401] F. M. Hildebrandt 208 
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’ 
