398 THE AMERICAN NATURALIST [Vol. XLIH 



100 times the weight of grains shelled from butt and 

 B = Total welghTorshelled corn from whole ear 



B is the percentage which the grain on butt and tip is of 

 total grain on the ear. This gives a measure of the ex- 

 tent to which ends of the ear are covered with grain. 



100 times the circumference of the cob at the middle 



c of the ear. 



Circumference of ear at middle. 

 This measures in effect the average percentage depth 

 of kernel (by difference). 



D 100 times the weight of th e cob. 



Total weight of shelled corn from the whole ear. 

 E = (circumference of ear at butt) — (circumference of 

 ear at tip). A measure of the shape of the ear. 

 F = 100— the observed percentage germination of grain 

 from this ear. 



All measurements and determinations are made on the 

 thoroughly dried ear and are recorded in centimeters and 

 grams. 



As an illustration of the way in which this index is cal- 

 culated a single example may be worked out in detail. 



Let us determine the value of I, for the ear of sweet corn 

 shown in Fig. 2, and recorded in our notes as ear No. 596. 



For this ear the following measurements were re- 

 corded : 



