1909.] Vegetable Assimilation and Respiration. 



43 



There can be no doubt, however, that with a carefully tested and thoroughly trust- 

 worthy planimeter, reading to the nearest O'l sq. cm., this method would be convenient 

 as a general method, and might be used for pieces cut from between the pi'ominent veins 

 of fairly large leaves without the error becoming prohibitive. In this way it would 

 have the advantage over the templet method, that the form of the piece cut out would 

 not be restricted, and a luore complete use of the lamina would be possible while still 

 avoiding the veins. 



On the other hand, when describing the results which were obtained in testing the 

 accuracy of the templet method, it wa^ jjointed out that the area of the prints was less 

 than the original area of the pieces printed, owing to changes of tension and to 

 shrinkage.* The magnitude of such changes will be less for entire half-leaves than for 

 cut pieces ; but in any case it is necessary to ensure as far as possible that the conditions 

 of tension in portions to be compared are the same, during printing, and to adopt exactly 

 the same technique, so that the times elapsing between the removal of the half-leaf, or 

 portion, and the finished print are not only as short as possible, but equal. So long as 

 the errors attect the measurement of control and experimental half-leaves in the same 

 degree the absolute error can be ignored, since its effect in the calculation of the result 

 for the unit of area is of the second order of magnitude. 



Besides these errors arising from the varying condition of the leaves themselves, there 

 are others which may l)e called purely mechanical. 



The leaves may not be perfectly smooth and level, and the pressure to which they are 

 subjected during printing may vary. The former difficulty is frequently encountered 

 with leaves like Troptfolum, which are sometimes waved at the edges, and in such a case 

 it was found necessary to cut triangular pieces from the waved parts, so as to make them 

 lie fiat. This is objectionable, both because of the danger of multiplying the circum- 

 ference error, and fiom the increased number of planimeter measurements involved in the 

 separate printing of the detached pieces. It is impossible always to exclude leaves which 

 show this waving of the edge, for under the initial conditions of turgidity it may not 

 be shown at all. 



If the same printing frame is used for both halves of a leaf, variations in pressure are 

 scarcely possible to an appreciable extent. 



The effect of varying pressure is more likely to be of account in the templet method : 

 but on general grounds, and from a consideration of the results of the test described 

 above, it can be assumed that when a leaf is flat the alteration of area caused by pressure 

 of the templet upon the leaf is very small, even if it is at all appreciable ; and if the leaf 

 is not flat the error is insignificant in comparison with that resulting from asymmetry. A 

 leaf of Catalpa was tested, in which one side was bulged between the veins, while the other 

 side was flat. Pieces cut by the same templet from each side differed in dry weight to 

 the very exceptional extent of 8 per cent, in favour of the bulged side, whereas the 

 average degree of asymmetry for Catalpa is ± 3'5 per cent. Such ei-rors as these can be 



using it for accurate work. To distinguish between errors due to lack of skill in following 

 the perimeter and those due to inaccurate graduation, it is sufficient to take a number of 

 successive readings in a clockwise direction, and then to traverse the same part of the 

 scale again in the counter-clockwise direction. By repeating this .several times, a number 

 of readings are obtained in each of several successive parts of the scale. 



The planimeter was only used in preparing the material foi' the earliest analyses 

 (Table I, p. 6), and since eight readings were averaged, the resulting error was 

 insignificant. 



* Pp. 39 and 40. 



