PHYSIOLOGY 177 



last century that the nature of the nourishment and nutritive processes 

 of plants was recognised. 



Plant nourishment is dependent upon the permeability of the cell 

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

 walls of living cells are permeated with " imbibed " water ; and to this 

 " imbibition water " in the cell walls, together with the physical char- 

 acter of the cell walls themselves, are due their flexibility, elasticity, and 

 ductility. 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 ; other- 

 wise, the water imbibed by ordinary cell walls is limited in amount. The walls 

 of woody cells take 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. 



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 to plant nutrition ; cell walls, 

 on the other hand, which are scarcely or not at all permeable 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, i.e. the lining of the cell wall. 

 Living protoplasm is not, however, like the cell walls, equally per- 

 meable to all substances in solution, but, on the contrary, completely 



EXCLUDES CERTAIN SUBSTANCES, WHILE ALLOWING OTHERS TO PASS 



THROUGH MORE OR LESS readily. Moreover, 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 entrance into the cell. Similarly 

 the inner protoplasmic membrane exercises a similar but often 

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

 plasm into the cell sap. The same determinating power is exercised 

 by these membranes in the transfer of substances in a reverse direction. 

 On account of the selection thus exercised by the protoplasm, it is 

 possible that, in spite of continued osmotic pressure, the contents of a 

 cell are often of quite a different chemical nature from the immediately 

 surrounding medium. To this same peculiar quality of the proto- 

 plasmic 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 



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