THE EVOLUTION OP PLANTS 



229 



habit. They are therefore in all probability adaptations 

 to that habit. Two more — stem anatomy and the ap- 

 parently terminal cotyledon in the embryo — may be 

 considered as direct consequences of such adaptations; 

 the stem anatomy acquiring its peculiar features from the 

 insertions of numerous broad-based leaves on a squat 

 subterranean axis, and the embryonic 

 cotyledonary number arising from the 

 congenital fusion of two ancestral cotyle- 

 dons. The seventh character — trimerous 

 floral symmetry — bears no obvious re- 

 lation to the geophilous habit, but is not 

 inconsistent with it." 



Recent evidence as to how monocoty- 

 ledony may have been derived from 

 dicotyledony has been furnished by a 

 study of the embryogeny of Agapanthus 

 umbellatus L'Her (Fig. 108), a South 

 African plant of the Lily family. 



The sequence of events is as follows.^ -"oj^ocotyledonous 



^ embryo. B,aicoty- 



As the massive pro-embryo enlarges the ledonous embryo, 

 r o t - e n d • elongates, thus remaining (Redrawn from 

 narrow and pointed; while the shoot-end P^° J J^^ ^- J" ^• 

 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 cotyledonary 

 zone. In this zone two more active points (frimordia) 

 appear and begin to develop. Soon the whole zone is 

 involved in more rapid growth, resulting in a ring or 



' The above description closely follows Coulter and Land. The origin 

 of monocotyledony. Bot. Gaz. 57: s°9-5i8, June, 1914- 



Fig. 108.— 

 Agapanthus 

 umbellatus . A, 



