ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 85 



has found that many protoplasts, when brought into contact with such 

 solutions, lose their vitality slowly, and that it is often very difficult 

 to decide at once whether they are still living or have undergone 

 more or less change ; the change from life to death may proceed very 

 slowly, extending over hours, or even days. The processes described 

 below can be followed with remarkable ease in the violet epidermal 

 cells of the under side of the leaf of Tradescantia discolor. 



If preparations of this object are left for some days in a neutral 

 weak plasmolytic solution of sodium chloride, or for a few hours in a 

 similar solution with addition of any poison, the following changes 

 are ordinarily observed. When the magnifying power is low, the 

 entire preparation appears to be still in a living and healthy con- 

 dition, the cell-sap retaining its colour in its original intensity. But 

 with stronger powers it is seen that the coloured cell-sap is sur- 

 rounded only by a thin layer of living protoplasm, while the outer 

 protoplasm, as well as the parietal layer and nucleus, are dead, and 

 adhere, as it were, in tatters to the inner layer, which is hyaline and 

 strongly refractive, and does not allow the colouring matter to pass 

 through it, even for some days. The outer protoplasm and nucleus 

 appear coagulated, opaque, and usually darkly coloured. This inner 

 layer of protoplasm acts like a living protoplast, contracting and again 

 expanding under different conditions ; it is obviously the wall of the 

 vacuole, which must be regarded as a special organ of the protoplasm. 

 In the cells of Spirogyra it may often be completely isolated after the 

 parietal layer, nucleus, and chlorophyll-bands are dead ; in a weak 

 plasmolytic solution it appears a tense hyaline, more or less globular 

 vesicle with smooth surface in one half of the cell, while the rest of 

 the protoplasm has collected into an amorphous lump in another 

 corner. The greater resistance of this vacuole-layer against injurious 

 agencies depends on its greater density and less permeability for 

 soluble substances, as compared with the rest of the protoplasm, and 

 especially with the parietal layer. 



The ordinary test for distinguishing between living and dead 

 protoplasm is the impermeability of the former to all coloured solu- 

 tions ; but De Yries finds that the protoplasm, in passing from one 

 condition to the other, goes through various stages, in which it is 

 permeable to some, but impermeable to other substances. Thus, for 

 example, acids, and easily diffusible salts, such as sodium chloride 

 and potassium nitrate, pass into the protoplasm more quickly and 

 easily than more sluggish salts like magnesium sulphate or than 

 sugar ; while pigments, as a rule, would cume in only in the last 

 stages. The author details a variety of observations on protoplasm 

 in the intermediate stage, when it is permeable for acids, but not for 

 sugar or pigments. 



De Vries gives the following rules, which should be observed in 

 plasmolytic experiments : — 



1. The degree of plasmolysis can only be determined during the 

 healthy, completely normal condition of the protoplasm. 



2. The preparations should remain in the solutions no longer 

 than is absolutely necessary for the experiments. 



