326 PHOTOSYNTHESIS 



The simplest method of making rough quantitative measurements of the 

 rate of photosynthesis in terrestrial plants is to determine the increase in the 

 dry weight of leaves during a given interval of time. One method of pro- 

 cedure is as follows: A representative sample of disks, usually lOO or more, 

 is cut from the leaves by means of a cork borer at the beginning of the period 

 for which the determination is to be made. These disks are transferred to 

 an oven and dried to constant weight. At the end of a chosen period of time, 

 the plant having been meanwhile exposed to the desired environmental con- 

 ditions, a second sample consisting of the same number of disks as the first 

 is cut from the leaves. Various precautions must be taken to insure the two 

 samples being as nearly comparable as possible. The dry weight of the second 

 sample of disks is also determined. Any gain in dry weight is considered to 

 represent carbohydrates which have accumulated in the leaves as a result of 

 photosynthesis. 



This method is subject to three principal errors : ( i ) the consumption of 

 a portion of the photosynthate in respiration, (2) translocation of some of 

 the products of photosynthesis out of the leaves, and (3) changes in the area 

 of the leaves (Thoday, 1909). The last error is due to variations in leaf 

 turgidity and is more marked on clear, sunny days. On such a day the area 

 of a leaf shrinks during the daylight hours. Hence, a disk cut out of a leaf 

 in the afternoon will include more cells than one of equal area removed 

 from the leaf in the early morning. Even if no photosynthesis occurs, the 

 former disk will possess a greater dry weight than the latter because it will 

 include more cell wall and protoplasmic material. A part of the gain in the 

 weight of leaves indicated by this method is often due, therefore, simply to 

 changes in the area of the leaves. Cutting out disks of the leaf tissue may 

 also induce metabolic changes in the residual tissue which will influence its 

 rate of photosynthesis. The gain in dry weight of leaves under conditions 

 favorable for photosynthesis is usually between 0.5 and 2.0 g. per square meter 



per hour. 



The "twin leaf" method described by Denny (1930) is a useful variation 

 of this method. This procedure can be followed only with species bearing 

 similar opposite leaves or leaflets. One leaf of a number of pairs is used 

 for the first determination of dry weight, the mates being taken for a similar 

 determination at the end of the experimental period. The accuracy of this 

 method will depend in part upon the number of leaves used in the matched 

 samples. Denny records that when 25 leaves of Salvia splendens were used, 

 the error due to a difference in the initial weight of the leaf samples was only 

 about I per cent. 



