192 DROSEBA. [CH. VIII 



raent in half a minute. In all experiments on Drosera 

 leaves should be selected which have good drops of 

 secretion on the glands, and which show a healthy red 

 colour. Leaves which are too old are of a dark red and 

 should be avoided. 



(239 A) Drosera : irritated by contact with inorganic 

 matter. 



The tentacles may also be excited by other substances, 

 e.g. by fragments of pounded glass placed on the glands. 

 It is well to use coloured glass, so that it may be possible 

 to see easily whether the fragments actually touch the 

 glands or only rest in the secretion. In the latter case 

 no movement occurs. 



Tfhe glands are not sensitive to a single touch: the 

 impact must be rapidly made, but may be hard enough to 

 bend the tentacle. Nor has the blow caused by falling 

 drops of water any stimulating effect. 



But water containing even the finest solid particles 

 produces movement. This may be shown by placing half- 

 a-dozen leaves in, distilled water rendered milky by a 

 little precipitated chalk, an equal number of leaves being 

 placed in pure distilled water^ for comparison. In ten 

 minutes the leaves in the emulsion should be well in- 

 flected. 



In Darwin's Insectivorous Plants a number of experi- 

 ments are given which show that excessively light bodies 

 resting on the glands cause inflection. Pfeffer has how- 



' Carefully distilled water should be used, and even this sometimes 

 causes after a time a certain amount of inflection. 



