224 
GENETICS: PEARL AND SURFACE 
of puberty to the growth of children. This does not necessarily mean 
that the two processes are analogous physiologically, but it is a fact of 
interest and worth further consideration from a physiological point of 
view. 
It appears probable that the corn plant grows in a series of cycles. 
Each cycle is characterized by the special development of one set of 
organs. They are, in order, the root cycle, the leaf cycle, the tassel 
cycle, and the ear cycle. 
With these general facts regarding the growth of the maize plant in 
hand the analysis of the variation in terms of individuals was undertaken. 
Individual plants and groups of plants having the same relative size at 
one stage of their growth, were followed as individuals through the re- 
FIG. 1. DIAGRAM TO ILLUSTRATE THE DIVISION OF A FREQUENCY CURVE INTO QUIN- 
TILES. IN TmS DIAGRAM A NORMAL CURVE HAS BEEN DIVIDED INTO 5 EQUAL AREAS 
BY THE FOUR PERPENDICULAR SOLID LINES. THE FREQUENCY IN ANY ONE OF THESE 
AREAS IS EQUAL TO THAT IN ANY OTHER. 
maining growth stages. We have endeavored to ascertain how such 
plants are distributed as to relative size in the successive growth stages 
and to discover the factors determining these distributions. 
To study these questions it is necessary to have a measure of the 
relative size of the plants at each growth stage. For this, each distribu- 
tion was divided into live equal areas or quintiles. In any distribution 
the relatively small plants are in quintile I and the relatively large ones 
in quintile V. Figure 1 illustrates the exact meaning of a quintile. 
The problem was first approached by studying the quintile distribu- 
tion of all the measurements, throughout the season, of a group of plants 
starting in a given quintile. It is shown that there is a strong tendency 
for the plants to remain in or near the quintile in which they started. 
As a measure of this tendency use is made of the root-mean-square devi- 
