410 



GROWTH OF PLANTS 



the particular leaves being studied before they use it as a basis for calcu- 

 lating leaf changes. If one has complete analyses of two similar samples 

 of leaves taken, say at dusk and dawn, he can calculate the percentage 

 change in any constituent during the night on the basis of either of these 

 fractions and get a correct picture. In this case -the absolute amount of 

 change in like samples will not be compared, but rather the percentage 

 change based on one or the other of the two constants. In using these two 

 methods of calculation one is not limited to twin-leaf or half-leaf sampling, 

 but can compare any tw^o similar samples of leaves taken at dusk and at 

 dawn or for any other period studied. 



Table 47. Comparison of DitTerent Methods of Computing Changes 



We have already said that the twin- and half-leaf methods give com- 

 parable results. Table 47 shows that the twin-leaf, the residual dry weight, 

 and the total nitrogen methods give comparable results. These analyses 

 are made on the basis of the twin-leaf method, and the results are then 

 calculated on the basis of residual dry weight and the total nitrogen. For 

 each of the three kinds of leaves reported in the table, three different com- 



