264 THE PROPERTIES OF PROTOPLASM 



The following are results which were obtained by Over ton, employ- 

 ins; the cells of spirogyra filaments: 



Isotonic concentration: 



Dissolved Molecular found, calculated, 



substance. weight. per cent. per cent. 



Cane-sugar . . 342 6.0 



Mannitol 182 3.5 3.19 



Glucose 180 3.3 3.15 



Arabinose 150 2.7 2.63 



Erythritol 122 2.14 



Asparagin 132 2.5 



GlycocoU 75 1.3 1.32 



The tf calculated" values were computed as follows: The isotonic 

 concentration of cane-sugar being 6 per cent, and its molecular weight 

 342, the concentration of an isotonic solution is evidently /$ = ^ 

 molecular. A ^y molecular solution of glycocoll would contain |^- 

 grams of glycocoll per liter, or 1.32 grams per hundred c.c. It will 

 be seen that.the experimental and the calculated values are exceedingly 

 close to one another and we may infer that, at all events so far as 

 limited periods of time are concerned, the protoplasm of spirogyra is 

 impermeable to the substances mentioned, although freely permeable 

 to water. 



This method of estimating the isotonicity of solutions, however, is 

 subject to several sources of error and uncertainty. In the first place 

 we must take into consideration the fact that the protoplasmic limit- 

 ing membrane must necessarily alter in form before we can perceive 

 any solution to be Hypertonic, or in excess of the isotonic concentra- 

 tion. Now the external limiting membranes of cells must undoubtedly 

 possess some degree of Elasticity, in consequence of which they must 

 themselves exert some pressure upon the cell-contents. The forces 

 leading to shrinkage of the protoplasm are not solely osmotic there- 

 fore, but to some slight extent elastic also, and we cannot positively 

 estimate the proportion of the total force which this elasticity com- 

 municates, since it will not improbably add a constant amount to 

 each osmotic pressure investigated. Isotonic solutions are therefore 

 isosmotic with one another, but not necessarily isosmotic with the 

 cell-contents. In red blood-corpuscles this is probably the origin of 

 the constant slight difference between the osmotic concentration of the 

 contents of the corpuscles and the surrounding medium or plasma, 

 amounting, according to Moore and Roaf, to a difference of freezing- 

 point depression of 0.02 to 0.03 C., or an osmotic-pressure difference 

 of from 0.24 to 0.36 of an atmosphere. -t 



In the second place, the Semipermeability of living cell-membranes 

 is, of necessity, never absolute. This becomes obvious when we 

 consider that the nutrition, and therefore, the maintenance and 

 growth of cells depends upon their intake of substances dissolved 

 in water. Unless a cell can be penetrated by the mineral or organic 

 substances which constitute the components out of which protoplasm 

 is built up, the progressive consumption of material and dissipation 



