XV S UMMA R V A ND CONCL USIONS . 5 ' 7 



ence between the two sorts of spores was less marked than 

 obtains in the other two classes. In the leptosporangiate 

 families, the Marsiliacea^ 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 chloro- 

 phyll, and is capable of independent growth ; but in the 

 Isoetaceai and Selaginelleae the formation of the female pro- 

 thallium is much more like that in the Spermaphytes, and 

 makes it extremely likely that from some such forms the 

 latter have been derived. 



The microsporangia of the Spermaphytes 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 Selaginella 

 approaches this condition, as the macrospore is retained within 

 the sporangium until germination is far advanced. The integu- 

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

 with the indusium. The young macrosporangium of Azolla is 

 extraordinarily like a developing ovule, and the closely invest- 

 ing indusium has all the appearance of an ovular integument. 

 The velum of Isoctcs is possibly of the same nature. 



The development of heterospory in several unrelated groups 

 of Pteridophytes at once suggests the possibility of a multiple 

 origin for the Sperm.aphytes. The radical differences between 

 Gymnosperms and Angiosperms, and the absence of any truly 

 intermediate forms, make it extremely probable that these two 

 great divisions have originated independently of one another, prob- 

 ably from different stocks, and it is by no means unlikely that 

 the same may be said of the Cycads, Conifers, and Gnetaceae. 



Except for their siphonogamic fertilisation, the Gymno- 

 sperms really are much nearer the Pteridophytes than they are 

 to the Angiosperms. As both the pollen tube and the seed- 

 formation are but further developments of heterospory, it is 

 quite conceivable that these might have arisen independently 

 more than once. The close resemblance between the Conifers 

 and the Lycopods, especially Selaginella, probably points to a 

 real relationship. The strobiloid arrangement of the sporo- 

 phylls, as well as the development of the prothallium and 



