EMBRYOGENESIS IN LHPTOS POR ANGI ATE FERNS 169 



considerable variation in the embryonic development from species to 

 species, in the Filicineae, in which there are not merely generic but also 

 familial, ordinal and sub-class dilTerences, a still wider embryonic 

 diversity might well be expected. To some extent this expectation is 

 realised. There are, indeed, difterences between eusporangiate and 

 leptosporangiatc embryos; and within some families, and sometimes 

 even genera, such difterences as the presence or absence of a suspensor 

 are found. Other dilTerences, such as the size and position of the foot 

 and the tardy organisation of the shoot apex, are of the kind already 

 encountered in Lycopodiwu. The extent of the vascular development 

 below the shoot apex also differs in different genera. No vascular tissue 

 is formed below the shoot apex in the embryo of the Marattiaceae and 

 Ophioglossaceae. In Matonia pectinata, on the other hand, a strong 

 and precocious vascular development can be discerned while the embryo 

 is still within its calyptra (Stokey and Atkinson, 1952). Of the two 

 groups of heterosporous ferns, both of which show specialisation in 

 their spore-producing members, the Marsileaceae have an embryogeny 

 which is essentially the same as that of leptosporangiatc ferns, whereas 

 the Salviniaceae have an exoscopic embryogeny. The Salviniaceae, 

 however, are highly modified in several respects. 



In general, the embryological data support the phyletic seriation 

 based on the morphology of the adult plant. Furthermore, as the 

 data of this and the preceding Chapter show, all fern embryos have 

 much in common in the major aspects of their organogenic development 

 and organisation. 



Comparison with Other Pteridophytes. In view of modern trends in 

 taxonomy, a comparison of ferns with other pteridophytes should be 

 deferred until the data for all the classes of vascular plants have been 

 reviewed. Some points of comparison with other pteridophytes may, 

 however, be considered at this point. 



An evident matter for consideration is the orientation of the embryo 

 relative to the archegonium. Equisetum, Psilotum, Isoetes, Ophio- 

 glossimi, Botrychium, Azolla and Salvinia, are all characterised by 

 exoscopic embryogeny. In Lycopodium, Selaginel/a, Botrychium obli- 

 quum, Helmimhostachys, Dauaea and Macroglossum the embryogeny is 

 endoscopic. In Marattia and some related genera, in which the arche- 

 gonia are formed on the lower side of the flat prothallus, the embryo is 

 endoscopic but has no suspensor. In leptosporangiatc ferns, the embryo 

 is neither exoscopic nor endoscopic: it is lateral, i.e. its axis is at right- 

 angles to that of the archegonium. The orientation and initial segmenta- 

 tion of the zygote appear to be primarily determined by inherent or 

 intra-prothallial factors, but the magnitude of these factors, and their 

 true significance in the embryonic development, have yet to be assessed. 



