THE VITAL PROPERTIES OF THE CELL 137 



As to whether a given substance will be absorbed or not can, ac- 

 cording to Pfeffer, who has carefully studied the subject, only be 

 decided empirically. 



The substances excreted by cells also vary. Just as with 

 absorption, excretion depends upon the special individual properties 

 of the living cell body. The red or blue-coloured petals of 

 phanerogamic flowers do not allow the concentrated solution of 

 colouring matter which they contain to become diffused into the 

 surrounding water as long as they are alive. However, as soon as 

 the cells die, the colouring matter commences to pass through 

 the cell-wall. 



In order to really understand all these complicated phenomena, 

 it would be necessary to possess an exhaustive knowledge of the 

 chemistry and physics of the cell. For the property, which I have 

 designated above as the power of selection, must in the last instance 

 be traced back to the chemical affinities of the very numerous 

 substances which, being formed during the process of metabolism, 

 are present for a time in the cell. The same thing, doubtless, 

 occurs here as with the absorption of oxygen and carbon dioxide, 

 which can only take place when, through metabolic processes, sub- 

 stances with chemical affinities for them are set free. It is on 

 this account that no carbon dioxide is taken up by plants in the 

 dark, although it is immediately absorbed, if, under the influence 

 of direct sunlight, the chemical process for which it is necessary is 

 started. 



The same thing occurs when living cells absorb aniline dyes. 

 Azolla, Spirogyra, the root-hairs of Lemna, etc., gradually draw 

 up into themselves so much colouring matter out of a very Aveak 

 solution of methylene blue, that they acquire a deep blue coloura- 

 tion, such as is seen in a 1 per cent, solution. The methylene blue 

 does not stain the protoplasm itself, but simply passes through it, 

 thus forming in the cell sap a solution of ever-increasing strength. 

 Hence the death of the cell, which would inevitably occur if the 

 poisonous methylene blue were to be collected in such quantities 

 in the protoplasm itself, does not ensue. This storing up in the 

 cell sap is caused by the presence in it of substances which, with 

 the aniline dye, form compounds, which osmose with difficulty. 

 Pfeffer considers that the tannin which is so frequently found in 

 })lant cells is a substance of this nature. This tannin, with the 

 aniline colour, forms compounds which are sometimes insoluble, 

 and hence are precipitated in the cell sap (methylene blue, methyl 



