44 LECTURE III. 



through the cell-wall. This is an instance, then, of the re- 

 sistance which the primordial utricle offers to the passage of 

 substances through it. More striking instances are however 

 afforded by cells which contain colouring matters in solution 

 in their cell-sap. Naegeli pointed out that certain colouring 

 matters (anthocyanin, erythrophyll) do not pass through the 

 primordial utricle of living cells. De Vries repeated Naegeli's 

 observations upon the parenchymatous cells of the Beet-root, 

 and found that after they had been left for as many as four- 

 teen days in water, neither the colouring-matter nor the 

 sugar which their cell-sap holds in solution had escaped from 

 them. He further shewed that a solution of sugar will readily 

 pass through the cell-wall, for when the cells of the Beet-root 

 are placed in syrup they become plasmolytic., When the cells 

 are killed, for instance by heating them above 60 C, or by 

 immersing them in alcohol, the cell-sap readily escapes from 

 them. This is in accordance with the well-known fact that it 

 is impossible to stain living protoplasm: it is when protoplasm 

 is dead that colouring-matters can penetrate into it. 



When we compare the osmotic properties of the pri- 

 mordial utricle with those of the cell-wall, the explanation of 

 the difference between them which at once suggests itself is 

 that the intermolecular interstices of the former (at least of 

 the outer firmer layer which we have termed the ectoplasm) 

 are smaller than those of the latter, so that substances which 

 can readily traverse the one cannot traverse the other, but 

 this explanation cannot be regarded as complete. It still 

 remains difficult to understand, as we shall see more fully in 

 the next lecture, how it is that a sufficient quantity of nutritive 

 salts can be taken up by the plant, and how such substances 

 as sugar can pass from cell to cell in the plant. Possibly, as 

 Pfeffer suggests, the osmotic properties of the primordial 

 utricle may vary from time to time, and possibly the saccha- 

 rine substances which are certainly transferred from one part 

 of the plant to another, travel in other chemical forms. But 

 we must bear in mind that we have here a living and not a 

 dead membrane to deal with, and consider that the laws which 

 regulate osmosis through the latter may be, and probably are, 



