1909.] 



Vegetable Assiiitilatuni and Respiration. 



25 



Loaves III and lA' on the same plant (I'esults not plotted) behaved in a 

 similar way. Leaf II, on the other hand, appeared fully expanded and turgid 

 right on till July 28, after a very trying windy morning. Even then so slight 

 were the visible signs of change that, unless they had been very cai^efuUy 

 watched for, the leaf would midoubtedly have been described as fairly turgid. 

 Nevertheless, between 3 1'..m. and 6 I'.M., during its recovery, the increase in 

 area was o"-4 per cent., and fiuni 5 a.m. to 3 p.m. tlie shrinkage must have 

 been at least 6 per cent. 



The good condition of Leaf II was ])artly due to its sturdier construction, but later 

 observations have shown that the ability to resist extreme conditions is largely a function 

 of the age of the leaf. It was observed on August 7, under by no means extreme 

 cou'litions, that Leaf I, which was situated fairly low on the plant, was completely flaccid, 

 the lamina hanging in folds against the midrib. Higher up on the same plant. Leaves III 

 and IV were only slightly waved, and towards the top of the plant were leaves which 

 appeared completely turgid. 



It was also observed that Leaf I became less and less able to withstand the heat 

 of the sun, and although on July 22 it was only slightly waved at midday, yet on 

 August 7, under conditions which were if anything less severe, it reached the completely 

 limp state just described. The same phenomenon was even more strikingh' shown by 

 Leaf II, which u]) to July 27 had not shown the slightest signs of flaccidity to the eye. 

 After the wind and sun of the 28th it quickly deteriorated and became early in 

 August as unable as Leaf I to withstand bright sunshine without collapsing. 



Referring for a moment to Leaf 5 it may be remarked how much less 

 shrinkage this leaf shows than Leaf I. Tlie curves for this leaf are much 

 more comparable with those for Leaf II. This is to be attributed in part to 

 similarity of age ; in part, as the curves for July 28 show, to the more 

 sheltered situation of Leaf Y, in which it was protected on that day from the 

 wind that continually agitated the plants in the open bed. 



Fig. 4 shows details of all six leaves for a day of very varying illumina- 

 tion, haze and sun alternating as stated in the figure. Corresponding with 

 these alternating periods of hazy and brighter weather, the slope of the 

 curves for Lea\ es I and II changes from steep to nearly horizontal, indicating 

 less rapid recovery in brilliant illumination. It will be observed that wliile 

 Leaves III and IV show a sunilar alternation, Leaves V and VI behave 

 differently, though they agree very closely indeed with each other. This 

 (Ufterence is doubtless to be attributed to disturbing factois peculiar to the 

 situation, but the records have not afforded any clue to the nature of those 

 factors. 



The rise in the curves for the following day, July 25 (fig. 5), shows in a 

 very striking way the effect of clouds and rain. At 9.30 the sky was 

 covered with thin grey cloud, through which the sun was only faintly 

 visible, and from 10.15 to 10.35 a light show-er of rain fell. During this 



