300 PHYSIOLOGY 



Imbibition. — When a plant is placed in dry air, water evaporates 

 from it and its \"arious parts shrink and shrivel. A little shrinkage occurs 

 when plants wilt on a hot, dry day. When water again enters in suffi- 

 cient quantity, they swell and regain their fresh look. The water may 

 even be driven out entirely from some plants, as certain mosses, and when 

 again wetted, the parts swell and regain partly or wholly their original 

 dimensions. The most obvious of these changes are due to the collapse 

 or expansion of the cells; but that they are not limited to alterations 

 in the dimensions of the cells may be shown by measuring a dry bit 

 of cell wall or a dry starch grain under the microscope, and after 

 wetting, remeasuring it. On examination it appears that almost every 

 substance in the plant body is capable of imbibing water, and of swelling 

 or shrinking as the proportion of imbibed water increases or diminishes. 

 The smaller the quantity of water the more difficult, and the larger the 

 amount the more easy it is to remove it. From the fully swollen gelati- 

 nous body of a sea weed, Laminaria, some water maybe extracted by the 

 pressure of the fingers, while the greatest pressure does not suffice to 

 squeeze it all out, and even by heating it is most difficult to remove the 

 last traces of water. 



Theoretical structures of organized bodies. — A study of the phe- 

 nomena of swelling by imbibition, and of the way in which cell walls and 

 starch grains affect polarized light, permits some inferences either as to 

 the form and position of the particles, or as to the existence of strain or 

 tension bet\\een them, by which they are slightly deformed or displaced. 

 These inferences lead to theories of the invisible structure of the cell 

 parts. The particles of which wall and protoplast are composed, it 

 seems probable, are surrounded by water. Whether these particles are 

 the chemist's molecules, linked together in a tense network, or aggre- 

 gates of molecules (micellae) having a crystalline form, which are 

 features of the two prominent theories, is of only remote significance. 

 In either case the water between them may increase or diminish in 

 amount; correspondingly, the particles approach or recede from one 

 another. When any water is present, it forms a connected whole, how- 

 ever irregular its distribution may be. The particles of the swollen stulT 

 also cohere, and remain so related to one another that when the water 

 is all removed, they regain the form they had before it entered. 



Swelling and solution. — In the recovery of the original form is a 

 practical but only a jjartial difference between the behavior of merely 

 swollen and of dissolved substances. In both cases water wanders in 



