94 PLANT PHYSIOLOGY 



on the other. The cups should of course be weighed when new to make 

 sure of the correctness of the stamped weights; and they should occasion- 

 ally be weighed thereafter. Some thin leaves show a tendency to stick to 

 the upper die, and hence the discs do not enter the cup. This difficulty can 

 be overcome by painting the face of the upper die with a thin glaze of water- 

 glass. 



An area cutter of fair efficiency may be adapted from a cylindrical cork- 

 borer working through the leaf against a cork, the discs remaining in the 

 tube; they may be supported in the test-tube by a wire diaphragm, and 

 weighed vials may then serve in place of the aluminum cups. 



Demonstration Methods. The increase of dry weight in leaves in 

 presence of light may readily be shown to a class by this method, though 

 naturally it cannot be made very striking. 



Other Methods of Study. Another method, quite different in principle, 

 of determining the amount of photosynthate formed in the leaf has teen 

 used by Brown and by Blackman, as described in their papers cited below 

 under Literature. They enclose the leaves in special chambers, supply 

 carbon dioxide in known amounts, and determine the quantity used by the 

 leaves in a given time, from which, of course, the amount of photosynthate 

 can be calculated. The results are much lower than those obtained by Sachs 

 by the leaf -area method just described, but the subject is not yet closed. 

 There is also an apparatus of similar aim described by Stone in Torreya, 4, 

 1904, 1. 



It will occur to the student as he completes the preceding 

 experiment, especially if he has reduced his results to grams 

 per square meter per hour (gm 2 h), that this result represents 

 the actual amount of starch. (or equivalent) made in that time; 

 and he will think thus to obtain one of those exact quantities 

 which it is the aim and the delight of the physiologist to secure. 

 Unfortunately, however, the result is seriously affected by a loss 

 due to the disappearance of some starch through translocation 

 and respiration, processes later to be considered. This loss can 

 in part be prevented, however, and in part computed; and if 

 the student can follow farther this very important, though 

 rather difficult line of study, he will be aided by the following: 



Suggested Experiment. Following the general method of the pre- 

 ceding experiment, prevent the translocation loss by the method which Sachs 

 describes (in his paper last cited, 372; also Detmer, 49), and compute the 

 respiration loss from the Table of Conventional Constants in Part III of 

 this book. The result should be compared with the corresponding figures 

 in the same Table of Constants. Compare also the references in Jost, 116. 

 Brown and Escombe, in a paper of 1905 cited below under Literature, 



