102 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



ether and chloroform, whilst others restrict it to those 

 which are non-saponifiable. As all substances which lower 

 the surface tension of a liquid tend to become more con- 

 centrated at the surface than throughout its bulk, it is 

 only to be expected that such fatty bodies would accumu- 

 late on the outer surface of protoplasm, and also on the 

 vacuole wall. A summary of these researches is given by 

 Czapek in " Chemical Phenomena in Life." 



It must be added that Ruhland (1909) has shown that 

 many dyes fail to penetrate the plasmatic surface though 

 freely soluble in lipoids, whilst others insoluble in lipoids 

 and forming colloidal solutions in water enter readily. 

 Methyl orange is an example of the latter class. 



SURFACE TENSION or PROTOPLASM. 



Czapek (1910) measured the surface tension of plant 

 cells by noting the diffusion from them of tannin, antho- 

 cyanin, etc., when in contact with various solutions; for 

 he found that media which brought about such changes 

 always had equal surface tensions, amounting to about 

 two-thirds that of water viz., that of 11 per cent, aqueous 

 ethyl alcohol. It was also shown by Czapek that emul- 

 sions of fatty bodies can cause injurious effects if their 

 surface tensions are sufficiently low, as in the case of leci- 

 thin or cholesterin. Czapek's view of the constitution of 

 the surface film of protoplasm is that it consists of fat in 

 the form of a very fine emulsion. The aqueous phase 

 filling up the spaces between the globules of fat contains 

 a colloidal protein solution. 



DIFFERENTIAL PERMEABILITY. 



It has been usual, in studying permeability, to assume 

 that one surface only need be considered namely, the 

 outer plasmatic membrane. Osterhout (1913, 3) em- 



