1910.] Vegetable Assimilation and Besjnration. 439 



equivalent to an increase of dry weight of 4'7 milligrammes per hour. My 

 results, by the dry weight method, are in fair agreement with this gasometrre 

 result,* but indicate more rapid assimilation in continuous sunshine ; and it 

 may be true for Catalpa, as well as for Hclianthus, that before the highest 

 rate of assimilation can be attained the leaves must be exposed to conditions 

 approaching full insolation. 



Expt. 7 exhibits within itself the effect of the closure of the stomata in 

 diminishing the rate of assimilation. In spite of the great irregularities due 

 to asymmetry errors, a general gradation is visible in the rates of increase 

 following the variation in the averages of the hygroscope observations. The 

 highest rates of increase are shown in the case of leaves with their stomata 

 most widely open. 



Calculating from the average result, on tlie assumption that the hygroscope 

 readings are proportional to the individual rates of assimilation, a closer 

 approximation to the rate of increase for Leaf 13, with its stomata most 

 widely open, is 5"l + r5 milligrammes. This is close to the average rate in 

 Expt. 6. It is to be noted that in Expt. 7 the sky was at first cloudy ; in 

 continuous sunshine yet a higher value would probably have been obtained. 



In Expt. 8 tlie leaves were still on the tree, and the average rate of increase 

 is much lower than in Expt. 6, although the leaves remained in appearance 

 turgid, and the sunshine was continuous throughout the day. This difference 

 is, in part, to be explained by less widely open stomata, for Brown and 

 Escombe's simultaneous experiments with attached and detached leaves, and 

 observations of my own with the horn hygroscope, both show that detached 

 leaves tend to open their stomata more widely than leaves still attached to 

 the tree. This point will be discussed more fully in the next section. 



Section IV. — Eemarks on the Kate of Asslmilation in Nature ; and on 

 THE Occurrence of Translocation During the Day. 

 In the previous sections it has been shown that very high rates of 

 assimilation are possible to leaves of Heliantkxis annuus when detached 

 from the plant, and that leaves of Catalpa also may under like conditions- 

 assimilate more rapidly than has hitherto been observed. It therefore- 

 becomes of importance to inquire how far the conditions necessary to these 

 high rates are actually realised in nature, i.e., in leaves which are still upo% 

 the plant. 



* Brown and Escombe's results for Catalpa by the dry weight method do not agree 

 with their gasometric results nor with my dry weight results, but vary up to 16'6 milli- 

 grammes per hour (average for four leaves). This is due, as has already been pointed 

 out {loc. cii., p. 52), chiefly to error from shrinkage in area, to the possibility of which 

 Brown and Eacombe were the first to draw attention. 



2 M 2 



