THE VITAL PHENOMENA OF CELLS 33 



From these discoveries on the phenomena of osmosis it follows that every 

 change in the constitution of the medium surrounding the cell or of the fluid 

 contained within it will have power to effect a change in the osmotic pressure 

 prevailing in the cell. This would mean also a change in the quantity of fluid 

 (degree of turgescence) as well as in the chemical constitution of the fluid 

 contained in the cell, or indeed in the constitution of the protoplasm itself, 

 according as the limiting layer of the protoplasm is permeable or not to the 

 substances present. 



Plant cells afford us the simplest examples of the influence of osmosis. 

 The cell membrane is permeable to gas, water and solutions. In the living 

 cell the membrane is impregnated with -water; hence all substances which are 

 dissolved in the water can penetrate the membrane and thus come into contact 

 with the outer layer of the protoplasm (primordial sheath) just inside the 

 membrane (cf. Fig. 16). 



Within the plant cell and surrounded by the primordial sheath we find 

 the cell sap, which is a watery solution of various salts, carbohydrates, etc. 

 The primordial sheath is permeable to water, but prevents entirely the en- 

 trance of certain compounds, behaves toward them in other words exactly like 

 a semipermeable membrane. 



If therefore the cell is bathed by a solution of a compound whose osmotic 

 pressure is greater than that of the cell sap, water passes out of the cell, the 

 primordial sheath is loosened and shrinks away from the cell membrane (plas- 

 molysis}. But if the cell is bathed by pure water, the water can pass through 

 the primordial sheath in the reverse direction and raise the internal pressure 

 above its usual level, whence we have the condition of cell turgor. 



The primordial sheath permits the entrance of certain other substances 

 more or less easily, and in the commerce between the cells, where transporta- 

 tion and exchange occur freely, it has the power to * change its permeability 

 according to circumstances. From this it follows that neither absorption nor 

 excretion on the part of plant cells is to be described as a simple osmotic 

 process. 



In the cells of the animal body, both the cell body and the nucleus permit 

 certain substances dissolved in water to pass through, but exclude others ; they 

 behave toward these substances like semipermeable membranes (Hamburger, 

 Hedin). To one and the same substance the permeability of different kinds 

 of cells may be considerably different, and it appears to vary with the same 

 kind of cells under different physiological conditions (Hamburger). 



From the investigations of Hamburger, Hedin, Koeppe and Gryns on the 

 permeability of red blood corpuscles to different substances, one may gather 

 that the K, Na, Ca, Sr, Ba, Mg ions, the different kinds of sugar, arabite and 

 mannite do not penetrate them at all ; and that they are but slightly permeable to 

 amino acids (glycocoll, asparagin, etc.). In fact they appear to offer a power- 

 ful resistance to the amido group in the amino acids, but toward the amido 

 group in the acid amides (acetamide, etc.) the resistance is not so great. The 

 red blood corpuscles are permeable to NH 4 ions, to free acids and alkalies, 

 and to alcohols in inverse proportion to the number of hydroxyl ions in the mole- 

 cule; also to aldehydes, ketones, ethers, esters, antipyrin, amide, urea, urethan, 

 bile acids and bile salts. The leucocytes of the blood and of the lymph glands, 



