258 



EVOLUTION OF PLANTS 



its upper surface. The peculiar digestive fluid which is 

 present in both cases is probably a direct product of the 

 plant itself, although it has been claimed that it is due 

 to the presence of certain bacteria, which are present in 

 large numbers. Whether the digestive process is due 

 to the secretions of the plant itself, or to the activity 

 of the bacteria, the products of digestion undoubtedly 



serve to supply the plant 

 ^ ^ ;, -,- -^i^r-. with nitrogenous food. 



The pitcher-plants. 

 Nepenthes (Fig. 58, B), 

 Sarracenia (Fig. 58, A), 

 and Darlingtonia, are 

 also striking examples of 

 these carnivorous plants. 

 All of these have the 

 leaves modified into 

 pitcher -shaped recepta- 

 cles, into which insects 

 are lured by the bright 

 colors of the leaves as 

 well as an abundant se- 

 cretion of a honey-like 

 substance. In most of 

 them the upper part of 

 the interior of the pitcher 

 is lined with stiff, down- 

 ward pointing hairs, be- 

 low which the wall is 

 very smooth, so that an 

 insect which has fallen in cannot escape. A fluid 

 is secreted by the pitcher, which partially fills it, 



Fig. 58 (Carnivorous plants). — A, 

 leaf of the common pitcher-plant 

 {Sarracenia purpurea) ; B, pitcher 

 of a tropical pitcher-plant (Nepen- 

 thes) borne upon the end of a ten- 

 dril, the opening protected by a 

 lid; C, leaf of a sundew {Drosera 

 longifolia) ; D, leaf of Drosera 

 which has captured a mosquito, 

 showing the way in which the ten- 

 tacles, ten, clasp the insect ; E, part 

 of a leaf of bladder-weed (Utricu- 

 laria), with the bladder-like trap, 

 !) ; F, a single vesicle of U tricu- 

 laria more enlarged. (Figs. A, B 

 after (ioebel.) 



