1. Lateral branch origin:
Lateral branches of
Equisetum stems arise
between leaves (i.e. branches alternate with leaves). In other plants
with lateral (as distinguished from terminal) branching, branches originate
in
leaf
axils (i.e. in the vertex of the upper angle between a leaf and the
stem from which it arises)
2. Stomatal structure:
Dayanandan (
1977)
observed that
Equisetum species "possess perhaps the most structurally
complex stomata in the entire plant kingdom" (p. 175). The stomata
of equisetum are so unique that "a single well-preserved stomatal apparatus
is all that is needed to identify the genus
Equisetum (even the two
subgenera) from among all other living plants" (
Dayanandan, 1977). The uniqueness
of
Equisetum stomata is the result of two characteristics (
Dayanandan, 1977):
1.) The two subsidiary cells overlie the guard cells completely, whereas
in other plants the guard cells are the superficial cells.
2.) "The inner tangential wall of each subsidiary cell develops 7
to 24 ridge-like thickenings, a feature not found in any other genus."
(
Dayanandan, 1977)
These unique features are nicely shown in illustrations from the plates
accompanying Milde (
1867) of:
1.
a stoma of
E. giganteum (subgenus
Hippochaete)
2.
a stoma of
E. bogotense (subgenus
Equisetum)
note: The subsidiary cells are illustrated transparently
so that the kidney shaped guard cells can be seen beneath.
3. Elaters on spores:
Each
Equisetum spore has four
strap-like structures called
elaters
attached to the spore surface at a common point. These elaters
are hygroscopic (i.e. they expand and contract with changes in humidity)
and probably function to help disperse the spores (
Hauke, 1963).
4. Silicon requirement:
Among terrestrial plants, only the horsetails have
been definitively shown to require
Silicon
as an essential, not simply beneficial, mineral nutrient (
Epstein, 1999). The requirement
for silicon has been shown for
Equisetum arvense (
Chen and Lewin, 1969 ) and for
E.
hyemale (
Hoffman and Hillson, 1979
), so this requirement appears to hold for members of both subgenera within
Equisetum .
5. Rhizome architecture (?):
Golub and Whetmore (
1948) excavated the rhizome system of
a colony of
Equisetum arvense to a depth of 2 m and found five
successive horizontal layers of rhizomes connected by vertical
rhizomes. This rhizome system extended below 2 m, but the
investigators did not excavate further. I suspect that this
"tiered" rhizome architecture may be unique in the plant kingdom.
Indeed, other rhizomatous plants generally have but a single
horizontal rhizome system layer (
Bell and Tomlinson, 1980
).