1909.] 



Vegetable Assimilation and Respiration. 



39 



Section VI. — On the Measurement or Area. 



The only importaut methods hitherto used for determining the area of the 

 portion of leaf of which the dry weight is required are Sachs' templet 

 method, and the planimeter method, used by Brown and Escombe, consisting 

 in the measurement of a photographic print by means of a planimeter. 



Two new methods have been devised in the course of this research. One 

 is the puncli method, a modification of the templet method depending upon 

 the use of a rotating punch ; the other is the stamping method, by which 

 shrinkage errors are eliminated. 



The Tenqjlrt Method. 

 In this method a piece is cut from the leaf of the same area as a 

 rectangular plate of wood, metal, or glass laid upon it. This is by far the 

 simplest and most rapid method. 



Sachs gave the error involved in his use of the templet as a few square millimetres in 

 an area of 50 or 100 square centimetres, or only a few ten-thousandth parts of the 

 whole.* This must, however, be taken as a rough estimate. 



Miillert states that in cutting an area of 40 sq. cm. with a templet the limit of the 

 error introduced into the dry weight per square metre varies from 0"25 to 0"66 gramme, 

 according to the prominence of the veins. This is of the order of 1'5 per cent, 

 and mucli too high. Since he does not say how he arrived at the estimate, no weight can 

 be attached to it. 



There is no perfectly satisfactoiy method of mea.suring the degree of accuracy of the 

 templet method directly. For instance, to cut out a given area from paper with 

 a templet and then measure it accurately would only give a minimum value for the error 

 involved in cutting a similar piece from a leaf. The leaf is held firmly only at the veins, 

 and it is difficult to avoid some small displacement of the extensible tissue which 

 intervenes. 



To obtain a maximum estimate of tlie error, pieces were cut in the usual way from 

 leaves, and photographic prints of these pieces taken immediately on " gelatino-chloride " 

 paper. The untoiied prints were subsequently measured by means of a scale of centi- 

 meti'es with a vernier reading to O'l mm. A number of measurements were made 

 of the longitudinal and transverse linear dimensions, and from these the area of the 

 print was estimated. 



Before discussing the results obtained in this way it is necessary to remark that the 

 only point of immediate importance is the degree of concordance between results obtained 

 with similar leaves. The area of the pieces at the time of printing was likely to be 

 different from their area when the templet was laid upon them and the cuts made, owing 

 to freedom from tension and some loss of water by evaporation. It is, therefore, in 

 accordance with expectation that the areas of leaf pieces given below are all less than 

 the area of the templet used in cutting them ; and also that this absolute error is greater 

 for Ildianthus annuus, which has been shown to shrink so rapidly. 



* Loc. cit. 



+ "Die Assimilationsgrosse bei Zucker- und Starkeblattern," 'Jahrb. f. wiss. Bot.,' 

 vol. 40, 1904, p. 443. 



