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 iembryogeny) 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.^ 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 fiat end the peripheral cells remain in a state of 

 more active division than do the central cells, and form 

 what is known as the coiyledonary zone. In this zone two 

 1 The above description closely follows Coulter and Land (1914). 



A, monocotyledon- 

 ous embryo; B, 

 dicotyledonous em- 

 bryo. (Redrawn 

 from photo by W. 

 J. G. Land.) 



