128 
EDMUND W. SINNOTT 
from this ancient even-bundled type to the odd-bundled angiospermous 
condition of the blade takes place by the fusion of the two central 
bundles or branches to form a midrib (Fig. 4, 5, c, d, e,f). This may 
occur in various ways and at various levels, but the result is always 
the production of a midribbed cotyledon unlike that characteristic of 
gymnosperms. The two bundles of which the midrib is composed 
often do not fuse, but run close together through the blade and diverge 
again widely near the tip of the cotyledon. 
Discussion 
It is therefore evident, from a study of only a few of its features, 
that the structure of the seedling displays various categories of char- 
acters differing in the amount of conservatism which they possess. 
The number of protoxylem groups and the level of transition (together 
with many other characters) vary from species to species and from 
genus to genus; the number of main veins in the cotyledon (whether 
one, producing a pinnate blade, or three, five or more, forming a 
palmate one), and the method of insertion of the cotyledonary traces, 
are much more constant and distinguish families or groups of families; 
the main type of venation of the cotyledon (whether midribbed or 
dichotomous) is still more constant and serves to distinguish angio- 
sperms from gymnosperms ; and finally the type of cotyledonary trace 
(the double bundle and its modifications) is essentially uniform 
throughout all seed plants. 
No general statement that the seedling as a whole is "conserva- 
tive" or "variable" can therefore well be made. Certain of its 
characters are highly conservative and certain others are highly vari- 
able, the emphasis placed on the one group or the other having led 
to the differences of opinion as to the general conservatism of the 
seedling. Indeed, it should be recognized that throughout the plant 
body it is not particular organs or regions which are less variable than 
others, but particular characters of the plant. In certain structures, 
especially the flower, these conservative characters are very noticeable, 
as in the relations of coalescence, adnation and number. Such a 
large number of other floral characters are highly variable, however, 
that it is obviously impossible to regard the flower as a whole as con- 
servative. So many conservative characters have recently been found 
in different parts of the plant as to suggest that when our knowledge 
of comparative plant anatomy is more complete, we shall find that no 
