386 SCIENCE PROGRESS 



rate of growth per unit leaf-area instead of per unit dry-weight, 

 and have made use of a third significant relation, Unit Leaf- 

 rate, which they define as the weekly rate of increase of dry- 

 weight in mgs. per cm.^ It is found^that the unit leaf-rate does 

 not undergo a perfectly definite type of variation as does the 

 relative growth-rate, but fluctuates about a mean value. The 

 general conclusion which emerges from a study of the correlation 

 between unit leaf-rate and various environmental factors is 

 that the unit leaf-rate is correlated more closely with tempera- 

 ture than with any of the other environmental factors, the 

 correlation coefficient with weekly mean temperature for the 

 year 1875 being r = 77. 



The E'ynployment of the Growth-rates of Standard Plants as 

 Indices for the Comparison of Different Climates. — Since 191 6, 

 when B. E. Livingston and F. T. McLean (" A Living Clima- 

 tological Instrument," Science, N.S., xliii, 191 6, p. 362) 

 proposed a method of employing the growth-rates of standard 

 plants as indices for the comparison of different climates as 

 these influence plant-growth in general, several papers have 

 appeared which give the results of studies of plant-growth to 

 which this method has been applied successfully. 



Hildebrandt (" Leaf-product as an Index of Growth in 

 Soy-bean," Johns Hopkins Univ. Circ, March 191 7, p. 202) 

 has pointed out that the dry-weight and leaf-area of soy-beans 

 four weeks old can be determined approximately from their 

 leaflet measurements, and that this plant should be a very 

 suitable one for use as a standard plant to indicate " climatic 

 effectiveness " in the manner suggested by Livingston and 

 McLean, since its growth-rate can be approximately determined 

 from easily obtained leaf measurements. 



In an attempt to test the above method for determining 

 some of the quantitative relations between climatological 

 conditions and the growth of plants, McLean (" A Preliminary 

 Study of Climatological Conditions in Maryland, as related to 

 Plant-growth," Physiol. Res., ii, 191 7, p. 129) investigated 

 the growth of soy-beans during the first two weeks and during 

 the first month from sowing at Oakland (in the mountains of 

 Western Maryland) and at Easton (on the eastern shore of 

 Chesapeake Bay) respectively. The following growth-measure- 

 ments were made after about two weeks' growth — stem height, 

 average number of leaves per plant, average length and width 

 of mature leaves, and the " leaf-product," which is obtained by 

 multiplying length by width for each leaf. After about four 

 weeks' growth these measurements were repeated, whilst, in 

 addition, the average leaf-area and the average dry-weight of 

 tops per plant were determined. It was found that the rates 

 of growth in terms of leaf-surface and in terms of dry-weight 



