6o8 
STRUCTURE AND LIFE HISTORIES 
much significance should be attached to that fact has 
been disputed by students of morphology. 
The gap between the stamen of Cycadeoidea and the 
type characteristic of modern Angiosperms is partially 
bridged by the genus Williamsonia (which has simple 
vs. pinnately compound stamens), and by another genus, 
Wielandiella, both older genera than Cycadeoidea. From 
this it has been inferred that the Bennet- 
titales are a lateral branch, further re- 
moved than their ancestors from the direct 
evolutionary stock of the Angiosperms. 
522. Origin of Monocotyledons. If 
the earliest Angiosperms were dicotyle- 
dons, as seems to be the case, the mono- 
cotyledons were probably derived from 
the dicotyledons by a process of simplifi- 
cation. Much light has been thrown on 
this question by a study of the develop- 
ment of the embryos (embryogeny) of 
certain plants. The case of Agapanthus 
umbellatus L'Her. (Fig. 430), a South 
African plant of the Lily family, may 
be taken as illustrating the nature of 
the evidence derived from embryogeny. 
The sequence of events is as follows. 1 As the mas- 
sive proembryo enlarges, the root-end elongates, thus re- 
maining narrow and pointed ; while the shoot-end widens, 
becoming relatively broad and flattish. At this broad 
and flat end the peripheral cells remain in a state of 
more active division than do the central cells, and form 
what is known as the cotyledonary zone. In this zone two 
1 The above description closely follows Coulter and Land (1914). 
FIG. 430. Aga- 
panthus umbellatus. 
A, monocotyledon- 
ous embryo; B, 
dicotyledonous em- 
bryo. (Redrawn 
from photo by W. 
J. G. Land.) 
