APOGAMOUS FERNS. THE GENERAL PHENOMENON 



which would at the same time be reproductively effective for its development by stages. 

 For similar reasons a simple qualitative mutational mechanism seems unlikely. We 

 seem rather to be dealing with the effects of a generalized disturbance of a quantitative 

 kind involving several processes. We are ignorant of the chemical nature of these 

 processes, though some further information on the mode of origin of a generalized 

 disturbance will emerge from the next chapter. 



Another point of interest to comment upon in passing is the surprising variety of 

 sporangial life histories which all these ferns possess and to note its evolutionary con- 

 sequences. If any of the spores produced by the types of sporangia described as (i), 

 (3) and (4) on pp. 163, 166 above should prove viable, a considerable saltation in 

 morphology and genetical constitution might result. That under certain circumstances 

 changes of this kind do, in fact, occur, will also be shown in the next chapter. 



SUMMARY 



A general description of the sporangial development found in all known cases of 

 apogamous ferns is given, with photographic illustrations principally selected from 

 Cyrtomium ( = Aspidium) falcatum sens. lat. There are four main types of sporangia, all 

 of which may be found together in one sorus. These may be designated according to 

 the number of spore mother cells undergoing meiosis as sixteen-celled, eight-celled, 

 eight-celled with partial cleavage and four-celled. Only the eight-celled sporangia are 

 normally effective in reproducing the species and they give rise to spores with the un- 

 reduced chromosome number. 



170 



