CHAPTER 11 



APOGAMOUS FERNS [cont,] 

 EVOLUTION OF THE SEPARATE SPECIES 



The nature and possible mode of origin of the various apogamous species listed on 

 p. 158 may next claim our attention. And here the reader should perhaps be warned to 

 expect a rather extreme measure of compression in the description of each case as it 

 arises, for only so can the results of an extended inquiry be brought within the compass 

 of a single chapter. Each species must be analysed on its own evidence without a pre- 

 conceived interpretation. But in the concise presentation of evidence it will scarcely be 

 possible to convey anything of the charm of the plants themselves or of the many details 

 of interest in their structure, behaviour or distribution which reward the investigator 

 for the labour involved in piecing the evidence together. In the colourless description 

 of species after species the impression of repetition and sameness may indeed pre- 

 dominate. This, it should be remarked, is, in a sense, what we are after. The differences 

 between species give personality to them, but only the resemblances are likely to show us 

 anything of the general principles which they all share, and the principles which we 

 may hope to discern in this context are those which underlie the development of 

 apogamy in ferns as a whole. 



Sources of information about the evolution and origin of an apogamous species are 

 of three, or perhaps four, kinds. The chromosome number alone may be highly informa- 

 tive if enough is known about the cytology of related sexual species; in the absence of 

 such knowledge chromosome number by itself is, of course, uninformalive. Of greater 

 value is observation of the mode of pairing of the chromosomes in the sixteen-celled 

 sporangia (type (i), p. 163 above). This can give a very great deal of information 

 regarding the cytogenetical make-up of the plant involved, even in the complete 

 absence of related species for comparison. Thirdly, significant differences can some- 

 times be detected in the relative frequency with which the various types of sporangia 

 appear. Observations of this kind are sometimes of value (notably in Dryopteris Borreri) 

 for confirming cases of suspected hybrids between an apogamous and a sexual species. 

 Lastly, as was shown by Dopp (1939) it is sometimes possible to synthesize hybrids 

 between apogamous and sexual species, and in such cases observation of chromosome 

 pairing should be as informative as in other cases of hybrids of known parentage. 

 Owing to the labour involved, very little work has yet been done using this last method, 

 but the first two, and to a less extent the third, have given information of considerable 

 value about almost every species. 



It will be convenient to start this chapter with Pteris cretica, which will serve both to 

 round off de Bary's account previously quoted and to supply us with a species more 

 amenable than most to cytological treatment and of interest as representing a genus not 

 of the Dryopteroid affinity. 



171 



