RECENT ADVANCES IN SCIENCE 223 



the conclusion that the temperature coefficient of carbon 

 assimilation is not approximately 2, as Blackman avers, but 

 rather approximates to unity, and so falls into line with the 

 temperature coefficients of other photochemical reactions. 

 Moreover, the critical examination of the data provided by 

 earlier workers leads to the same conclusion. In their second 

 paper Brown and Heise show that the data at present avail- 

 able regarding the relation between light intensity and carbon 

 assimilation indicate that there is no direct proportionality 

 between light intensity and assimilation, but for each in- 

 crease in light intensity there is a progressively smaller aug- 

 mentation of the rate of assimilation. This decrease of the 

 rate of augmentation continues until a point is reached at 

 which further increase in light intensity produces no measur- 

 able increase in assimilation. 



The bubbling method, which so far, in spite of its exten- 

 sive use, has, owing to its many inaccuracies, yielded few 

 results of any value, has lately been much improved as a result 

 of extensive researches undertaken in Kniep's laboratory. 

 A paper by Hilda Plaetzer, " Untersuchungen iiber die Assimila- 

 tion und Atmung von Wasserpflanzen " (Verhandl. phys. med. 

 Ges. z. Wilrzburg, 45, 31-102, 191 7), in which these improve- 

 ments of the method are embodied, is of special interest. The 

 investigation deals largely with determinations of the " com- 

 pensation point," i.e. the light intensity where assimilation and 

 respiration just balance each other so that no gaseous ex- 

 change takes place. When water-plants with an intercellular 

 system, e.g. Elodea canadensis, Cabomba caroliniana, Myrio- 

 phyllum spicatum, were used as experimental objects, the 

 bubbling method was employed. When water-plants without 

 an intercellular system — e.g. Spirogyra, Cladophora, Fontinalis 

 — were used, the assimilation and respiration were ascertained 

 by determining oxygen and carbon dioxide content of the water 

 in the experimental vessel. At a definite temperature the 

 compensation point is different for the various species. It is 

 not clear what factors condition this, but it is not merely the 

 magnitude of the respiration. The position of the compensa- 

 tion point for any one species depends on the temperature. 

 The light intensity decreases with the temperature, as seen 

 from the following numbers : 



Spirogyra at 2o°C.,- 174 H.K. ; at 5°C, 26*7 ; Cladophora 



