146 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



by the osmotic pressure of the external medium. This, 

 according to the measurements of Raciborski (1905) by 

 the vapour pressure method, amounts to 965-3 atmos- 

 pheres, at 20, that of a saturated solution of sodium chloride 

 at 18 being 375 atmospheres. From the latter solution 

 these air-dry seeds were observed to absorb 7 per cent, of 

 their own weight, when equilibrium was reached. Thus 

 the intake of 7 per cent, of water, in addition to the 8 per 

 cent, already present, suffices to lower the forces of capil- 

 larity, imbibition and osmotic attraction on the inside by 

 590 atmospheres, the difference between 965 and 375 

 This rapid alteration was studied further, the results being 

 shown in Fig. 17. 



SUMMARY OF RESULTS. 



These researches by Brown, Schroder, and Shull, show 

 conclusively that physiologists can no longer assume that 

 cell walls are permeable to all substances alike. Since the 

 behaviour of Xanthium and Hordeum to the same salt is 

 quite different in a number of cases, it appears that Arm- 

 strong's application of his hydrone theory to explain the 

 selective permeability of Hordeum is not supported by 

 Shull's experiments. For instance, Hordeum and Xan- 

 thium alike hold back sodium chloride, but silver nitrate 

 is only kept out by the former. 



LOCALIZATION OF THE SEMI-PERMEABLE LAYER IN 

 SEED COATS. 



In Xanthium the semi-permeability has been traced to 

 the inner and middle layers of the testa. The outermost 

 is incapable of this function. The middle layer is several 

 cells thick, whereas the inner is only one cell thick, 

 except in the chalazal region. This last layer is believed 



