VEGETATION OF STILL WATER 31 



become inflected ; that in two cases the blades of the leaves were 

 so much curved inwards that they formed little cups enclosing 

 the drops of milk ; that the leaves expanded on the third day. 

 Altogether, Darwin experimented with nitrogenous fluids on 

 sixty-four leaves, and found that sixty-three responded. Care 

 has to be taken to select young and active leaves, as old ones 

 have not sufficient power of response. These experiments cer- 

 tainly seemed to show that non-nitrogenous matter did not 

 stimulate the leaves of the Sundew ; to place the matter beyond 

 a doubt, Darwin tested the same leaves with bits of meat, and 

 found they did respond, though not so readily as fresh leaves, 

 for their powers had been somewhat impaired owing to the experi- 

 ments with non-nitrogenous material. Other experiments were 

 performed to ascertain the nature of the process. Darwin 

 found that the glands in the knob of the tentacles of Sundew 

 have the power of secreting a ferment analogous to the pepsin 

 contained in the gastric juice of animals ; this ferment in the 

 presence of an acid dissolves nitrogenous compounds. The 

 length of time during which the tentacles remain inflected depends 

 partly on the quantity of the substance given ; they remained 

 inflected longer over large bits of meat than over small ones, 

 and only the tentacles on the same side as that on which the 

 meat was placed bent in, whilst those on the opposite side 

 remained distended. To repeat some of Darwin's experiments 

 as recorded in his book on Insectivorous Plants would give a 

 far better idea than any description of the response of these plants 

 to the stimulus of nitrogenous objects. 



Insectivorous plants may be arranged in two groups : 

 (i) Those that catch insects by means of traps ; (2) those that 

 perform movements in the capture of their prey. To the first be- 

 long the Bladderwort ; to the second, the Sundew and Butterwort. 

 There are three fairly common species of Bladderwort. The most 

 common is the Utricularia vulgaris, which is found in deep pools. 

 It has not true roots, but root-like floating branches, sometimes 

 nearly a foot long. The leaves are very much divided, and bear 

 bladders. The flower-stem is six to eight inches high, bearing 

 a few large yellow flowers, which are generally over by the end 

 of August, or even earlier. The smallest Bladderwort (Utri- 



