CHAPTER 13 



THE GENUS EQUISETUM 



Turning now away from the ferns to consider some representatives of the other great 

 groups included with them in the Pteridophyta we may take as our first example of a 

 ' microphyllous ' (small-leaved) group, the genus Equisetum. 



The very curious appearance of the Horsetails has already to some extent been illu- 

 strated in Chapter 2 and other examples will ht found on p. 211 and on p. 226. So 

 striking are they that most European languages are rich in popular names for them, and 

 they are familiar playthings of most country-bred children, at least in the west of the 

 Continent. Their relationship to the ferns is by no means obvious to the layman, and it 

 rests, indeed, primarily on community of hfe history and on the structure of the sexual 

 generation, details which can scarcely be observed outside a laboratory. An affinity with 

 the Lycopods or 'clubmosses' is perhaps easier to detect, both groups possessing small 

 leaves in contrast to the 'megaphyllous' (large-leaved) ferns, but closer inspection re- 

 veals so many differences in anatomy, morphology, form and position of the reproduc- 

 tive structures of both generations that the relatively isolated position of the Horsetails 

 has long been recognized by assigning them to an independent group, the Equisetales, of 

 equal rank to the Fihcales (ferns) and Lycopodiales (clubmosses) and of an antiquity at 

 least commensurate with these. 



The Equisetales have existed almost from the earliest times at which fossil plants have 

 been preserved. From beginnings traceable with difficulty in the rocks of the Devonian 

 they achieved in the Coal Measure period a burst of evolutionary development which 

 they never repeated. The Calamites of the Coal Measure forests were large trees present 

 in great numbers, both of individuals and of species, while the variety of cone structures 

 actually found must denote the existence of a far greater number of generic types which 

 are lost and which greatly exceed anything that the one living genus might lead one to 

 expect. That the range of early genera may have included Equisetum itself side by side 

 with its arboreal relatives is suggested by the finding of a small fertile specimen of 

 Carboniferous age described under the name o{ Equis elites Hemingwayi by Kidston (1892), 

 in which the cone structure is identical with that of the living genus except for the larger 

 size of the sporangia and scales. Unfortunately, the specimen only shows the external 

 form of the plant and not the anatomical structure, without which the generic identity 

 with living species is uncertain; nevertheless, the evidence is clear enough to suggest 

 that Equisetum itself may extend in unbroken sequence from the Coal Measures to the 

 present day, and if indeed it did so, we should have to regard it not merely as the sole 

 living representative of a large and ancient group but as itself the oldest living genus of 

 vascular plants known to science. For this reason alone a comparison of its cytological 

 constitution with that of the recent ferns is likely to offer points of unusual interest. 



In contrast to the age of the genus and the wealth of extinct species traceable at many 

 geological levels above the Carboniferous period the existing species, some two dozen in 



Mpc 209 ^* 



