Notes, Observations, and Ideas Regarding the Giant Horsetails

Contents:

1. Phototropism and negative geotropism of stems
2. Branch orientation
3. Rhizome orientation
4. Stem, branch, and rhizome allometry
5. Regularity of branching
6. Number of leaves per whorl (a record?)
7. Stem biomechanics (Capability of stems to be free-standing)
8. Rhizome biomechanics
9. Rhizome hairs
10. Intercalary meristems and stem and rhizome elongation
11. Hydathodes and guttation
12. Misidentification of living plants and herbarium specimens
13. Gametophyte sexuality controversy
14. Giant Equisetum  in the classroom
15. Increasing horticultural interest in Equisetum
16. Scientific interest in Equisetum


Phototropism and negative geotropism of stems

   Developing stems and branches of giant Equiseta have strong phototropic responses.  A stem will begin to lean towards a light source, though I have not yet measured the speed of this response.  I grow some giant horsetails under fluorescent lights indoors and when new stems grow above the light source, they will begin to reorient and eventually start growing down or the the side towards the light source.  Miguel Porto (personal communication) has also observed this phenonomenon.  As the stem reorientation occurs, the stem (of E. giganteum and E. x schaffneri, but apparently not E. myriochaetum) eventually loses its upright stiffness and begins to lay on its side below the light source, exposing the whole stem length to the artificial light source.  Hence, the phototropic response can apparently be stronger than the negative geotropic response of giant Equisetum stems.  In the wild, this marked phototropism probably helps position newly developing stems in the best available light environment.  This would be an important adaptation because giant horsetails are quite susceptible to shading by other vegetation ( Hauke, 1969a ).  

    When stems become mature, the photoropic response diminishes markedly and is eventually lost.  Srinivasan et al. ( 1979 ) observed an analogous decrease in negative geotropic response of maturing Equisetum hyemale stems and attributed this decrease to increasing silica deposition in the once-supple intercalary meristems.  The loss of phototropic response is probably due to the same mechanism of increasing silica deposition in the nodal regions.

    Srinivasan et al. ( 1979 ) found that the negative geotropic response of Equisetum hyemale stems was mediated by auxin.  It would be interesting to investigate the hormonal basis of the phototropic response in Equisetum species and to discover how the negative geotropic and phototropic responses interact.


Branch orientation

    The orientation of giant horsetail branches can be quite variable although all the branches in a whorl generally assume the same orientation.  The branches may be ascending (i.e. orthotropic) or  descending or horizontal (plagiogropic) or grow essetially straight out from the stem at an acute or obtuse angle (for example, see illustration of E. x schaffneri in Mickel and Beitel ( 1988 )).  Light probably has an important influence on branch orientation due to the strong phototropic response of developings stems and branches.  For example, I seen the branch whorls on an E. myriochaetum clone grow in a strongly ascending manner underneath artificial overhead light and grown descending when the stems grow above the level of the lights.  However, proximity of a whorl to the stem apex also seems to affect branch orientation.  Whorls nearer the stem apex sometimes seem to have less of a tendency to ascend.  For example, a tall stem photographed by Miguel Porto in Costa Rica exhibited ascending proximal branch whorls and markedly less ascending distal branch whorls (See " In the Land of the Giant Horsetails ", part 2, second photo from top).  Perhaps this pattern is due to an effect akin to apical dominance which is induced by auxins produced at the stem apex.  I have also observed (in several herbarium specimens and the one living fertile clone I have seen) that fertile whorls (i.e. whorls with branches bearing cones) of E. myriochaetum are often strongly ascending, whereas sterile whorls lower down on the same stem are much less strongly ascending (or actually plagiotropic).  Another branch orientation pattern that sometimes occurs involves branches emerging in a downward orientation and then becoming ascending.  This phenomenon can be seen in this giant horsetail in Peru and one of the stems photographic in " In the Land of the Giant Horsetails ", part 3, sixth photo from top.  This branch orientation pattern demonstrates that the final orientation of branches can be quite different than the initial angle at which the branches emerge from the stem.  

    Furthermore, there appears to be a significant genetic component to branch orientation.   I have seen an E. giganteum clone and an E. myriochaetum clone growing in the same greenhouse under identical lighting conditions and the branches of the E. giganteum were strongly ascending while those of the E. myriochaetum were nearly horizontal or descending.  
 

Rhizome orientation
   
    Because I grow Equisetum species in translucent containers, I have had the opportunity to observe rhizome growth.  When growing rhizome tips reach the walls of containers, they generally turn downards.  However, rhizome shoots occassionally grow at an upward angle through the substrate surface, turn green, and redifferentiate into aerial stems.  On rare occassions, a rhizome will actually grow at an upward angle through the substrate surface and then turn back downard and grow back into the substrate.  My E. giganteum clone from Jamaica and my E. x schaffneri clone from Peru  have each formed an "arch" of rhizome above the substrate that resulted from just such a change in apical orientation.  My other horsetails (including E. myriochaetum) have not exhibited this phenomenon so far.  Interestingly, McClure ( 1966 ) observed a somewhat similar phenomenon in bamboo.  McClure noted that when a bamboo rhizome tip emerges from the soil (such as when growing through the side of a bank), it at first exhibits a negative geotropic response and attempts to reenter the soil.  If this is unsuccessful, then the rhizome tip will eventually turn upward and differentiate into an aerial stem (culm).  However, the phenomenon I have observed in E. giganteum actually involves a rhizome growing up vertically or at an angle out of the substrate and then reorienting, rather than simply emerging horizontally into the air as in McClure's bamboo observation.


