543 



with the thouij;ht that this method, of cutting otit discs and 

 weighing them dried, is not a correct or ade(|uate means of 

 determining the amount of carhon assimilation during the day. 

 There is more or less truth in these assertions, for it is difficult 

 to determine the rate of translocation, and hence it is only a test 

 of the accumulation of products. It is possible that it is a slight 

 shock to the leaf to have a couple of round discs of one sq. cm. 

 area cut from it, and i)erhaps there is a slight tem])orary effect 

 on the metabolic activities of the remaining portion of the leaf. 

 But, nevertheless, for the purposes of these studies this method 

 is wholly satisfactory and adeciuate. It is safe to assume that 

 the shock to one-half the leaf is not greater nor less than to the 

 other half, and hence the value of the comparison is not 

 imjiaired whether there is a slight shock to the leaf, or not. 

 Again, since light seems to have somewhat of an inhibitory 

 effect on the diastase action in digestion and removal of starch, 

 and since translocation of sugars appears to be somewhat 

 increased in rate when the temperature rises and it appears, 

 further, the presence of a dust coat or surface coating of any 

 sort tends to slightly increase the temperature and, therefore, 

 slightly accelerate translocation of sugars, we may conclude 

 safely that in the coated half of the leaf the rate of trans- 

 location is at least not reduced, and when we consider all the 

 facts it appears that probably it is substantially unchanged. 

 We may, therefore, conclude that the weight of accumulated 

 dry matter in each half of a hundred or more leaves is, at 

 least, a good comparative test when we are seeking to deter- 

 mine the ratio of photosynthetic activity in two parts of the 

 same leaf. Whether or not this method is adequate for testing 

 the total assimilation of carbon during a given period of time 

 is a question with which we are in this study not in the least 

 concerned. 



The other of the two principal methods used was the 

 examination microscopically of microtome sections of leaves 

 stained in iodine. This, of course, is a check only on the starch 

 and not on all carbohydrates. Leaves were treated as outlined 

 above, a portion of the upper surface being coated and the 



