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 who 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 



