xvii SUMMARY AND CONCLUSIONS 603 



separately from forms like the Marattiaceae, and not from the 

 Osmundaceae as is usually assumed, although there is evidence 

 of a not remote relationship with the latter. 



The affinities of the Gleicheniaceae Cyatheaceae and Polypo- 

 diaceae are very apparent. The Hymenophyllaceae, while prob- 

 ably of pretty ancient origin, form an aberrant group which 

 has become a good deal changed on account of its peculiar habit 

 of life. The Polypodiaceae are par excellence the modern Fern 

 type. 



The two heterosporous families, the Marsiliaceae and Sal- 

 viniacese, are independent developments. The former are prob- 

 ably allied to the Schizaeaceae, the latter to Cyatheaceae or 

 Hymenophyllaceae. 



The development of heterospory in the different groups of 

 the Pteridophytes is of especial interest, from its bearing upon 

 the question of the origin of the Spermatophytes. That hetero- 

 spory arose in a number of widely remote groups is unques- 

 tionable. While among the living Pteridophytes it is confined 

 to the Ferns and Lycopods, the very perfect fossil remains of 

 Calanwstachys show that heterospory was also developed' in 

 the Equisetineae, although apparently the difference between 

 the two sorts of spores was less marked than obtains in the other 

 two classes. In the leptosporangiate families, the Marsiliaceae 

 and Salviniaceae, although there is great reduction in the size of 

 the prothallium, its development is essentially the same as in 

 their homosporous relatives, and the female prothallium, if 

 unfertilised, usually develops chlorophyll, and is capable of 

 independent growth; but in the Isoetaceae and Selaginellaceae 

 the formation of the female prothallium is much more like that 

 in the Spermatophytes, and makes it extremely likely that from 

 some such forms the latter have been derived. 



The microsporangia of the Spermatophytes do not differ 

 essentially from those of the heterosporous Pteridophytes, and 

 the microspores (pollen spores) are shed before germination. 

 The macrospore (embryo-sac), however, is retained within the 

 macrosporangium (ovule), where it remains during the whole 

 period of germination. Among the Pteridophytes Sclaginclla 

 approaches this condition, as the macrospore is retained within 

 the sporangium until germination is far advanced. The 

 integument of the ovule is, with very little question, homologous 

 with the indusium. The young macrosporangium of Azolla is 



