72 
AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
observed, for the zone within which the hexarch condition persists is narrow 
and its level is variable; and there is necessarily more or less variation in the 
level at which the sections are cut. 
2. Base of Hypocotyl. In the series of sections of the base of the hypo- 
cotyl made at Storrs, the number of double vascular strands (each of which 
is derived from a primary root bundle and corresponds to a pole of the root) 
and the number of intercalary strands were recorded separately. There is 
no difficulty in distinguishing between these two categories of bundles, 
since the latter are almost invariably without protoxylem elements and are 
irregularly placed. 
The original data for the five lines are condensed in table 2. The 
number of bundle pairs (the primary double bundles) is given in parenthesis, 
and the number of intercalary bundles, if such are present, follows the + 
sign outside the parenthesis. 
There are three outstanding features in this table. 
First, the wide range of variation in the number and in the combinations 
of primary double bundles and intercalary bundles in both normal and 
abnormal plants observed when reasonably large series of seedlings are 
sectioned. It is clear that an anatomist w^ho deals with only a few seedlings 
may obtain an altogether inadequate picture of the conditions which actually 
prevail in the species under investigation. 
Second, notwithstanding the wide range of variation there are conspi- 
cuous modal classes in both normal and abnormal seedlings. In the normal 
plants these fall in all cases on four primary double bundles, without 
intercalary bundles, or with but one intercalary bundle; and in the trimerous 
plants, on six primary double bundles without intercalary bundles. 
Third, the plants which are externally dimerous and trimerous are also 
clearly differentiated in internal morphology. The internal characters 
are, however, transgressive. It is impossible in some cases to distinguish 
from sections of the hypocotyl base alone between plants which superficially 
fall into the strictly alternative classes of dimery and trimery. 
For purposes of more detailed analysis these formulae must be split up 
into their component elements. 
A. Primary Double Bundles. The distribution of the number of primary 
double bundles in the five lines considered is shown in table 3 for dimerous 
and trimerous seedlings. These frequencies, reduced to a percentage basis, 
are represented graphically in figure 13. This shows that in all five lines 
the modal number of primary double bundles is two higher in the trimerous 
than in the dimerous plants. In the dimerous plants the modal class is in 
all cases 4; in the trimerous seedlings the modal class is 6. There is, there- 
fore, a profound reorganization in the vascular anatomy of the seedling 
upon the assumption of a trimerous external organization. 
Limiting our attention to primary double bundles and judging from modal 
classes only, an increase of fifty percent in the number of vascular elements is 
