112 Pearl and Surface. 



An exaniination of Fig. 3 will make it clear that in the growth 

 of the corn plant the decrease in variability is not equal in all periods 

 of growth. In other words the change in variabilitj' cannot be rei)re- 

 sented by a straight line. In general, there is a marked increase in 

 variability during the early stages of growth, i. e., up to June 19. This 

 is followed ])y a very rapid decrease up to June 29. During the 

 tasseling period there is another very marked increase in variability and 

 tills is again followed by a gradual decline until a stable condition is 

 reached at maturity. 



The cause of the increase in variability up to June 19 is not 

 clear. Series A and C show a consistant increase in variability up to 

 this date, while series B shows a decrease at the second measurement, 

 but again increases at June 19. In discussing the growth curves 

 (p. 106 foot note) it was noted that there was a slight sag in the curves 

 at June 19. It was suggested that this may have been caused by the 

 hoeing which the corn received a few days previous. Such a disturbance 

 would also tend to increase the absolute variability. Referring to Fig. 2 

 it is seen that the standard deviation increases up to June 19 and then 

 remains practically constant until the plants begin to tassel. If the 

 increased variability were due to hoeing or other environmental circum- 

 stances acting for a short time only we should not expect to get such 

 a gradual rise in these curves. It seems, therefore, that some biological 

 significance is to be attached to this increased variability in the early 

 growth period. 



It is well known that in the growth of animals there is a period 

 of increased variability following birth. This is to be explained by the 

 fact that the young animals are becoming adjusted to a new mode of 

 life. Some become adjusted earlier than others and tend to grow faster. 

 Later when all have become accustomed to the new environment they 

 tend to grow at the same rate and hence maintain the same variability. 

 In the early growth stages of the corn plant the chief growth must 

 occur in the root system. Plants must vary considerably, in the deve- 

 lopment of the roots and in the supply of food material obtained. 

 Plants which develop their root systems earliest will grow faster 

 than the others and hence increase the variability. After about the 

 middle of June all the plants have developed their root systems and 

 tend to grow at the same rate. This appears to be the most plau- 

 sible explanation of the increase in variability at the beginning of the 

 season. 



