Botany. 159 



It appeared therefore of importance to ascertain accurately the 

 quantitative relations of glycerin to tissues, and this has been 

 attempted by deVries in the paper referred to. His method is 

 essentially the same as that employed by him in his previous 

 series of plasmolytic experiments, and the material is the same, 

 namely, — the red epidermis of the petiole of Begonia manicata. 

 From the epidermis of the upper side of the petiole about twelve 

 good microscopic slices are taken, and subjected at once to the 

 action of the plasmolytics, such as potassium nitrate or sugar, of 

 different degrees of strength. After two hours it is possible to 

 detect in the sections placed in the stronger solutions the begin- 

 ning of the plasmolysis, and afterwards one may trace its further 

 progress in the weaker ones. From these observations deVries 

 determines the isotonic coefficient for glycerin to be 1*78. 



When this figure is compared with the coefficients of some 

 other substances, and also with the figures obtained by Raoult 

 for the lowering of the freezing-point, the curious relations are 

 confirmed. 



Glycerin, _ - 1'78 _ 17-1 



Cane-sugar, __ 1*88 18-5 



Inverted sugar, 1-88 19-3 



Malic acid, _ 1 '98 18-7 



Citric acid, 2*02 19-3 



Tartaric acid, 2-02 _19"5 



deVries has also measured the permeability of the protoplasts by 

 glycerin, but he gives only a few results as an example of the 

 working of his general method. 



(2) Assimilation by colored and variegated leaves. — E^gelma^tn" 

 to whom we are indebted for a method of quantitatively estimating 

 the effects of different rays of the solar spectrum in assimilation, 

 has extended his observations to variegated leaves, and to those 

 which have a uniform color other than green. (Bot. Zeit., xlv. 

 no. 25.) Passing over his studies as to the structure of the paren- 

 chyma of the leaves, and merely stating that he distinguishes 

 between colors which depend (l) on the cell-wall, (2) on the cell- 

 sap, and (3) on the plastids themselves, it may be said that he 

 presents results confirming to a certain extent his former views 

 as to the relation of the absorption-bands and assimilative activity. 

 Without dwelling upon the possible errors of his method, it is 

 enough to call attention to a marked advantage which it has over 

 any other in the determination of the amount of activity in a sin- 

 gle cell or group of cells. The method, it will be remembered, 

 consists essentially in the application of a small direct-vision spec- 

 troscope under the stage, throwing the light through the dia- 

 phragm upon the under side of an object placed in the field. By 

 the activity of bacteria in the water surrounding the object 

 (assuming that this activity is due to the elimination of oxygen 

 during the process of assimilation), Engelmann endeavors to ascer- 

 tain the relative amount of assimilation in different parts of the 

 spectrum. When, as in the case of large leaves or fragments of 



