists of insects, insectivorous plants. The 

 catching of animals is done 



373. I. By pitfalls and traps. — (a) The various pitcher 

 plants furnish a fine e^ifample of well-devised pitfalls. The 

 leaves of these plants have a deep, trumpetlike tube making 

 up the body of the leaf ; or they carry at the end of a long 

 petiole a deep cup with a lid, as in the tropical pitcher plants 

 (fig. 2 2o; see also fig. loi). The tube is one-third or half 

 full of water, in which are always found numbers of dead or 

 dying insects. The sides of the tube without are often made 

 attractive by gaudy colors or by lines of sweet secretion, 

 which draw both flying and crawling insects. Within, its 

 surfaces are either excessively smooth, so as to afford no foot- 



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