172 GROWTH 



in pll from 77 to 17 is without effect, but at pH 1-2 the 

 growth is much slower and the coefficient of utilization falls 

 to 0-29. The influence of temperature will be considered 

 later. 



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 response 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 dry weight and total leaf-area of the 

 buckwheat 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 

 analysed the growth of the maize in terms of dry weight, leaf 

 area and time, and employed 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 ratio curve is the leaf area in square centimetres 

 per gram of dry weight plotted against time. 



The growth rate of the maize varies much in magnitude at 



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



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



