576 SCIENCE PROGRESS 



referring to the capacity of the cell for allowing the passage 

 of a particular substance into it from the external medium, 

 or out of it into the external medium. Such a use of the term 

 is not ideal, but it is the most general, and in the present state 

 of our knowledge a more exact use is scarcely possible. 



In regard to permeability phenomena it is impossible to 

 generalise to any extent, for not only will the permeability of 

 the cell to different substances be different, but plant cells 

 differ among themselves both morphologically and physiologi- 

 cally, and may exhibit different permeability properties. 



In the living plant permeability phenomena always have to 

 do with solutions, and the permeability to the solvent may be 

 perfectly different from that to the solute. The solvent is 

 always water ; in this review of recent advances in our know- 

 ledge of permeability we will consider first the question of 

 permeability to the solvent. 



Permeability to Water: (a) Seed- Coats. — Denny {Bot. Gaz. 

 63* 373-97 > 1 9 l 7) has investigated the effect of temperature 

 on the passage of water through the seed-coat of the pea-nut 

 {Arachis hypogcea). The measurements were made by means 

 of an osmometer in which the membrane separated pure 

 water and a solution. Contrary to the results previously 

 obtained by A. J. Brown for barley, Denny found the tempera- 

 ture coefficient (Q10) lower than that usual in chemical reactions 

 (2 to 3) ; it was, however, higher than the temperature coefficient 

 of diffusion (about 1*3). The coefficients were found to de- 

 crease with rise of temperature, e.g. in one set of experiments 

 Q10 between 5*2° C. and i5"2° C. was i'628, while between 

 35° C. and 45 C. it was 1*344. 



A surprising result obtained by Denny was that water 

 passed through the membrane from the outer surface more 

 rapidly than in the reverse direction. Great variations in the 

 permeability were exhibited by the seed-coats of different species, 

 and it is shown that the thickness of the membrane is not the 

 determining factor. 



In a later paper the same author {Bot. Gaz. 63, 468-85, 

 191 7) concludes that lipoid substances, tannins, and pectins 

 are important in determining the permeability of seed-coats to 

 water. This conclusion is based on the change in permeability 

 brought about by extracting different seed-coats with various 

 solvents. Thus in the case of peanut and almond, extraction 



