MOTION 



37 



menon may be studied in the so-called salivary corpuscles (Fig. 6, II) which are 

 dead leukocytes that have entered the saliva and have absorbed much water. The 

 delicate molecular movement is displayed in this case by the fine granules situated 

 in the immediate vicinity of the nucleus. Brown, who discovered this phenome- 

 non in the cells of plants (1827), attributed it to the vibration of the molecules 

 themselves and regarded it therefore as 

 active. Wiener and Exner, however, 

 have proved that it is passive, and that it 

 represents an instability similar to that 

 exhibited by the molecules of any liquid. 

 The latter are never at a standstill, but 

 always change their position and con- 

 stantly move toward and away from one 

 another. 



Movements by swelling of the cell-walls 

 are produced whenever the constituents 

 of a dry, expansible body are brought into 

 a moist medium so that they can attract 

 molecules of water. The latter are stored 

 in between them and force them apart 

 until the body as a whole increases 

 markedly in volume. As an example of 

 this type of motion might be mentioned 

 the so-called resurrection-plants found in 

 desert regions. These plants may remain 



in a perfectly dried up condition for several years, their leaves being folded to- 

 gether like the fingers of a closed hand. When brought into a moist environ- 

 ment, they immediately unfold and assume definite shape. 



Movements by changes in the cell-turgor are observed chiefly in plants. In- 

 side the walls of the different cells is found a delicate protoplasmic sac, formed by 

 the so-called primordial utricle. The latter is filled with a liquid, the cell-sap, the 



U 



FIG. 6. BROWNIAN MOTION. 

 7. Closterium; with vacuole. II. Sali- 

 vary corpuscle. (Verworn.) 



FIG. 7. SENSITIVE PLANT (Mimosa pudica). (Verworn after Detmer.) 

 A. Resting position. B. Stimulated. 



concentration of which is varied by the addition of certain chemical substances 

 which are formed in the course of the vital activities of these cells. As a result 

 of the osmotic influx of water, the pressure in the primordial sac is increased. If 

 the concentration of the medium is increased, water is abstracted from the cell. 

 Variations in the pressure of the cell-sap may also be brought about by the con- 

 traction of the primordial utricle. Of greatest importance at this time, is the 



