CONSERVATISM AND VARIABILITY IN SEEDLING OF DICOTYLEDONS I29 
one region possesses a very much larger proportion of these slowly 
changing features than does any other. Of course we must recognize 
that a given character may be variable in one family or group and 
conservative in another. This emphasis on the single character, 
however, rather than on the whole structure, in a consideration of 
variability, is clearly in harmony with modern conceptions of heredity. 
The very fact that there are such things as "conservative char- 
acters," which for some reason have become so firmly fixed in the 
germ-plasm that they have been consistently less variable than others 
throughout large groups of organisms during evolutionary history, is, 
of course, what makes it possible for us to recognize relationship and 
to construct a ''natural system" of classification. A clear demon- 
stration of this principle is one of the chief contributions of phylogeny 
tr those sciences w^hich are concerned with the method of evolution. 
Summary 
1. The present paper is a comparative study of the structure of 
the seedling throughout the Dicotyledons, with a view to determining 
the deg.ee of conservatism which it exhibits. 
2. Certain features, notably the number of protoxylem clusters 
and the level of transition from root to stem structure, were found 
to be very variable, thus confirming the results of other investigators. 
3. Of much more constancy was found to be the relation between 
the vascular system of the hypocotyl and that of the epicotyl. Two 
main types were recognized; that in which the cotyledonary trace 
makes but a single gap in the epicotyledonary system, and that in 
which more than one gap (usually two, three or four) is produced. 
4. The venation of the cotyledon, whether of three main palmate 
veins or of a single strong midrib wuth weak side veins, was found to 
be very constant. 
5. An odd number of veins in the cotyledon (a midrib, usually 
with one or more pairs of main lateral veins) was found to characterize 
the seedling of all dicotyledons, and to distinguish it from that of 
gymnosperms with broad cotyledons. 
6. The most conservative character in the anatomy of the seedling 
is the structure of the cotyledonary trace, which throughout dicotyle- 
dons is a double bundle or a modification of it, a type universal among 
seed plants and also, at least primitively, in the foliage leaf of ferns 
and gymnosperms. 
