194 BOTANY I-AUT i 



the cell walls to liquids and gases. Although impervious to solids, the 

 cell walls of living cglls are permeated with " imbibed " water ; and to 

 this " IMBIBITION WATER " in the cell walls, together with the physical 

 character of the cell walls themselves, are due their flexibility, elasticity, 

 and extensibility. The permeability of cell walls for imbibition water 

 is only possible within certain limits, so that they thus retain the 

 character of solid bodies. 



Treated with certain chemical reagents (potassium hydrate, sulphuric acid, etc.) 

 cell walls become swollen and gelatinous, or even dissolve into a thin mucilaginous 

 .slime. This change in their character is due to an increase in the amount of their 

 imbibition water, induced by the action of the chemicals; otherwise, tin- wati-r 

 imbibed by ordinary cell walls is limited in amount. The walls of woody cells taku 

 up by imbibition about one-third of their weight ; the cell walls of some seeds and 

 fruits and of many Algae absorb many times their own volume when dry. 



THE CELL WALLS ARE NOT ONLY PERMEABLE TO PURE WATER, 

 BUT ALSO TO SUBSTANCES IN SOLUTION. This fact, that the cell wall 



offers no resistance to the diffusion of crystalloid bodies when in 

 solution, is of the utmost importance for the nutrition of the plant : 

 cell walls, on the other hand, which are scarcely or not at all per- 

 meable to liquids (cuticularised walls), take no part in the absorption 

 of plant-nourishment, except in so far as they may still be permeable 

 to gases. 



In order that liquids may enter by osmosis into the living cell, 

 they must first pass through the protoplasm, and in the first place 

 through the outer limiting membrane of the protoplast which is in 

 contact with the cell wall. LIVING PROTOPLASM is not, however, like 

 the cell walls, equally permeable to all substances in solution, but, 

 on the contrary, COMPLETELY EXCLUDES CERTAIN SUBSTANCES WHILE 



ALLOWING OTHERS TO PASS THROUGH MORE OR LESS READILY. Moir 



over, it is able to change its permeability according to circumstances, 

 and thus THE OUTER PROTOPLASMIC MEMBRANE HAS THE POWER OF 

 DECISION whether a substance may or may not effect an entr.up 

 into the cell. The wall of the vacuole exercises a similar but often 

 quite distinct power over the passage of substances from the proto- 

 plasm into the cell sap. The same influence is exercised by these 

 membranes on the transfer of substances in a reverse direction. 

 On account of the selection thus exercised by the protoplasm, 

 the contents of a cell, in spite of continued osmotic pressure, 

 are often of quite a different chemical nature from the immedi- 

 ately surrounding medium. To this same peculiar quality of the 

 protoplasmic membranes is also due the SELECTIVE POWER of cells, 

 manifested by the fact that different cells, or the roots of different 

 plants, appropriate from the same soil entirely different compounds ; 

 so that, for instance, one plant will take up chiefly silica, another 

 lime, a third common salt. The action of Seaweeds in this respect 



