PHYSIOLOGICAL STUDIES 53 



temperature coefficient decreases with increasing tempera- 

 ture in the range o° to 30° C. At low intensities of illumina- 

 tion a smaller temperature coefficient was observed which 

 is consistent with the interpretation that the over-all rate is 

 then limited by the photochemical process, the coefficient of 

 which, bv analogy with in vitro photochemical reactions, is 

 probably small (see Fig. 4.6). With low concentrations of 

 carbon dioxide, when the rate of the over-all process is 

 determined by some dark process, such as Process i, the 

 temperature coefficient is greater and values of a fourfold 

 increase for a rise of 10° C. have been reported by Warburg 

 for the alga Chlorella. Warburg gave the name of 'Blackman' 

 reaction to the temperature-sensitive dark process respon- 

 sible for the high temperature coefficient. With a foliage leaf 

 in air the process of diffusion may limit the over-all reaction 

 and in this case of course the apparent temperature coeffi- 

 cient may be appreciably different from that observed with 

 algae. 



Poisons. The type of mechanism first proposed by War- 

 burg has also been of value in interpreting the effect of com- 

 pounds which depress the rate of photosynthesis. Such 

 compounds may be divided into two classes: (i) Poisons 

 which are presumed to combine with some specific com- 

 ponent in the photosynthetic mechanism and (2) narcotics 

 which are thought to affect the environment of the reaction, 

 as for example by reducing the 'surface' available for re- 

 action. To determine the class of any particular depressant 

 it is necessary to determine its effect on photosynthesis 

 under a wide range of conditions. 



Cyanide, in the form of undissociated hydrocyanic acid 

 molecules, is a powerful depressant of photosynthesis in 

 the class of poisons. In Chlorella photosynthesis is almost 

 completely inhibited by concentrations of cyanide too small 

 to affect respiration. In other organisms, e.g. the alga 

 Scenedesmus, photosynthesis is inhibited to a smaller extent 

 than respiration and in Hormidium it has been claimed 

 (although without subsequent confirmation) that photo- 

 synthesis may be even stimulated by dilute concentrations 

 of cyanide. With Chlorella, Warburg (1919) found the 



