l82 BOTANICAL GAZETTE [February 



Evidence is also presented that such plants as Pleris aquilina and Pinus 

 often succeed in competition owing to their dead foliage excluding the light 

 from their competitors, causing etiolation and decay. 



In a more recent paper Farrow^^ has examined the retrogression begun 

 by rabbits and continued by sand blasts. This retrogression shows exactly 

 the reverse order of the succession inaugurated by irrigation, being particularly 

 noticeable in the Agrostis vulgaris giving place to Festuca ovina wherever the 

 sand blast became intensive. Once begun, bare areas tend to increase, the 

 sand assisting in destroying the vegetation both by direct attack and by remov- 

 ing the substratum, leaving clumps of grass upon the tops of small hummocks 

 which are being constantly undermined. With the checking of wind erosion 

 in such bare areas Polytrichum and Cladonia become agents of stabilization 

 and revegetation. — Geo. D. Fuller. 



Photosynthesis. — Osterhout and Haas^'' summarize as follows a piece of 

 work on the dynamics of photosynthesis. " Ulva which has been kept in the 

 dark begins photosynthesis as soon as it is exposed to sunlight. The rate of 

 photosynthesis steadily increases until a constant speed is attained. This 

 may be explained by ass,uming that sunlight decomposes a substance whose 

 products catalyze photosynthesis or enter directly into the reaction. Quanti- 

 tative theories are developed to account for the facts." The rate of photo- 

 synthesis was determined by the rate at which a portion of Ulva rendered sea 

 water basic to phenolphthalein. Since the dissociation of carbonic acid is 

 very slight, change of reaction is a very crude way of measuring the amount 

 present. There is also the possibility of other exchanges of more strongly 

 dissociating materials that could modify the reaction of the water. In the 

 face of excellent and very accurate methods for the quantitative determination 

 of carbon dioxide it seems hardly justifiable to use this questionable method for 

 a study of either respiration or photosynthesis. It is also doubtful whether 

 sufficient regard has been given to other possible limiting factors of the rate 

 of photosynthesis in these experiments. If, in spite of the defects of experimen- 

 tation, the general conclusion proves true, it is a contribution of great signifi- 

 cance and aids in confirming Willstatter's view that the presence of a 

 catalyzer is a common internal limiting factor to the rate of photosynthesis. — 

 Wm. Crocker. 



Organic plant poisons. — Brenchley^^ finds hydrocyanic acid very toxic 

 to pea and barley seedlings in water cultures. Hydrocyanic acid in concentra- 

 tions of I part to 100,000 proved rather quickly fatal for peas and somewhat 



'3 Farrow, E. P., On the ecology of the vegetation of Breckland. V. Character- 

 istic bare areas and sand hummocks. Jour. Ecology 6:144-152. 1918. 



'4 Osterhout, W. J. V., and Haas, A. R. C, Dynamical aspects of photosynthesis. 

 Proc. Nat. Acad. Sci. 4:85-91. 1918. 



^5 Brenchley, Winifred E., Organic plant poisons. I. Hydrocyanic acid. 

 Ann. Botany 31:447-456. 1917. 