Stem, branch, and rhizome allometry

    Stem size in a giant horsetail clone can vary considerably.  For example, a small rhizome division might begin sending up stems only a few millimeters in diameter and new rhizomes will likewise be rather small.  After a plant has become established in a large container, the size of the rhizomes and stems will gradually increase as the clone accumulates more energy reserves.  However, if a plant is kept in a small container, then its rhizomes and stems will be limited in size.  The relationship between stem diameter and height has not been investigated.  Taller stems will likely tend to be larger than shorter ones.  However, because the tallest stems tend not to be self-supporting ( Hauke, 1963 ; John Mickel, personal communication), it seems plausible that they will not necessarily be thicker than many stems that aren't as tall.  It would be very interesting to compare the maximum height attained by the exceptionally thick-stemmed north Chilean E. giganteum plants with the maximum heights of more typically sized clones in a controlled environment.

    My informal observations suggest that, unlike aerial stems, branches do not vary much in size.  Even on small stems, branches are the same size as they are on larger stems.  Therefore, one generally sees proportionally fewer branches per node on small stems than large stems.


Regularity of branching
  
    Although giant  Equisetum species are generally regularly  (i.e. radially symmetrically) branched ( Hauke, 1963 ),  there nevertheless appears to be  some variation in  branching  regularity.  Richard Moyroud ( 1991 )  notes that  a  specimen  of   E. myriochaetum from Mexico growing at the New York Botanical Garden  appeared to be  "less regular in branching habit".   Similarly  Richard Moyroud and I have observed that  a Jamaican clone of Equisetum giganteum  currently in cultivation is  distinctly irregular in branching  habit.  More typically,  a   Mexican clone of E. myriochaetum  at the University of California Botanical Garden is very regular in branching  habit as is the populations of E.  giganteum in northern Chile and the  giant horestail population along the road between Tarapoto and and Moyobamba, Peru.  The regularly branched clones are certainly the most striking and beautiful due to their remarkable symmetry.


Number of leaves per whorl (a record?)

    Smith (1981) mentioned that Equisetum giganteum stems can have as many of 56 aerial stem ridges.  Since the number of ridges corresponds to the number of leaves in a whorl (Jenman, 1898), this would indicate that E. giganteum can have whorls with 56 leaves (perhaps even more in the case of the exceptionally large plants in northern Chile).  Michael J. Shields (personal communication) has suggested that E. giganteum may well attain the highest number of leaves per whorl of all extant vascular plants.


Stem biomechanics (Capability of stems to be free standing)

    Spatz et al. ( 1998 ) studied the biomechanics of a giant horsetail that they designated as Equisetum giganteum(?)1 .  The investigators found that E. giganteum(?) has a turgor-based support structure that is distinct from the lignification-based support found in hollow-stemmed grasses. The results of the study demonstrated stems taller than ~2.0-2.5 m were not mechanically stable (i.e. they buckle without external support).  Taller stems required support of neighboring stems (facilitated by intertwined side branches) to remain upright.  The stems of the clone they studied were relatively thin,  only reaching ~1 cm at the widest.  It would be valuable to know whether the plant studied by Spatz et al. (1998 ) was truly Equisetum giganteum because many of the plants in cultivation appear to be E. myriochaetum.    Speck et al. ( 1998 ) found that another member of the subgenus Hippochaete , E. hyemale, had some biomechanical properties quite distinct from those of the E. giganteum(?) that Spatz et al. ( 1998 ) studied.


Rhizome biomechanics

    Treitel ( 1943 ) carried out some interesting experiments on the rhizome biomechanics of several Equisetum species from both subgenus Hippochaete and subgenus Equisetum.  He investigated rhizome elasticity, breaking stress, and breaking strain.  The results of the study indicated that different Equisetum species often had markedly different stress-strain curves.  The investigator attributed these differences to differences in amount of strengthening elements in rhizomes (e.g. suberization and schlerenchyma cells) that are in turn due to differences in soil environment (wetter vs. drier soil) and to rhizome maturity.  The two species for which Treitel calculated the modulus of elasticity had very different values of this parameter, with E. fluviatile (subgenus Equisetum) having much more elastic rhizomes than E. scirpoides (subgenus Hippochaete ).  He attributed E. fluviatile 's greater elastiscity to its much wetter habitat and hence its lesser need for strengthening tissues in its rhizomes.  It would be interesting to learn how the biomechanics of the rhizomes of the giant horsetails compare with those of other Equisetum species and how these rhizome mechanical properties correlate with soil properties and rhizome anatomy and allometry.


