DIV. ii PHYSIOLOGY 223 



drying and remain productive for a long time. In this case also all vital manifesta- 

 tions cease in the dry condition. 



Many seeds lose their power of germination after having been kept dry for only 

 one or a few years ; others even after a few days ; and others again cannot endure 

 drying at all. It must not be forgotten that in all these instances a certain amount 

 (about 9-14 per cent) of hygroscopic water is retained by plants even when the 

 air is quite dry. Over the sulphuric acid of the desiccator seeds retain for weeks 

 6 per cent or more of their weight of water. Even drying at 110, or the action 

 of absolute alcohol, can be borne by some spores and seeds. 



Absorption of Water 



Absorption of Water by the Cell. All parts of a plant and all 

 the parts of its individual cells are saturated with water. The cell 

 membrane has the water so freely divided between its minute particles 

 that the water and the solid substance are not distinguishable under 

 the highest magnification. If the water is allowed to evaporate, air- 

 filled cavities do not appear in its place, but a contraction of the 

 cell wall takes place. On the other hand, the absorption of water by 

 dry or not fully saturated cell walls causes a swelling of the latter. 

 The increase in volume which a body undergoes as the result of the 

 introduction of fluid is termed IMBIBITION ( 8 ) ; the amount taken up 

 is limited for a particular temperature. There are substances which 

 swell in alcohol or xylol ; the vegetable cell wall, however, swells in 

 water. The walls of lignified cells absorb about one-third of their 

 weight of water, while those of many Algae and some seed-coats 

 and pericarps absorb several times their weight. This takes place 

 with considerable energy, and can therefore overcome considerable 

 resistance. 



The air-dry protoplasm of many seeds and spores imbibes water 

 and swells just as does the cell wall. Like gum arabic, however, it 

 loses the characters of a solid body and passes into a colloidal solution. 

 This is the condition of the protoplasm, as a rule, in the actively 

 living cell, though certain portions may have a firmer consistence. 

 Colloidal solutions have, indeed, always the tendency to pass from 

 the fluid (sol) condition to the gel condition. 



The cell sap is always a molecular solution of crystalloids in 

 water, but may also contain colloids. 



Only a cell which is not completely saturated for water can 

 withdraw water from its surroundings. It is thus necessary to be 

 clear as to what is meant by a cell being saturated for water. For 

 the cell wall the answer is simple ; the wall is saturated when the 

 maximum of swelling has been reached. It is much more difficult to 

 determine the limits of water capacity for the protoplasm and cell sap. 

 Taking the latter first, it may be assumed for the sake of simplicity 

 that it is a solution of crystalloids, and that it is enclosed by the 

 cell wall only without an intervening layer of protoplasm. If a tube 



