INTRODUCTION 



archegonium is formed, there may be in some cases a consider- 

 able number, and owing to the large amount of nutritive 

 material in the spore, in case an archcgonium is not fertilised, 

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

 for a long time at the expense of the food materials that 

 normally 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 

 Marsiliaceae, 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 heterpsporous forms the first divisions in the 

 germinating spore take place while it is still within the 

 sporangium, and may begin before the spore is nearly fully 

 developed. In other cases the sporangia become detached 

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

 sporangium, 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 Spermaphytes, 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 Azo/la, for example, and that the seed is 

 simply a further development of the same structure. 



