401] F. M. Hildebmndt 203 



criterion of plant growth, dry weight of tops, can be obtained 

 but once for any individual plant, since the plant is destroyed 

 during the determination. Also, the accurate determination 

 of leaf area is very difficult unless the plants are destroyed. 

 On the other hand, as McLean has emphasized, leaf dimen- 

 sions may be obtained repeatedly during the development 

 of the plant without serious danger of inflicting injury. It 

 may therefore be of considerable importance if leaf area, and 

 even dry weight, can be satisfactorily estimated for soy-bean 

 by the employment of the leaf-product as an index. 



The general procedure followed in obtaining the observa- 

 tional data upon which are based the results here considered 

 has been described by McLean, who conducted all the cultures 

 personally (see his paper cited above). For the present pur- 

 pose it is sufficient to state that cultures, each of 6 soy-bean 

 plants, were started from the seed every two weeks through- 

 out the summer season, at each of the nine stations employed, 

 and that plant measurements were taken after about two and 

 after about four weeks of growth. Dry weight and actual 

 leaf area were determined only for the four-week periods, the 

 plants being then destroyed, but the lengths and breadths 

 of all leaflets were obtained for both the two-week and the 

 four-week periods. Consequently, to study the correlation 

 between total leaf area and total leaf -product per plant, only 

 the four-week data are available, and these are the ones here 

 considered. Thus, each of the nine stations is represented 

 by a series of consecutive four-week culture periods, each 

 period overlapping on to the next preceding and next follow- 

 ing one. A large number of different sets of climatic con- 

 ditions is thus represented by the whole series for the nine 

 stations, which includes 97 4- week culture periods in all. 



The leaf measurements here dealt with have all been ob- 

 tained by the writer from photographic contact prints made 

 by Dr. McLean from the fresh leaves immediately after these 

 were removed from the plants. Areas were obtained from the 

 same prints with a planimeter. The leaflet length was taken 

 from tip to junction of blade and petiole for each leaflet, and 



