254 PHOTOSYNTHESIS 



of water at 100° C. by the colloidal elements of the cell contents." The 

 discrepancy may also be partially due to differences in the symmetry in 

 venation and the thickness of the two halves of the leaf. Brown and 

 Escombe's studies show that such differences show a mean of ± 2.2 per 

 cent. They also found that leaves of Catalpa bignonioides undergo changes 

 in area as the result of insolation, amounting to as much as 3 per cent and 

 Thoday has obtained similar results with Hclianthus annuus in which 

 there was a diminution in area of more than 5 per cent when conditions 

 favor rapid transpiration. 



Both these sources of error are of considerable magnitude, for if a leaf 

 underwent a reduction in area during insolation so that its area as meas- 

 ured after illumination was less than at the beginning of the experiment 

 the dry weight per unit area would be increased. On the basis of their 

 studies of the half leaf method. Brown and Escombe conclude: "The 

 average error observed due to differences in symmetry amounts to 2.2 per 

 cent and that due to change of area to 1.1 per cent. Assuming that the 

 accumulated errors from all sources in a Sachs experiment amounted to 

 2.0 per cent with a leaf having a dry weight of 0.50 gram per square 

 decimeter, this would lead to an over- or under-estimate of the matter 

 assimilated of 0.010 gram per square decimeter for the total time of the 

 experiment, and if the duration of the experiment were five hours, to an 

 apparent assimilation or depletion at the rate of 0.002 gram per decimeter 

 of leaf per hour. But this is about the average amount of true assimilation 

 observed for the leaf of Catalpa bignonioides by the direct method of 

 determining the carbon dioxide assimilated, so that an under-estimate of the 

 area of only 2 per cent in the insolated half leaves would on the Sachs 

 weighing method give an over-estimate of the assimilation of 100 per cent 

 of the true value, whereas it would only affect the results obtained from 

 our method of carbon dioxide absorption to the extent of a 2 per cent." 



Ganong^^ has devised a leaf area cutter by means of which discs of 

 one square centimeter can be cut from the leaves. This to a measure 

 avoids the errors due to lack of symmetry of the two halves of the leaf. 

 It is, however, only a partial improvement on the original method. 



The most convenient method of determining leaf areas is to place the 

 leaves or portions thereof which are used on blue print paper in a printing 

 frame and thus make a direct print of the leaf. The area of the photo- 

 graphic image of the leaf can then be determined by means of a planimeter. 



In conclusion it may be stated that while the method of determining 

 the rate of photosynthesis from the amount of dry material present in the 

 leaves offers the simplest means of estimating this activity and that for 

 many forms of investigation this simplicity is highly desirable, in its 

 present form it is not reliable. Aside from the sources of error arising 

 from differences of symmetry and area of the leaves the two very impor- 

 tant factors of respiration and translocation of synthesized material offer 

 difficulties which we have at present no means of overcoming. In order 



^'Ganong, Bot. Gaz., 39, 150 (1905). 



