I INTRODUCTION 7 



the prothallium, even if it does not form chlorophyll, may grow 

 for a long time at the expense of the food materials that nor- 

 mally are used by the developing embryo. In strong contrast 

 to the slow growth and late development of the reproductive 

 organs in the homosporous forms, most of the heterosporous 

 Pteridophytes germinate very quickly. The Marsiliacese, in 

 which the female prothallium is extremely reduced, show the 

 opposite extreme. Here the whole time necessary for the 

 germination of the spores and the maturing of the sexual 

 organs may be less than twenty- four hours, and within three or 

 four days more the embryo is completely developed. 



That heterospory has arisen independently in several widely 

 separated groups of Pteridophytes is plain. The few genera 

 that still exist are readily separable into groups that have 

 comparatively little in common beyond possessing two sorts of 

 spores; but each of these same forms shows much nearer 

 affinities to certain widely separated homosporous groups. 



In some of the heterosporous forms the first divisions in the 

 germinating spore take place while it is still within the sporan- 

 gium, and may begin before the spore is nearly fully devel- 

 oped. In other cases the sporangia become detached when 

 ripe, and the spore (or spores), still surrounded by the spo- 

 rangium, falls away from the sporophyte before germination 

 begins. In these respects the heterosporous Pteridophytes 

 show the closest analogy with the similar processes among the 

 lower Spermatophytes, where it has been shown in the most 

 conclusive manner that the ovule with its enclosed embryo-sac 

 is the exact morphological equivalent of the macrosporangium 

 of Selaginella or Azolla, for example, and that the seed is 

 simply a further development of the same structure. 



