2l6 



Elementary Biology. 



FIG. in. LEAK OF Drosera WITH 



TENTACLES PARTIALLY INFLEXED. 



(Dr.rwin.) 



strand reduced to its simplest elements, surrounded by 

 parenchyma, and covered by epidermis. Should a fly 

 chance to alight on an uninjured leaf, it is immediately 

 caught by the viscid secretion on the glands of the short 

 -central tentacles. Gradually, apparently in response to a 

 stimulus conveyed from the centre of the leaf to the peri- 

 phery, the much longer marginal tentacles begin slowly to 



curve over their victim. The 

 fly soon becomes drowned in 

 the secretion produced by the 

 large number of tentacles 

 which ere long will have folded 

 over it. The entire movement 

 takes about a quarter of an 

 hour. 



The secretion, however, 

 seems to have the power not 

 only of killing the insect, but 

 afterwards of digesting it that 

 is to say, of rendering the 

 insoluble nitrogenous com- 

 pounds composing its body 

 soluble and easily absorbed. 

 The ferment (proteolytic) in 

 the secretion acts in presence 

 of an acid also found in the secretion, and is therefore, as 

 we shall see afterwards, quite comparable to the ferment 

 found in the stomachs of animals. 



The tentacles may be made to bend inward by a simple 

 touch, but not by wind or rain. Contact with a solid or the 

 absorption of the minutest trace of a nitrogenous fluid is, 

 however, the most efficient stimulus. 



Sarracenia purpurea. The extraordinary leaves of this 

 plant are particularly well adapted to fulfil the function of 

 catching and destroying insects and such like, which may be 

 unfortunate enough to tumble into their widely open trum- 



