840 ASSIMILATION. 



them. Darwin found that movement was caused by the contact 

 of a particle weighing only .0008 milligram (Tghs of a grain). 

 When a tentacle has been excited by 

 contact with a solid particle, there is seen, 

 after some hours, a remarkable change in 

 its cells near the gland. "Instead of be- 

 ing filled with homogeneous purple fluid, 

 they now contain variousl}' shaped masses 

 of purple matter suspended in a colorless or 

 almost colorless fluid." ^ Tentacles which 

 have been thus acted on b}' contact of par- 

 ticles have a mottled appearance, and can 

 be picked out with ease from all the othei's. 

 The change of contents is termed by Darwin 

 aggregation. " The little masses of aggre- 

 gated matter are of the most diversified 

 shapes, often spherical or oval, sometimes much elongated, or 

 quite irregular with thread or necklace-like or club-formed pro- 

 jections. They consist of thick, apparently viscid matter, w^hich 

 in the exterior tentacles is of a purplish, and in the short discal 

 tentacles of a greenish, color. These little masses incessantly 

 change their forms and positions, being never at rest. . . . 



" Shortly after the purple fluid within the cells has become 

 aggregated, the little masses float about in a colorless or almost 

 colorless fluid ; and the layer of white granular protoplasm which 

 flows along the walls can now be seen much more distinctly. 

 The stream flows at an irregular late, up one wall and down the 

 opposite one, genei-ally at a slower rate across the narrow ends 

 of the elongated cells, and so round and round. But the current 

 sometimes ceases. The movement is often in waves, and their 

 crests sometimes stretch almost across the whole width of the 

 cell and then sink down again. Small spheres of protoplasm, 

 apparently quite free, are often driven by the current round the 

 cells ; and filaments attached to the central masses are swayed 

 to and fro, as if struggling to escape. Altogether, one of these 

 cells, with the ever-changing central masses and with the layer 

 of protoplasm flowing round the walls, presents a wonderful 

 scene of vital activitj'." ^ 



' Darwin ; Insectivorous Plants, 1875, p. 39. 

 ' Darwin : Insectivorous Plants, pp. 40, 42. 



Fig. 154. Drosera rotundifolia. Leaf with the tentacles on ono side inflected over a 

 bit uf meat placed on the disc. (Darwin.) 



