APOGAMOUS FERNS. THE GENERAL PHENOMENON 



Figs. 1 70 and 1 7 1 summarize some of these facts for one particular sorus of Cyrtomium 

 Fortunei in which sixteen-celled (Fig. 170a), eight-celled (Fig. 170^) and four-celled 

 sporangia (Fig. 170c) were found in full meiosis simultaneously. This is somewhat 

 unusual, although four-celled sporangia are sufficiently common to be hsted as a separate 

 type. Their frequency almost certainly varies with the genetical condition of the plant, 

 but only once have they been seen in such relative abundance as to dominate the repro- 

 ductive picture. This was in a monstrous form of Dryopteris Borreri, almost certainly a 

 descendant of a hybrid between that species and one of its sexual relatives; but un- 

 fortunately nothing further is known about this plant except a dried frond and some 

 cytological preparations. 



It is clear that diversity of this degree between the developmental histories of the 

 different sporangia in a sorus is a matter of great cytological, genetical and physiological 



^^4$: *%ill* ' 



a 



Fig. 171. Details of chromosome pairing in two of the sporangia of Fig. 170, from sections, x 1500. 

 a A sixteen-celled sporangium showing irregular pairing, b. An eight-celled sporangium showing 

 very regular pairing, since the chromosome number in this has been doubled by the abnormal 

 premeiotic division. 



interest. Much further work is likely to be necessary before the causal mechanism will 

 be understood, but a few further facts of a genetical kind will be forthcoming at the end 

 of the next chapter, while others of the many purely cytological points of interest must 

 be left aside for consideration elsewhere, since they are irrelevant to the evolutionary 

 study which is the main purpose of this book. 



Even in this Hmited context, however, the situation revealed is of unusual interest. 

 In the life history of the apogamous ferns a very remarkable compensating process is 

 present which is curiously difficult to equate with anything of a comparable nature in 

 other groups of plants or animals. It is, moreover, a complicated process in which 

 purely cytological aberrations in the sporangia must occur in the same organism as the 

 morphological aberrations in the prothalU which result in apogamy, or the continued 

 existence of the species would be impossible. That the somatic organization of the 

 leptosporangiate ferns lends itself rather easily to these aberrations is shown by the 

 relative frequency with which identical behaviour is found in isolated examples from 

 quite unrelated genera. That the whole abnormality must have arisen in each case 

 suddenly seems almost inescapable, since it is difficult to visualize any mechanism 



169 



