Equisetaceae - horsetail family
This family is composed of 15 species in one genus, Equisetum, worldwide.

Mount Desert Island is home to 3 species in 1 genus. Click on a link below or scroll down for more information.
   Equisetum (3 species)
      Equisetum arvense - field horsetail (occasional [see note 1 at bottom of page])
      Equisetum fluviatile - river horsetail (uncommon)
      Equisetum sylvaticum - wood horsetail (common)

Equisetum (horsetail)
Mount Desert Island's three species of horsetails can be distinguished from each other by their branching. Equisetum fluviatile is either unbranched or has short, seemingly random branches that are somewhat stubby and appear mutant. Equisetum arvense has a whorl of branches at each node (except on the fertile stems described below), as does E. sylvaticum. The branches of the latter, however, are themselves branched. Species of Equisetum that are unbranched are often called scouring-rushes.

Equisetum arvense (field horsetail) may be mistaken for two different species if one observes the stems in early April and then later in the season. This is because the stems are dimorphic; that is, they have two different forms. In early April (on Mount Desert Island) the fertile stems appear. They are flesh-colored and have no branches.


At their apex is a strobilus, a reproductive structure that contains spores.


Later in April the photosynthetic stems appear. They are green and have a whorl of branches at each node. These branches are very short at first but soon grow to be several inches long.


The reproductive stems soon wither, and only the green, branched stems are evident during the rest of the season.

Equisetum fluviatile (river horsetail) - [information to be added]

Equisetum sylvaticum (wood horsetail) is a rather delicate looking horsetail.


It is, as noted above, the only species of Equisetum on MDI whose branches are themselves branched.

Note:
   1. Frequency designations are from the paper "Vascular flora of the Acadia National Park region, Maine" by Craig W. Greene, Linda L. Gregory, Glen H. Mittelhauser, Sally C. Rooney, and Jill E. Weber, published in the spring 2005 issue (vol. 107, No. 930) of Rhodora: Journal of the New England Botanical Club.