BENNETTITALES 



83 



No recognizable trace of a female gametophyte has been observed, 

 for all of the sections of strobili have shown either very young and 

 structureless ovules or mature seeds with fully developed embryos. 



4. The embryo 



One of the characteristic features of Bennettitales is the large 

 dicotyledonous embryo completely fiUing the embryo sac, in fact 

 filling the whole seed to the testa, with the 

 exception of the nucellar beak (figs. 67-69). 

 The numerous sections of strobili that have 

 been made show that this remarkable em- 

 bryo, much the larger part of whose bulk 

 consists of two cotyledons, was quite uniform 

 in appearance and was very favorable for 

 preservation. Unlike the embryo of any 

 other known gymnosperm, it destroyed 

 practically all of the endosperm during its 

 intraseminal development. Unusual as this 

 is, however, it is merely a difEerence of 

 degree, since the embryos of all gymnosperms 

 encroach more or less upon the endosperm. 

 The contrast in this feature between Bennet- 

 titales and Cycadofilicales is all the more 

 remarkable from the fact that the former 

 group is believed to have been derived from 

 the latter. If this view is a true one, a 

 remarkable change in the intraseminal de- 

 velopment of the embryo has been supposed 

 to have taken place during the passage of 

 cycadophytes from the Paleozoic into the 

 Mesozoic; but it should be remembered that 

 probably the embryos of Bennettitales are 

 in evidence because the strobili remained attached to the plant (p. 47). 



It has also attracted attention that this bulky, dicotyledonous 

 embryo gives no evidence of the long, slender, and tortuous suspensor 

 characteristic of all living gymnosperms except Ginkgo. Those who 

 are familiar with sections of the seeds of Cycadales and Cnuiferales, 



I J.. 



Fig. 67. — Bennettites 

 Gibsonianus: mature seed 

 showing embryo with two 

 somewhat unequal coty- 

 ledons; the testa shaded 

 with parallel radial lines; 

 at the base of the seed and 

 in the pedicel the vascular 

 supply is sho^vn. — After 

 Solms-Laubach (5). 



