THE 
AMERICAN NATURALIST. 


Vor. XXXIX. May, 1905. No. 461. 

AFFINITIES OF THE GENUS EQUISETUM. 
DOUGLAS HOUGHTON CAMPBELL. 
Tue genus Equisetum, as the survivor of an ancient race, 
which has otherwise completely disappeared, is of peculiar inter- 
est to the botanist. It is not strange that numerous attempts 
have been made to trace its history, and to determine its rela- 
tionships to the other pteridophytes, recent and fossil. 
The existing species of Equisetum, about twenty-five in num- 
ber, are distributed over nearly the whole world — Australia 
alone, among the larger bodies of land, has no species in its 
flora. Some species, e. g., Equisetum arvense, have a very wide 
range, while others, like Egazsetum giganteum, are confined to a 
smaller area. The genus is sometimes subdivided, but the 
differences are of minor importance, and more commonly all 
the species are relegated to the single genus Equisetum. 
There is a general agreement in the anatomical structure, per- 
haps the most marked difference being the variation in the 
relation of the endodermis to the vascular bundles. Thus, in 
certain species, like Eguzsetum telmateia, the ring of vascular 
bundles in the internodes is surrounded by a common outer 
endodermis, while in others, e. £» Equisetum hiemale, each 
strand has its own sheath; while in a third type, represented 
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