392 SCIENCE PROGRESS 



the growth of Wheat in Culture solutions. The addition of 

 Agar was found to have a stimulating effect on growth when 

 the nutrient solution had a low concentration, but the reverse 

 effect when the concentration of nutrient salts was raised. 

 The addition of fine quartz sands caused a decrease in dry 

 weight which was the most marked when the finest-grained 

 sand was employed, and a similar depression was observed 

 with both Ferric and Aluminium hydroxide. It is suggested 

 that the concentration of the nutrient substances becomes 

 reduced through the presence of the adsorbent. Increased 

 weight with colloidal silica appears to be due to direct absorb- 

 tion of this substance, a result in agreement with the conclu- 

 sions of Gregoire. Other workers have, however, found that 

 finely-divided material tends to promote growth, and this has 

 been explained as an outcome of the adsorption of toxins. 



In an interesting account of studies on Pasture, Jenkin 

 (Pasture Studies, Bangor, 19 19) describes the changes of vegeta- 

 tion in an area laid down to grass. All the areas studied 

 showed a decrease in the proportion of sown grasses, but an 

 increase of the grasses not sown. The influence of the method 

 of treatment, and the character of the original seed mixture, 

 are emphasised. 



Blackman (Ann. Bot.) calls attention to the fact that the 

 rate of growth increment, in the earlier phases, follows the 

 compound interest law, and is therefore dependent on the 

 initial capital, as represented by the size of the seed and the 

 efficiency of the plant to form new material. Similarly, at 

 any given phase the amount of material present is an impor- 

 tant factor in determining the increment of the future. 



PLANT PHYSIOLOGY. By Prof. Walter Stiles, M.A., University 

 College, Reading. (Plant Physiology Committee.) 



Influence of Environmental Factors on Growth and Development 

 of Plants. — During the last few years a vast amount of work 

 has been published dealing with the influence of various factors 

 of the environment on the development of plants ; conse- 

 quently it will only be possible here to review a representative 

 selection from recent researches on this question. 



The chief environmental factors are light, temperature, and 

 humidity. In regard to light the measurement offers con- 

 siderable difficulties. D. T. MacDougall and H. A. Spoehr 

 (Science, 45, 616-8, 191 7) point out that the only reliable 

 measurements of solar radiation available to plant physiologists 

 have been made with the Angstrom and Abbot type of pyrheli- 

 ometer. This records in heat units the total insolation, but is 

 not sufficiently sensitive to the violet end of the spectrum. 



