INTRODUCTION TO THE PROBLEM 



apospora Cropper or ' Asplenium Filix-foemina var. clarissima ]onts\ In all these cases, 

 whether genetically normal or not, the result of the aposporous development is a 

 prothallus with the same chromosome number as the parent sporophyte. Such pro- 

 thalh, in the case of the horticultural varieties, are generally apogamous, and nuclear 

 change in them is ehminated from the life cycle. Where apospory has been induced in 

 genetically normal plants, the sexual function of the induced prothalh may be un- 

 impaired; the consequence of this is a polyploid series. 



A special case of abnormal life history to which the name apospory cannot strictly be 

 applied is sometimes found in association with permanent apogamy in ferns. Pteris 

 cretica and Cyrtomium falcatum are the classic examples, and in these the morphology of 

 the spores is retained, but the meiotic process is rendered ineffectual by a sequence of 

 unusual and highly interesting cytological happenings in the sporangium. Spores with 

 the unreduced chromosome number are formed, and these give rise to apogamously 

 reproducing prothalli. Again the nuclear basis for aUernation of generations has been 



eliminated. 



Some photographs in illustration of the phenomena of apogamy and apospory are 

 appended in Figs. 7 and 8, and examples of all these types of abnormal life history will 

 be examined below. Some of them are very instructive from an evolutionary point of 



view. 



With this amount of introduction we may turn to the plants themselves. Fig. 9 is a 

 reproduction, translated into English where necessary, of a fairly recent summary 

 of the probable relationships of the main living and fossil types. It is not necessary to 

 analyse it in detail,, for certain obvious general conclusions can be obtained at a glance. 

 The preponderance of extinct compared with living forms is striking and would doubt- 

 less be still more marked if account could be taken of extinct forms which have perished 

 without trace. It is also obvious that, in the living forms, we are dealing with the end- 

 products of phyletic lines which have long been separated. Of their somewhat im- 

 poverished modern representatives only the ferns can compare at all favourably with the 

 weahh of the known or presumed ancestral types, and the ferns {'Pteropsida') enormously 

 outnumber all the other living representatives added together. 



The living members of the Pteridophyta may be hsted in the somewhat more 

 famihar terminology adopted by Bower as follows : 



PsiLOTALEs: two living genera, Psilotum and Tmesipteris, both mainly tropical or sub- 

 tropical, each with one, or at most two, species. 



Lygopodiales (Clubmosses) : four living genera. Lycopodium, world-wide distribution, 

 about 185* species. Phylloglossum confined to Australia and New Zealand, one 

 species. Selaginella, world-wide distribution, over 700 species. Isoetes, world-wide 

 distribution, 50 species. 



Eq,uisetales (Horsetails) : one living genus, Equisetum, world-wide distribution, except 

 Australasia, about 25 species. 



* These numbers and the following are quoted from Willis's Dictionary (1925). 



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