THE ANATOMY OF A HYBRID EQUISETUM^ 
* Ruth H olden 
The genus Equisetum has been the object of a considerable 
amount of study on the part of a great many botanists. As a rule, 
each has chosen certain features for especial investigation, with the 
result that each has adopted a different scheme of classification de- 
pending on what characters he himself deemed of most significance. 
Consequently, there is not a little divergence of opinion as to what 
features are of specific and what are of varietal importance. The 
confusion is increased by the large number of forms which differ from 
each other only in slight, though seemingly constant characters. 
For example, Mr. A. A. Eaton, who was the systematic authority on 
this genus in America, concluded that the true Equisetum hiemale L. 
is not found on this side of the water, but is replaced by E. hiemale 
var. ajffine (Engelm.) A. A. Eaton which differs in that the ridges of 
the stem are rounded instead of biangulate. As he expressed it 
(Fern Bulletin, v. H, p. 7) ''the subgenus Hippochete is a multitude 
of races of which species are the centers of variation." Accordingly, 
he established a number of so-called varieties, which, as far as external 
features are concerned, bridge over by gradual steps all the gaps 
between different species. Thus, E. arvense L. grades into E. littorale 
Kiihl through E. arvense var. diffusum and E. littorale var. arvensiforme. 
Duval Jouve (Histoire Naturelle des Equisetum de France), on the 
other hand, has done much to show that what have been described as 
separate varieties, or even separate species, are often merely fluctu- 
ations dependent on some environmental difference. For example, 
E. fluviatile L. was made a different species from E. limosum L. by 
Linnaeus, because the former has numerous branches, while the latter 
has none. Duval Jouve, however, claims that one grows in deep water 
and one in shallow, and that both species, with all the intermediate 
varieties, spring from the same rhizome. Milde (Monographia Equise- 
torum) goes further still, and considers that since E. ramosissimum 
Desf., E. hiemale, E. trachyodon A. Br., E. variegatum Schleich., 
^ Contributions from the Phanerogamic Laboratories of Harvard University, 
No. 65. 
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