350 SCIENCE PROGRESS 



a dull reflection image, while the pyrenoids appear as bright 

 spots. The cell wall itself is optically homogeneous. 



The oscillating movement of the particles both in the proto- 

 plasm and cell sap, already referred to, is undoubtedly of the 

 nature of a Brownian movement. Since the great impetus given 

 to the study of colloids by Siedentopf and Zsigmondy's work, 

 this phenomenon has been brought into fresh prominence. 

 Discovered by a botanist, Dr. Brown, in 1827 (after whom it is 

 called), it was shown to extend to particles of extremely minute 

 and ultimately of ultramicroscopic size, though here the move- 

 ment is very much more rapid. It has been shown that the 

 rapidity of motion varies inversely with the size of the particles, 

 and, as a result chiefly of Perrin's beautiful researches, it has 

 been shown almost without doubt that the movement is a direct 

 expression of the actual molecular movement in the surrounding 

 fluid. Zsigmondy observed that the minute particles present in 

 liquid colloid solutions— of the nature of " sols " — showed such a 

 Brownian movement in a very striking manner. 



Spirogyra is quite good for the stud3' of this Brownian move- 

 ment, for the larger sap particles can be seen to oscillate much 

 more slowly than the minute particles of the protoplasm, 

 illustrating the variation of the rate of movement with variation 

 in size of the particles. 



Very similar appearances are given by other cells examined. 

 Mongeotia, for example, shows a similar structure, with well- 

 marked Brownian movement, but on account of the character of 

 the chloroplast is not quite so suitable for observation. 



The staminal hairs of Tradescantia , used by Gaidukov, have 

 a cell wall which is optically heterogeneous, and this interferes 

 with the clear observation of the cell contents. Cells in four 

 different states of vitality were examined. 



1. Young cells without sap vacuoles. — Particles with strong 

 Brownian movement were present in the protoplasm. 



2. Older cells with vacuoles and streaming protoplasm. — In 

 spite of which the Brownian movement was clearly seen. 



3. Dying cells. — A moderately active movement could be seen. 



4. Dead cells. — The protoplast had coagulated and the con- 

 stituent particles were motionless. 



In the young cells the Brownian movement is more difficult 

 to see, on account of the closer aggregation of the particles in 

 the complex. 



