Because the three giant horsetails appear similar in overall habit, and because Equisetum species exhibit considerable morphological plasticity ( Hauke, 1963 ; Schaffner, 1928 ), more stable anatomical characters are used to distinguish between the species (see table below). The most important diagnostic characters (branch ridge patterns, stomatal patterns, and endodermal patterns) can only be observed under high magnification and many of characters of E. x schaffneri overlap with its parent species ( Hauke, 1963 ). Therefore, accurate identification of giant horsetails can be problematic. As a result, both dried specimens in herbaria (Stolze, 1983 ; Husby, personal observation) and living specimens in botanical gardens (Moyroud, 1991 ; Husby, personal observation) are often misidentified (for further discussion of misidentfication, see Notes, Observations, and Ideas ).
Below is a synopsis of the key characteristics useful for distinguishing between the giant horsetails. The information is from Dr. Richard L. Hauke's ( 1963 ) monograph of Equisetum subgenus Hippochaete , unless otherwise referenced. Dr. Hauke's work was based upon intenstive study and analysis of a large number of herbarium specimens.
| E. giganteum |
E. myriochaetum |
E. x schaffneri |
|
| Type locality |
Jamaica (or Hispaniola 1 ) |
Vera Cruz, Mexico |
Orizaba, Mexico |
| Distribution 2 ( more detailed distribution information ) |
Cuba, Jamaica, Hispaniola, Central
American (Guatemala, El Salvador, Honduras, Nicaragua ( Luis Diego Gómez,
1985 ), Costa Rica, Panama), and South America(Venezuela,
Colombia, Ecuador, Peru, Bolivia, Brazil, Paraguay, Uruguay,
Chile, and Argentina) |
Mexico, Central America, Colombia,
Ecuador, and Peru |
Mexico3
, Central America, Colombia, Venezuela 3 , Ecuador, Peru |
| Habitat |
"...along rivers or in swampy
places, usually shaded" |
"In swampy places or along rivers
and streams, ususally at the edge of or within forested areas..." |
"...in springy, marshy places,
or along rivers and streams." |
| Altitudinal range |
150 - 2600 m |
200 - 3000 m |
500 - 3000 m |
| Maximum stem height 4 |
5 m |
8 m |
4.5 m |
| Maximum stem diameter 4 |
2.4 cm |
1.8 cm |
2.2 cm |
| Branch ridge pattern |
"...square or flattened in profile,
in Brazil, Peru, and southern South America tending to be irregular." |
sawtooth pattern oriented apically |
"...sawtooth to irregular." |
| Main stem stomatal pattern |
"...in bands of 3-4 (rarely 2-3
or 4-5)" |
"Stomata in one line on each
side of the groove." |
"...in bands of 1-2 (occassionally
2-3)" |
| Endodermal arrangement |
"Cross section with separate
endodermis around each vascular bundle (individual endodermises)" |
"...double common..." |
individual endodermises |
| Sheath teeth persistence |
"mostly persistent" ( Tryon, R. M. and Tryon,
A. F., 1982 ) |
"...usually thin and brown to
white, drying and breaking off to produce a clipped appearance at
the top of the sheath, or (especially in South America), the bases
or much of the teeth persisting" |
"...mostly smoothly shed." |
| Cones |
have a "short (about 0.5 mm)
but distinct apiculum" |
"blunt, or the branch cones frequently
with a slight apiculum" |
"acute or with slight apiculum" |
1. Proctor ( 1985 ) and Lellinger ( 1989 ) gave the type locality as "presumably...Hispaniola", whereas Hauke (1963) gave the locality as Jamaica.
2. Only two other Equisetum species reach the tropics in Latin America, E. hyemale (which reaches Guatemala) and E. bogotense (From Costa Rica to Chile and Argentina) ( Moran and Riba, 1995 ; Stolze, 1983 ).
3. Interestingly, this hybrid is found in Mexico, where
its parent, E. giganetum, is not known to be present,
and in Venezuela, where its other parent E. myriochaetum
is not known to be present. Hauke (1963) hypothesized that
this unexpected phenomenon may be due to occassional viable spores
being produced by E. x schaffneri and the resulting plants
persisting vegetatively. Viable spores have been observed for
other Equisetum subg. Hippochaete hybrids ( Krahulec et al., 1996
), so this hypothesis appears plausible.
Rolla M. Tryon and Alice F. Tryon ( 1982
) have suggested that E. x schaffneri may not be a hybrid at
all, but rather an intermediate form of a single polymorphic horsetail
species E. giganteum (with the currently recognized E.
giganteum and E. myrichaetum representing extremes
of this species). However, Hauke(
1963 ) notes in support of E. x schaffneri's hybrid
status that its spores are not viable in contrast to those of the other
two species. The status of E. x schaffneri could probably
be settled by attempting to synthesize E. x schaffneri from
a cross of E. giganteum and E. myriochaetum in
the laboratory, as has been done for other subgenus Hippochaete
hybrids ( Ducket, 1979
). Molecular taxonomic analysis would likely
shed additional light on this question.
4. These numbers are based on collector's notes and measurements of herbarium specimens (See How large are giant horsetails? )