io8 GROWTH 



60 mgs. whilst the loss of dry matter due to respiratory pro- 

 cesses is 14 mgs., leaving a balance of 46 mgs., which is equi- 

 valent to 1 6 -5 per cent, of the dry weight of the plant. A 

 similar relation is shown by Oxalis, a shade plant, but the 

 amounts are much smaller; -8 mg of carbon dioxide being 

 assimilated per 50 sq. cm. per hour at 20 C, whilst the loss 

 due to respiration is ! 5 mg. at the same temperature for the 

 same units. 



A disposal to go further may be evinced and to select 

 the number or weight of offspring as being the true 

 measure of a naturally growing organism's growth, since the 

 selfish needs of the parent are thereby eliminated. For 

 obvious reasons such a measure is impracticable except in 

 special cases where reproduction takes place with extreme 

 rapidity, as for instance in bacteria, or where crop yield in re- 

 sponse to methods of cultivation is required. 



The employment of the dry weight method has a disad- 

 vantage in that it forbids the study of progressive change in 

 one and the same member, since to find the dry weight, 

 the plant or plant member must be killed. For this reason 

 other indices sometimes must be employed. Thus change in 

 the size and area of leaves in certain investigations serve as a 

 reliable measure, a fact which becomes evident when it is 

 realized that an increase in the size of a leaf, or of the entire 

 chlorenchyma system of the plant, means an increase in the 

 plant's factory and all that this connotes. Thus Johnston * 

 found that the total dryweight and total leaf-area of the buck- 

 wheat ran on parallel lines during the season February to 

 October, the greatest rapidity of growth occurring in the 

 summer months. 



In further illustration the observations of Briggs, Kidd, and 

 West,f based on the investigations of Kreusler and others on 

 the maize, may be considered. Briggs, Kidd, and West analyse 

 the growth of the maize in terms of dry weight, leaf area, and 

 time, and employ the relative growth rate and the leaf area 

 ratio. The relative growth rate curve is the weekly percentage 

 increase in dry weight plotted against time, and the leaf area 



* Johnston: "Johns Hopkins Univ. Circ.," 1917, 211. 



f Briggs, Kidd and West: "Ann. Appl. Biol.," 1920, 7, 103, 202. 



