554 SCIENCE PROGRESS 



Aloe ferox, is described by Professor Chamberlain {Bot. Gaz., 

 November). 



Ecology. — ^The destruction of Mosses by Lichens is described 

 by McWhorter {Bot. Gaz., November). This is usually due to 

 smothering, but sometimes apparently to actual parasitism, 

 Amphitoma being cited as an example. 



In an extensive and fascinating paper Erdtman {Arkiv. fiir 

 Boianik, March 17, 1921) describes the occurrence of pollen 

 of various trees in the successive layers of peat in a number of 

 moors and fens of Sweden. Both the statistical results with 

 respect to pollen and the distribution of vegetative organs on 

 the peat seem to indicate a succession from below upwards, 

 through Fen with Sedges, Menyanthes, Cladium, etc. ; Fen 

 Carr ; Pine ; and ultimately Sphagnum. 



Although there is some irregularity in the percentage curves 

 for the different species of pollen, most of these seem to indicate 

 an earlier phase in which deciduous trees were more abundant ; 

 whilst the recent peat tends to show an increase of Pinus and 

 Picea corresponding to a diminution or total absence of the 

 deciduous species, especially Corylus and others generally asso- 

 ciated with less acid conditions. 



PLANT PHYSIOLOGY. By Prof. Walter Stiles. M.A.. Sc.D.. 



University College, Reading (Plant Physiology Committee). 



Osmotic Pressure in Plants. — Considerable additions to our 

 knowledge of osmotic pressure and allied phenomena in plants 

 have been made during the last few years by J. A. Harris and 

 a number of co-workers in America, and by Ursprung and Blum 

 in Switzerland. As is well known, there are two chief methods 

 available for the determination of osmotic pressures in plants — 

 the plasmolytic method, which gives a measurement of the 

 osmotic pressures of individual cells, and the method which 

 consists in expressing the sap from tissues or organs and measur- 

 ing the freezing-point lowering of the fluid so obtained. The 

 determinations of Harris and his collaborators have been made 

 by the latter method, those of Ursprung and Blum by the 

 former. 



With regard to the magnitude of osmotic pressures in plant 

 cells, many observations have indicated that a mean value 

 would be somewhere in the neighbourhood of 10 to 15 atmo- 

 spheres. While values considerably higher than the latter 

 number have been recorded from time to time, Harris, Gortner, 

 Hofman, and Valentine (" Maximum Values of Osmotic Con- 

 centration in Plant-tissue Fluids," Proc. Soc. Exper. Biol, and 

 Med., 18, 106-109, 1 921) have found exceptionally high values 

 in plants growing in the neighbourhood of the Great Salt Lake, 

 the value of 153-1 atmospheres found in the case of the sap of 



