On the Occurrence of Centripetal Xylem 
in Equisetum. 1 
BY 
ARTHUR J. EAMES. 
With Plate XLV. 
A MONG living Vascular Cryptogams perhaps no group more readily 
attracts attention than the Horse-tails. These plants, with their odd 
appearance and peculiar organization, clearly proclaim themselves a stock 
distinct from other recent Pteridophytes. The few species, about twenty- 
five in all, do not vary greatly among themselves, and constitute a single 
genus, Equisetum. Although reduced in numbers in the present epoch, the 
Equisetales formed a large and important part of the flora of the earth 
during Paleozoic time. In the Devonian period well-developed and 
specialized Equisetaceous forms already existed ; through the succeeding 
Paleozoic periods, especially during the Carboniferous, flourished the tree- 
like Horse-tails with secondary thickening, called Calamites. After the 
Permian age there lived other members of the Equisetales, resembling 
always more and more in succeeding time the existing genus. In spite of 
continued reduction, the living representatives of the Equisetales resemble 
the Calamites very strongly : form, structure, and anatomy, both of stem 
and reproductive branches, are in many important features very similar. 
From these ancient, arborescent Horse-tails, our present genus has without 
question descended, and of that great and flourishing group its few wide- 
spread species are the only remaining representatives. Equisetum , because 
of this isolated position among existent plants, and because of its unusual 
structure, has been the subject of much study. Particularly has its 
organization in comparison with that of Calamitean fossil remains been of 
interest since the latter have become better known from abundant speci- 
mens, and the cryptogamic rank of all has been firmly established. P"or, 
in the anatomical study of plants, the object, as in similar investigation in 
the animal kingdom, is not merely to reveal the relationship of living genera, 
but especially to fix phylogenetic lines. For such determinations, the 
preserved remains of forms that existed in long-past time must hold great 
interest ; a knowledge of their organization becomes a necessity to a clear 
understanding of their evolution. 
1 Contributions from the Phanerogamic Laboratories of Harvard University, No. 19. 
[Annals of Botany, Vol. XXIII. No. XCII. October, 1909.] 
