ABSORPTION OF MATERIALS IN GENERAL III 



pressure, to replace the arbitrary one of an atmosphere. A ionie is the pressure 

 exerted upon a surface of i sq. cm. by i dyne (the well-known unit representing 

 the force necessary to give a velocity-acceleration of i cm. per second to a mass 

 of I g.). The terms dekatonie, hectotonie, kilotonie and myriotonie (10,000 

 tonies) are employed for greater pressures. A myriotonie (M) is about one- 

 one-hundredth of an atmosphere." 



§5. Absorption of Dissolved Substances. — Only a few direct experiments 

 upon the entrance of dissolved substances into the cell are available. Some con- 

 clusions concerning the mechanism of absorption may be drawn from plasmolytic 

 experiments with salt solutions. Every substance entering the cell must pass 

 through two membranes, the cell wall and the protoplasmic membrane. Most 

 dissolved substances easily penetrate the cell wall, but the protoplasm is imper- 

 meable, or nearly so, to many of these. 



The osmotic properties of the protoplasmic membrane are similar to those 

 of Pfeffer's precipitation membranes. Only the 

 living protoplasnTTs here meant, however; dead pro- 

 toplasmic membranes have entirely different proper- 

 ties. Thus pigments are persistently retained within 

 the cell sap by the living protoplast, but these and 

 other dissolved substances diffuse out very rapidly 

 after the cell is dead. Like precipitation membranes, Fig. 69. — Cell of zygnema 

 the protoplasmic membrane is not completely im- '^*'^ crystals formed by 



11 , T^ , -w^r «• , methylene blue. 



permeable to most substances. For example Pfeffer' 



succeeded in introducing useless and even injurious, substances (such as apiline 

 dyes) into the living cell. He found that the following pigments penetrated: 

 methylene blue, methyl violet, bismarck brown, fuchsin, cyanin, safranin, 

 methyl green, methyl orange, tropaeolin 00 and rosolic acid. The concentra- 

 tions of the solutions employed were very low (from o.ooi to o.ooooi per cent.) . 

 Some of the dyes, (e.g. methylene blue) first enter the cell sap and color it, but 

 form crystals after a time; Fig. 69 shows an alga cell (Zygnema) with crystals 

 formed by methylene blue. Other dyes (e.g., methyl violet) stain the proto- 

 plasm itself. In neither case is the cell fatally injured. 



Overton^ studied a number of different dyes and found that the permeability 

 of the protoplasm to these varied according to their chemical constitution. 

 Basic aniline dyes readily enter the cell but most of their sulphuric acid deriva- 

 tives penetrate either not at all or very slowly. Dyes that have accuinulated 

 in the cell diffuse out when the cells are placed in water, this outward passage 

 being accelerated by the addition of o.oi per cent, of citric acid to the water.^ 



^ PfefEer, W., Ueber Aufnahme von Anilinfarben in lebenden Zellen. Untersuch. Bot. Inst. Tubingen 

 2: I7P-33I. 1886-1888. 



2 Overton, E., Studien iiber die Aufnahme der Anilinfarbe durch die lebende Zelle. Jahrb, wiss. Bot. 

 34: 669-701. 1900. 



» Pfeffer, 1886-88. [See note i, this page.] 



"* This unit has never come into general use and it is now highly improbable that it ever will. 

 Pressures are generally stated in terms of millimeters or centimeters of a mercury column or in 

 atmospheres, an atmosphere being 760 cm. of mercury. It seems undesirable to state osmotic 

 pressure in any other terms than those already used for other kinds of pressure. — Ed. 



