Abstract
A method called “dot counting method” for the rapid determination of leaf areas is described. Photo. shows the application of this method for the measurement of leaf area of a poplar tree in the field. The surface of the transparent plastic plate had been marked with dots keeping a certain distance, here 1cm., with each other. The leaf is being put randomly between this plate and another white one. The number of dots overlapping the leaf coresponds to the leaf area.
The accuracy of the method was studied with the leaf of Populus deltoides. Deviation of the value was determined for the leaves of different area. The difference in distance between the dots affected to the accuracy. Fig. 1 and Fig. 2 show the standard deviation observed with dots in distance of 0.5, 0.7, 1 and 1.5cm. in which a dot is equivalent to the area of 0.25, 0.49, 1 and 2.25cm2. respectively. It is apparent that the narrower the distance of dots was the more the deviation decreased. Standard deviation depressed with the decrease of leaf area. But the rate of it was so gentle that the deviation expressed as coefficient of variation increased rapidly.
The measurement with narrower distance of dots is very laborious and the replication of determination with more wide one may be desirable in practice. The deviation of mean value that was obtained by the determinations replicated 2, 4 and 9 times with the dots of 1 cm. in distance is shown in Fig. 1.
The shape of leaf was supposed to influence the accuracy. For the estimation of leaf areas of Ginkgo biloba, Lithocarpus edulis, Liriodendron tulipifera and Liquidamber styraciflura, this method was available with degree of deviation that occured for the poplar. (Fig. 1) But the deviation was too great for the measurement of leaf area with complicated shape like as Thujopsis dolabrata to apply.
The accuracy of “dot counting method” was compared with other methods used widely. (Table 1) The “planimeter method” in which a copy of leaf is made and outline of that is traced with planimeter, gave the most accurate value. By the “paper weighing method” in which a copy of leaf is cut out and the area is calcurated from the weight of it, unexpected deviation was observed. Unevenness in weight per unit area of the paper used to make a copy was looked as a cause of this deviation. (Table 2) The accuracy of “dot counting method” is possible to be the same or superior to that of “paper weighing method” if the proper use was made.
In “dot counting method”, time spent for measurement was very short. For an instance, only 40 seconds by dots of 0.5cm. in distance, 20 seconds by 0.7cm., 12 seconds by 1cm. and 8 seconds by 1.5cm. Were needed for a determination of cut leaf with area of about 80cm2., while by application of “planimeter method” 4.5 minutes and “paper weighing method” 5 minutes were spent for the same leaf area.
A point that “dot counting method” is excellent in comparison with other methods is able to apply to attached leaves in situ without any trouble. Table 3 shows the changes of the leaf area of a 1-year-old poplar cuttings in the growing season of 1956 observed by this method.
