INTRODUCTION TO THE PROBLEM 



than the polyploid changes, and whilst clearly being more important, they have another 

 remarkable characteristic, namely, that the changes involved are always of a low 

 numerical order. The fundamental haploid numbers of genera known in the family are 

 limited to 5, 6, 7, 8, 9, 11 and 15, and though one might have expected that the poly- 

 ploid species could in turn have experienced comparable changes to give rise to new 

 genera with, say, basic haploid numbers of the order of 82 or 121, they do not, in actual 

 fact, appear to have done so. The two types of change would thus appear to be not 

 merely different in kind and in importance but in some sense, and for reasons which 

 are not at once self-evident, to be mutually incompatible. 



Since macroevolution would seem at the very least to require the successive origin of 

 genera rather than the blind multiplication of species, this antagonism between two 

 different types of evolutionary processes was felt to be possibly a matter of considerable 

 importance. It was also a matter on which further hght might be expected to be 

 thrown if some comparative data relating to a longer period of evolutionary activity 

 could be obtained. This was the reason why attention was first directed to the 

 Pteridophyta. 



In contrast to the Cruciferae, which may be regarded as a representative sample of 

 the dominant vegetation of to-day, all relatively new in a geological sense and still 

 in active development, the Pteridophyta are a mixture of newish forms such as some of 

 the leptosporangiate ferns, and forms of extreme antiquity, the last survivors of the 

 dominant vegetation of periods before the flowering plants existed. It is only necessary 

 to recall the obvious structural affinity between Equisetum and the Carboniferous 

 Calamites, or between the existing Lycopods and the fossil Lepidodendroids, or the still 

 more distant Devonian Drepanophycus and Silurian Baragwanathia ; or the very probable 

 affinity between the living Psilotales and the Devonian Psilophytales ; or between all the 

 ferns and the Carboniferous Coenopterideae, to realize that in this group we have a record 

 which cannot be equalled by that of any other types of living plants, and that they have 

 existed in some form since the earliest times from which the vegetation of the land has 

 been preserved. 



Here, therefore, if anywhere, should the cumulative effects of different sorts of 

 evolutionary mechanisms, working over long periods of time, be discernible. 



To the non-botanical reader, if such there be, to whom the above enumeration 

 provides merely a list of names, it may be helpful to mention briefly some of the 

 principal characteristics which unite an otherwise very varied collection of plants into 

 one great group. The members of the Pteridophyta are all vascular plants, possessing a 

 water-conducting system composed of characteristic lignified cells known as tracheids. 

 This lignified tissue differentiates them at once from the lower groups of land plants such 

 as the mosses or fungi and accounts for their relative ease of fossilization ; the tracheidal 

 structure is, however, a relatively primitive type of tissue, and is certainly more ancient 

 than the system of continuous tubes, the wood vessels, to be found in the Flowering Plants. 

 Seeds also are absent from the Pteridophyta, thereby distinguishing them from 

 both the higher groups of land plants which reproduce by seed, namely, the Gymno- 

 sperms and Flowering Plants. Reproduction in the Pteridophyta is by spores which are 

 liberated from sporangia borne on a variety of organs. They may be on the backs or 



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