Feb., I92I] HARRIS AND OTHERS — SEEDLINGS OF PHASEOLUS 
99 
have entered it from the hypocotyl.^^ Immediately above the cotyledons 
the six remaining bundles approach, closing the cotyledonary gaps and form- 
ing a ring, the six members of which almost immediately divide, giving rise 
to the modal number, 12, which persists throughout the length of the 
epicotyl. It is apparently the disappearance of the intercalary bundles as a 
conspicuous feature of the topography which results in the lowered variabil- 
ity of bundle number in the epicotyl as compared with the hypocotyl. 
If this conclusion be true, we should find the least difference in the 
variability of number of bundles in the central regions of the first two inter- 
nodes in the lines in which intercalary bundles are least conspicuous as a 
feature of the vascular topography. As a matter of fact, this condition is 
strongly supported by the results for the five lines investigated. Turning 
back to table 6, showing the constants for number of intercalary bundles, 
we note that lines 75, 93, and 98 have on the average from 0.60 to 0.83 
intercalary bundle per (normal) plant. These are the lines showing a 
relative difference of 55 to 65 percent in the standard deviations as com- 
pared with 36 percent in line 143 with an average of 0.31 intercalary bundle, 
and of only 0.73 percent for line 139 which has an average of only 0.07 
intercalary bundle per plant. The differences in the coefficients of variation 
for hypocotyl and epicotyl are from —9.3 to — 11. 4 percent in the three lines 
with from 0.6 to 0.8 intercalary bundle per plant, —7.5 percent in line 143 
with an average of 0.31 intercalary bundle, and only —1.7 percent in line 
139 with an average of only 0.07 intercalary bundle. 
In the trimerous seedlings the relationship between the variation of the 
number of bundles in the hypocotyl and in the epicotyl is just the reverse of 
that found in the normal type. Variability as measured by the standard 
deviation is significantly higher in the epicotyl of all lines studied. The 
same is true if the coefficient of variation be used as a measure of variability, 
although the differences for lines 75 and 143 are not large. 
The anatomical explanation of this fact seems to be found in the pecu- 
liarities of behavior at the cotyledonary node. As pointed out above 
(p. 70), the epicotyledonary ring is typically made up of nine strands 
instead of the six characteristic of the normal plant. There is, therefore, 
in the modal case an increase of fifty percent in the number of bundles in 
the epicotyledonary ring of the trimerous plant as compared with the 
dimerous plant. Many of these bundles, but not all, divide to form the 
bundle system characteristic of the main course of the epicotyl. It is this 
variability in the extent of division of the bundles of the epicotyledonary 
ring which, in connection with the low variability of the hypocotyl due to 
the formation of but few intercalary bundles (except in lines 139 and 143, 
where the number is about the same in normal and abnormal seedlings), 
brings about the great variability in the bundle number of the mid-region 
This statement is based on a more detailed anatomical study of a portion of the 
seedlings. 
