1909.] Vegetable Assimilation and Respiration. 



53 



They show a greater increase the shorter the time of experiment, which is just the 

 apparent eflect that the division of a positive error over a varying number of hours 

 would produce. The point cannot, however, be pressed, and the fact may have some 

 other explanation ; but it is worth while to mention the possibility, especially as the 

 results are so much higher even than those obtained in the expei'iments already discussed. 



Section IX. — Conclusions. 



1. Carbon analyses have shown that, except for minor differences, the same 

 result is obtained for the rate of assimilation by the half-leaf method, whether 

 increase of dry weight or of carbon content is measured. The dry weight 

 method is therefore not vitiated by any large indeterminable errors such as 

 would arise if varying quantities of water were retained by the colloids of the 

 leaf after drying it at 100° C. 



The calculation of the equivalent intake of carbon dioxide from the increase 

 of dry weight can, however, only be approximate, as too little is known of the 

 very variable composition of the products of assimilation. As a rule it will 

 probably be advisable to determine the increase in ash content and deduct it 

 from the increase in dry weight. 



2. The tendency of the method to give results which are too high is amply 

 explained by shrinkage in area of the experimental half-leaf through loss of 

 turgor during insolation. Shrinkage under insolation is a general phenomenon : 

 it is shown in varying degree by all the leaves examined. Leaves of 

 Hclianthi's amiuus often diminish in area by more than 5 per cent, between 

 early morning, when the air is moist, and midday, when the hot sun and dry air 

 favour rapid evaporation. Eobust leaves of this plant may show to the eye 

 little sign of flaccidity, even though they are 4 or 5 per cent, less in area than 

 when fully turgid ; hence errors from shrinkage cannot be avoided by mere 

 inspection. 



The leaves of Helianfhus anm'vs are extraordinarily sensitive to changes in 

 the intensity of illumination, and react to them with great rapidity. A leaf was 

 observed to increase or decrease in breadth by nearly 2 per cent, within 10 

 minutes when the sun passed behind a cloud, or appeared again after having 

 been for a time obscured. 



Since this source of error has been practically ignored in all the work 

 hitherto done by the dry weight method, few of the results are trustworthy. 

 Thus the rate of assimilation found by Sachs for attached leaves of Heliantlius 

 annuus, viz., 9 milligrammes per square decimetre, may easily have been 

 2 milligrammes in excess of the true rate ; while in his similar experiment 

 with Cucurhita fcpo the whole of the apparent gain may have been illusory. 



On the other hand, Sachs' result for detached leaves of Heliantlius annuus, 

 16 milligrammes per sqi;are decimetre, although so high, is not open to the same 



