230 HEREDITY AND EVOLUTION IN PLANTS 



tube, but with the primordia still evident. The cotyle- 

 donary zone continues its growth until a tube of con- 

 siderable length is developed, leaving the apex of the pro- 

 embryo depressed. At this stage either one of two things 

 may occur. As the cotyledonary zone continues to grow, 

 the two primordia on the rim of the tube may continue 

 to develop equally, forming two cotyledons ; or one of the 

 primordia may cease to grow, resulting in an embryo of 

 only one cotyledon; in other words, the entire cotyledo- 

 nary zone may develop under the guidance of only one 

 growing point. It is not that one cotyledon is eliminated, 

 but the whole growth is diverted into one. There thus 

 develops what appears to be an "open sheath" and a 

 ' 'terminal " cotyledon. 



In other words, according to Coulter and Land, mono- 

 cotyledony is not the result of the fusion of two cotyledons, 

 nor of the suppression of one; but is simply the con- 

 tinuation of one growing point on the cotyledonary ring, 

 rather than a division of the growth between two growing 

 points. In a similar way, polycotyledony is the appear- 

 ance and continued development of more than two growing 

 points on the cotyledonous ring (Cf. p. 222, and Fig. 104). 



We are not in possession of enough facts to construct 

 a genealogical tree showing the derivation of Mono- 

 cotyledons from Dicotyledons, nor the derivation of the 

 original Angiosperm stock, but the table of Arber and 

 Parkin (Table IV, p. 231) shows in a very general pro- 

 visional way a possible course of events, and the ap- 

 proximate geological period when the various advances 

 were made, beginning with the Paleozoic Cycadofilices 

 (Pteridosperms) . 



The first step in the immediate evolution of the Angio- 



