222 GENERAL PHYSIOLOGY 



ticles suspended in it. As is well known, the molecules in a liquid 

 are conceived to be in constant motion, crowding together, bound- 

 ing against one another, pushing away, moving off and again col- 

 liding. This motion of the molecules cannot be seen even with 

 the strongest magnifying powers, for liquids appear homogeneous 

 because their molecules are too small to be perceived even micro- 

 scopically. But the result of the motion can be observed in small, light 

 granules suspended in the liquid; if the molecules possess the 

 given kind of motion, they must strike the particles continually, 

 so that with their delicate mobility the latter are put into a 

 trembling, dancing motion. Hence, the so-called Brownian 

 molecular movement of small granules is a purely passive move- 

 ment caused by the constant slight impulses given to the granules 

 by the dancing molecules of the liquid. An excellent proof of the 

 correctness of this view is afforded by the fact that the Brownian 

 movement gains in intensity with increasing temperature of the 

 liquid. This might have been predicted from the fact that the 

 motion of the molecules of the liquid is greater the higher the 

 temperature ; it finally becomes so great that the individual 

 molecules are driven violently apart, that is, the liquid evaporates. 



&. Movements ly Swelling of the Cell-walls 



Movements that are caused by swelling of the cell-walls 

 constitute a variety intermediate between passive movements and 

 all those mentioned below, which latter depend upon the activity 

 of living substance. The phenomenon of swelling, as is well 

 known, is due to the fact that between the molecules of a dry, 

 expansible body, brought into a moist environment, molecules of 

 water become stored, being attracted so strongly by the molecules 

 of the body that they force the latter powerfully apart ; during 

 the process the volume of the body becomes markedly increased. 

 If the swollen body comes again into an environment free from 

 water, e.g., dry warm air, it gradually gives off its water, diminishes 

 its volume proportionately and shrinks; upon being again moistened, 

 it swells again. The organic products of the metabolism of plants, 

 especially the cellulose walls of plant-cells, are peculiarly prone to 

 swell. This is not associated in any way with the life of the 

 plant-cell, but goes on for an indefinite time in the cellulose of 

 dead cells, in the same manner as in that of living cells. In 

 order that a movement in one direction may be brought about by 

 the increase in volume caused by the swelling or by the decrease 

 in volume caused by the drying of an expansible object, such as 

 the stem of a leaf or a membrane, the different sides of the object 

 must be capable of swelling differently, one side strongly, the 

 other feebly or not at all. Were all parts equally capable, there 

 would result a uniform enlargement toward all sides. If, however, 



