EMBRYOGENESIS IN G YMNOSPERMS 185 



exalbuminous — a remarkable condition in gymnosperms (Wieland, 

 1906; Scott, 1923). 



Relationships. Chamberlain (1935) has argued that the Filicales, 

 Pteridospermae (Cycadofilicales) and Cycadales may be regarded as a 

 genetic line sufficiently well established by the morphological facts of 

 fossil and living species. He points to the very fern-like character of 

 the leaf in such genera as Stangeria which was once placed in the 

 Polypodiaceae. He also attempts to explain the very considerable 

 difference between the spore-producing members of ferns and cycads 

 on the grounds that 'extremely rapid changes may take place in 

 reproductive structures without any noticeable change in the leaves.' 

 It is certainly a fact that there are well marked similarities between 

 eusporangiate ferns, pteridosperms and cycads, while the probable 

 evolutionary sequence in the elaboration of the ovule (p. 172) will not 

 readily be set aside. But to show, on a strictly factual basis, at precisely 

 what point the divergence between the ferns and pteridosperms took 

 place, and to specify the morphological characters of the common 

 ancestor, are tasks of very great difficulty, the more so when the 

 possibilities of parallel evolution are borne in mind. 



In the foregoing survey, the nutritional status of the ovum at and 

 after fertilisation has been treated in some detail; for there is good 

 support for the view that nutrition is a major factor in the embryonic 

 development. If we assume that the cycads evolved from a eusporangi- 

 ate fern source, then among the critical genetical changes would be 

 those which determined the differentiation of a single megaspore and 

 its retention within the sporangium. Thereafter the nutritional status 

 of the prothallus, ovum and zygote would be radically modified. The 

 genetical changes envisaged, though of great importance, were not 

 necessarily of very great magnitude ; but in the circumstances indicated 

 above, they could be productive of very extensive changes in the em- 

 bryonic development. For whereas the eusporangiate fern embryo has 

 its inception in a small ovum, nourished by a small thalloid gameto- 

 phyte, the embryo of the nascent cycad would have its inception in a 

 large ovum, and v/ould draw upon the nutritional resources of a rela- 

 tively massive, vascularised sporophyte during its development. 



GINKGOALES 



It is purely as a matter of convenience that the Ginkgoales are 

 treated at this point: any relationship between this group and the 

 Cycadales is a very ancient and remote one. Chamberlain (1935), and 

 more particularly Florin (1949), have associated the Ginkgoales with 

 the Coniferophytes. 



The ovum nucleus in Ginkgo biloba is fertilised by the nucleus from 



