SPECIAL METHODS. 241 



cell-sap, continued the motions due to their weight after the 

 same treatment (cf. Zimmermann II, 68). 



436. Wakker (I) has also used abnormal plasmolysis with 

 a lofo solution of saltpeter, containing eosin (cf. 432 and 

 433), for the same purpose. It is often to be determined 

 with certainty by direct observation whether the bodies in 

 question lie in the vacuples isolated by this method. Be- 

 sides, a further confirmation of the conclusions reached by 

 direct observation may be obtained from the movements 

 taking place in such preparations when the slide is placed 

 vertically. 



19. Aggregation. 



437. Complicated changes of arrangement take place 

 within the cells of the glandular hairs of Drosera rotundi- 

 folia in consequence of chemical stimulation, which consist 

 essentially in the origin of rapid circulation-currents in the 

 protoplasm and the breaking up of the large central vacuole 

 into a large number of small vacuoles which gradually con- 

 tract more and more. This process, discovered by Darwin 

 and studied in detail especially by H. de Vries (II), will be 

 termed exclusively, in the following account, aggregation, 

 in agreement with de Vries, though Darwin and various 

 other authors use the same term also for the artificial pre- 

 cipitation which accompanies the process, but also occurs in 

 very many plants. 



438. The tentacles of Drosera are especially adapted to 

 the observation of aggregation, since their vacuoles stand 

 out very sharply on account of their red cell-sap. Aggre- 

 gation may be produced in them by bringing a leaf of the 

 plant in contact with an insect, a bit of cooked albumen, or 

 the like. It may also be observed in isolated tentacles 

 which have been placed in a \<f> solution of ammonium car- 

 bonate. Aggregation then begins at the bases and at the 

 tips of the tentacles, and may be especially well followed in 

 its separate stages at their middles (cf. Fig. 61). 



439. According to recent investigations of Bokorny (IV), 



