108 Pearl and Surface. 



Fig. 1 also shows that the gi'owth ceases rather abruptly. This 

 abruptness is still more marked in the case of individual plants than 

 in the means. Observations in the field indicate that this cessation of 

 g-rowth is very closely associated with the blooming of the tassel. These 

 observations open the question as to the cause of the termination of 

 growth in a corn plant. Stated in general terms, it seems most probable 

 that after the fertilization of the ear the growth forces of the plant 

 are directed towards the formation of the grain instead of the elongation 

 of the internodes. The immediate causes of this change in the direction 

 of growth present a physiological problem which will not be dealt with 

 in the present paper. 



Fig. 1 brings out certain differences between the several series of 

 measurements which are of interest. Considering first the leaf measure- 

 ments we note that the average height of series C which consists of 

 plants from different mother ears is considerably higher than either of 

 the other series. At the beginning of the measurements, Juue 12, this 

 series showed an average height of only 4 millimeters above that of 

 series A. Thus the subsequent difference in the height of series C was 

 probably not due to the unconscious selection of more vigorous plants. 



Series A and B run nearly parallel with the exception of the 

 points near June 29 and near the end of the growth period (July 20 

 to 27). Now it so happened that there was a marked difference in the 

 time of maturity of these two rows. Row No. 131 (series A) was the 

 earliest maturing row in the field, while row No. 133 (series B) was 

 one of the very latest rows. These differences are shown very clearly 

 in the two curves. Thus the majority of the plants in series B tasseled 

 later than those in series A. This accounts for the lag of the former 

 curve at the first of July. At the end of the growing season the tassels 

 matured much earlier in series A and these stopped growing while 

 series B was growing at almost the same rate as before. 



A similar difference is to be noted in the curves for tassel heights. 

 Thus series E, from the same plants as series B, shows a marked lag 

 behind the other two series until near the end of July. At this time 

 the curve for series E crosses both the other curves and results in 

 plants whose final average height is considerably greater than that of 

 the other series. This of course, comes about from the fact that the 

 plants in series E had a longer growing period. The difference between 

 series D and E is shown very nicely in the curves, and indicates a 

 genotypic difference in these several respects. Series D corresponded 



