XXXII. ESTIMATIONS OF THE CHANGES IN 



DRY WEIGHT OF LEAVES OF 



HELIANTHUS. 



J. Thompson and W. W. Pendergast. 



The experiments, the results of which are given below, were 

 undertaken for the purpose of making a comparative estimate 

 of the changes in the dry weight of leaves during periods of 

 daylight and darkness, in connection with some extensive ob- 

 servations upon the variations and total amount of mineral 

 matter in leaves stems and roots. 



The material was taken entirely from the "Russian Sun- 

 flower," beginning at the time when the first heads were open- 

 ing, July 21, and continuing for 18 days thereafter until August 

 8, 1895. 



The following conditions were observed in the selection of 

 material: 



Perfect fully grown leaves, generally the fourth or fifth from 

 the apex of the shoot, were used. Above this the leaves were 

 in a state of rapid enlargement, and below this they were more 

 or less injured by the action of the wind. By means of a piece 

 of glass of the required size placed on the lower side of the 

 leaf, and a sharp knife, fairly uniform areas of the leaf were 

 obtained. Two samples of each leaf were taken, one on either 

 side of the midrib, and equidistant from it and the base. As a 

 check on the equality of the two pieces thus obtained, which 

 were to be contrasted in every test, a number of pieces were 

 cut from fresh leaves and weighed immediately, showing in no 

 instance a variation of more than .0007 gram from the average, 

 a variation too small to affect the general results presented 

 below: 



