298 



MORPHOLOGY OF GYMNOSPERMS 



giganlea. Among the Cupressineae the range is two to five, two being 

 usual in Thuja, Callitris, Widdringtonia, and Actinostrohus; two or 

 three in Lihocedrus; and the whole range of two to five in the various 

 species of Jimiperus and Cupressus. Among the Araucarineae, 

 Agathis has two cotyledons, and Araucaria two to four. These num- 

 bers will doubtless vary upon wider acquaintance with the forms, but 

 they serve to illustrate that the fluctuations occur not only between 

 tribes, but also within tribes and genera, and even species. 



Figs. 367, 368. — Araucaria hrasiliana: a proembryo filling the entire egg; only 

 the shaded cells belong to the embryo proper, the wedge-shaped mass of cells below 

 being the embryo cap; fig. 368, later stage, showing the wedge-shaped mass of cells 

 being thrust aside; X153. — After Strasbueger (25). 



The explanation of this variation is difficult, but there seem to be 

 two alternatives : either the polycotyledonous condition is derived from 

 the splitting of two ancestral cotyledons (141, 154), or the dicotyle- 

 donous condition is derived from the fusion of more numerous 

 cotyledons. In other words, it is a question whether polycotyledony 

 is primitive or derived. 



The argument for the derivation of polycotyledony from dicotyle- 

 dony has been developed by Hill and De Fraine (141, 154). This 

 belief dates from Adanson and Jussieu, and was supported by 



