438 Mr. J). Thoday. Experimental Researches on [Mar. 1, 



These differences have, of course, to be divided by the number of hours 

 of insolation to find their effect upon the observed rate of increase of dry 

 VP eight. 



The corresponding data for Expts. 7 and 8 were calculated from the results 

 of asymmetry tests by the templet method given in the earlier paper.* 



It will be observed that the maximum asymmetry error would account for 

 practically the whole range of variation among the individual results. For 

 instance, in Expt. 6, Leaves 8 to 12, the maximum error of 7 milligrammes 

 on either side of the average rate would give — 1'7 milligrammes and 

 + 12"3 milligrammes. The considerable variations, however, in the hygroscope 

 readings on the different leaves indicate that the true rate of assimilation 

 itself varied from leaf to leaf, so that part of the variation in the results is to 

 be accounted for in this way. 



This is true, even in Expt. 6, where almost all the leaves preserved a 

 uniformly turgid appearance throughout the time during which the experi- 

 ment lasted.f 



Catalpa, however, is unlike Hclianthus, in that it is not possible to 

 gauge accurately the condition of the stomata, nor even the turgidity of a 

 leaf, from its external appearance ; for loss of turgidity is not at first 

 accompanied by readily visible symptoms^ The hygroscope readings which 

 were made at intervals on each leaf are therefore of the greater value. 



A comparison between Expts. 6 and 7 shows that when signs of flaccidity 

 do appear the effect on the assimilation is marked. In Expt. 6, corresponding 

 with the uniformly turgid appearance of the leaves, the average rate of 

 assimilation was 5"3 + 0"7 milligrammes per hour ; whereas in Expt. 7, in 

 which several of the leaves became flaccid, the much lower average rate, 

 3'5 + l"0 milligrammes, was found. 



Taking into account the variations indicated by the hygroscope readings, 

 greater rates of increase than 5"3 milligrammes per hour are to be expected 

 in fully turgid leaves. The rate of increase for Leaf 11, with an average 

 hygroscope reading of 40, should be about 6'4 + 0"9 milligrammes per hour. 



These figures may be compared with Brown and Escombe's highest value, 

 obtained during four hours of intermittent sunshine. Measuring directly tlie 

 amount of carbon dioxide absorbed by a detached leaf enclosed in their 

 experimental case, they obtained a rate of assimilation approximately 



* Loc. cii., p. 18. 



+ The only notewortliy exception was Leaf 12, which flagged visibly in the afternoon, 

 and for which the average hygroscope reading is correspondingly low. The low average 

 reading for Leaf 4 is due to its having postponed the opening of its stomata till later 

 than the other leaves. 



