422 Mr. D. Thoday. Experimental Researches on [Mar. 1, 



been able, after introducing appropriate modifications, to use the method 

 with precision, and to obtain results whose degree of approximation to the 

 truth can be justly gauged. 



The vexed question of the highest rate of assimilation possible to detached 

 leaves of Heliaiithus annmis in the open air first claimed attention. Measure- 

 ments of the gain of dry weight by these leaves on bright sunny days 

 (described in Section II) have clearly proved that Sachs' original value was 

 not excessive, and suggest some of the conditions which must be fulfilled if 

 such rapid assimilation is to be possible. 



Numbers for the dry-weight gain in Catalpa hignonioides have also been 

 determined, and form a valuable contrast with those for Helianthus. 



Attention has been paid throughout to the part played by the stomata in 

 regulating the rate of assimilation ; and in Section IV are discussed the 

 connected questions as to how far the numbers obtained with detached leaves 

 represent what is occurring with leaves on the plant, and whether any 

 translocation of the accumulating products of photosynthesis takes place 

 during the day. 



Section II. — On the Eate of Assimilation of Caebon Dioxide by 

 Leaves of HeliantMis annmis in Bright Sunshine. 



(i) The experiments described in this section were undertaken with the 

 object of checking Sachs' experiment with detached leaves of Helianthus 

 annuus, in which he obtained the high value 16'5 milligrammes per square 

 decimetre (1'648 grammes per square metre) per hour as the rate of increase 

 of dry weight on a bright sunny day. Brown* and Morris,t in a similar 

 experiment, found the much smaller rate of increase of 10 0 milligrammes ; 

 and Brown and Escombe,J determining the intake of carbon dioxide by 

 direct measurement, failed to obtain a higher rate of assimilation in their 

 experimental glass cases than was equivalent to a gain of about 5'5 milli- 

 grammes of dry substance per square decimetre per hour. 



Notwithstanding this lack of confirmation of Sachs' result, a study of the 

 details of his procedure in this experiment§ leaves no ground for assuming 

 that any known kind of error entered to vitiate it seriously. He guarded 

 against the only source of error which might have made the result far too 

 high, the shrinkage in area of the experimental half-leaves, by floating them 



* Loc. cit., p. 25. 



t ' Journ. Cheiii. Soc.,' 1893, pp. 626—8. 



I ' Roy. Soc. Proc.,' 1905, B, vol. 7G, p. 44, Table I. 



§ Thoday, D., loc. cit., p. 32. 



