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AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
of the hypocotyl are quite lost in the cotyledonary nodal vascular complex, 
and thus do not affect the variability of the dimerous plants; and (b) that 
the doubling of the primary epicotyledonary bundles which almost invariably 
occurs in the normal seedling may not always take place, at least not at as 
low a level as the central region of the epicotyl, in the abnormal type. 
Conclusions 
The results of the foregoing morphological and biometric analyses 
justify the emphasis at this point of certain general considerations. 
1 . External differentiation such as that which characterizes dimerous 
and trimerous seedlings of Phaseolus vulgaris is accompanied by profound 
differences in internal structure. 
2. Anatomical characters are by no means constant. On the contrary, 
they are very variable even in series of individuals which are genetically 
highly homogeneous. Morphological investigations based on limited 
series of individuals may, therefore, result in inadequate conceptions. 
3. Variation in anatomical structure is not constant for the plant as a 
whole, but may differ from region to region or from organ to organ. Thus 
in the regions of the seedling here under consideration, hypocotyl and epi- 
cotyl differ widely in the variability of bundle number. Furthermore, 
differences in variability from organ to organ or from region to region are 
not constant, but may be conditioned by other morphological features. 
To illustrate from the case in hand, the variability of bundle number of 
normal seedlings is higher in the hypocotyl than in the epicotyl. In seed- 
lings with three cotyledons and three primordial leaves, just the reverse is 
true. These differences in biometric constants are readily understandable 
in the light of a knowledge of comparative morphology. 
4. The results of this study emphasize the importance of the use of both 
biometric and comparative methods to supplement each other in any attack 
upon the problems of general morphology or of morphogenesis. 
