222 EMBRYOGENESIS IN PLANTS 



general embryological data, then, the gymnosperms could share a 

 common ancestry with other vascular plants, the concensus of morpho- 

 logical evidence pointing to ancient ferns, or possibly ancient fern 

 prototypes, as the ancestors of the Phyllospermae. In their embryonic 

 shoot apices, these gymnosperms show the eusporangiate type of 

 construction. Other gymnosperm embryos are reminiscent of those 

 pteridophytes with apices of an intermediate character between the 

 eusporangiate and leptosporangiate organisations. 



The inception of the gymnospermous phyletic lines may have 

 taken place during the Lower Devonian or even in Silurian times. In 

 a very general way we may perhaps envisage the ancestral forms as 

 members of a prototypic pteridophyte stock, some of which were in a 

 state of active mutation and therefore had a considerable potentiality 

 for morphological innovation. Some descendants from this ancestral 

 stock became elaborated as the eusporangiate ferns and others which 

 showed still more advancement along the same general lines became 

 the leptosporangiate ferns. There is evidence that some of these early 

 ferns soon advanced to the heterosporous condition, e.g. Archaeopteris 

 and Stauropteris. Among these ancient ferns, and also in species in 

 other lines in which the elaboration of the shoot rather than the leaf 

 was the conspicuous morphological innovation, were some in which 

 early genetical changes led to the differentiation of sex in separate 

 spores and sporangia, to heterospory, and to the retention of the 

 megaspore. Given these innovations at an early stage in the evolution 

 of vascular plants, important changes in the nature of the megaspore 

 and in the morphological development of the embryo would necessarily 

 follow, largely in relation to the impact of nutritional factors. And 

 assuming further genetical changes of other kinds, it becomes possible 

 to envisage how a number of lines, or phyla, of seed-bearing plants 

 could have originated from an early pteridophytic stock. 



