208 DROSERA. [CH. VIII 



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

 no movement occurs. 



The 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- 

 ever proved ^ when sufficient care is taken to prevent the 

 vibration of the room reaching the leaf, that a smooth 

 particle of glass which is comparatively heavy may rest 

 on the gland without producing movement of the 

 tentacle. 



(240) Drosera: inflection caused hy dilute solutions^. 



Very dilute solutions of ammonium phosphate cause 

 inflection. Prepare with good distilled water a solution 

 of ammonium phosphate in the proportion of I to 50,000 

 (2 centigrams to a liter). Fill 10 watch-glasses with the 



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

 causes after a time a certain amount of inflection. 



2 V liter suclmng en aus d. hot. Institut zu Tilhingen, i. p. 513. 



3 Insectivorous Plants, p. 153. 