Rhizome hairs
   
    I have occasionally noticed very fine hairs on rhizomes and on the bases of very young  aerial stems of giant Equisetum plants.  These hairs can be especially apparent near the substrate surface on stems and rhizome tips that have newly-emerged from the substrate.  Other Equisetum species, such as E. telmateia and E. diffusum have much more "hairy" rhizomes than the giant Equiseta.  These hairs are all very fine and remind me of root hairs.  I wonder whether theses hairs may pehaps play a role in the absorption of water or minerals?


Intercalary meristems and stem and rhizome elongation

    Most stem lengthening in Equisetum species is produced by intercalary meristems above each node and this growth pattern produces a relatively rapid lengthening of the stem ( Stewart and Rothwell, 1993 ).  Interestingly, this is a process similar to that which occurs in bamboo, which also have stems that lengthen primarily via intercalary meristem growth ( Judziewicz et al., 1999 ).  The nature of stem elongation in Equisetum is easy to observe.  In developing stems, the region of the internode close above a node is noticeably lighter green than the internode further away from the node.  This is because the internode tissue nearer the node is more recently generated by the intercalary meristem and is therefore less mature than tissue farther away.  For an example of this phenomenon, see the lower nodes of this young E. giganteum stem from Panama.

    Two types of elongation meristems are found in Equisetum rhizomes.  French (1984) found that the three subgenus Equisetum species he studied had uninterrupted meristems "charactersized by acropetal internode maturation".  In contrast, the four species of subgenus Hippochaete that he studied had intercalary meristems in their rhizomes.


Hydathodes and guttation
    Equisetum species, like many other plants, have hydathodes (Johnson, 1936 ).  In Equisetum, these are structures that are associated with veins on the leaf and/or sheath ( Johnson, 1936 ) and serve as exit routes for xylem water when  there is positive hydrostatic pressure (called root pressure) in the xylem ( Nobel, 1991 ).  The exit of this xylem water, termed guttation, results in the formation of small droplets in the vicinity of the hydathodes.  Guttation occurs when transpiration is nil, such as under very high relative humidity conditions or at night ( Nobel, 1991 ).  This phenomenon may serve to prevent flooding of mesophyll tissue in leaves (Johnson, 1936 ).  Johnson (1936 ) studied the anatomy of hydathodes in many Equisetum species and noted that the hydathodes of E. giganteum are "confined to the leaf and sheath bases."

    Equisetum species readily exhibit guttation, most markedly in young stems ( Johnson, 1936 ).  I frequently observe guttation in the developing stems of my cultivated giant horsetails (and other Equiseta ) at night and in the early morning (when evaporative potential is low).  This usually takes the form of droplets forming near the tips of stems and branches.  During late June 2002, I observed a very impressive and beautiful exhibition of guttation in young Equisetum palustre stems on a very humid overcast day in the Upper Peninsula of Michigan.  The giant horsetails also exhibit impressive guttation.
 

Stem developmental abnormalities

    Schaffner ( 1933 ) discussed six types of rare developmental abnormalities that he had observed in Equisetum stems during his years of studying the genus.  These are:  1.) Flexuous (i.e. sinuous) stems, 2.)  Short internodes (i.e. nodes are bunched together on the stem), 3.) dichotomously branching stems, 4.) Stems with spiral sheaths, 5.) Sterile and semisterile cones, and 6.) Proliferated cones (i.e. cones that continue to grow vegetatively from their tips).  Of these interesting abnormalities, I have only observed two in my limited experience with giant horsetails.  I once observed a new shoot on a specimen of what was probably E. myriochaetum with a single spiral sheath winding around it.  Unfortunately, I only saw this stem when it was a few centimeters long and when I next had an opportunity to visit the plant (several months later) the spiral sheathed stem was nowhere to be seen.  Apparently, this stem had perished.  I have also observed a trichotomously branched stem in Equisetum gignateum.


Misidentification of herbarium specimens and living plants

    Historically, there appears to have been a tendency to label most large, regularly branched horsetails from Latin America as Equisetum giganteum.  I have seen many giant horsetail herbarium specimens, originally identified as E. giganteum, that were later determined to be E. myriochaetum (and sometimes E. x shaffneri ).  An example of this is an herbarium specimen of E. myriochaetum from Guatemala that was labeled E. giganteum until Dr. Richard Hauke determined the specimen to be E. myiochaetum .  Stolze ( 1983 ) also mentioned the frequent mislabeling of giant Equisetum herbarium specimens.  The problem of mislabeling is largely due to the fact that the giant horsetails look quite similar in overall habit and the diagnostic characters that distinguish them are mostly microscopic (see discussion of taxonomy ) (Hauke, 1963 ).  

    A similar tenedency to identify all giant horsetails as E. giganteum appears to hold sway for living accessions in many botanical gardens.  For example, the two clones labeled Equisetum giganteum that Moyroud ( 1991 ) obtained from botanical gardens (one in garden the U.S. and one in Europe) were both determined to be E. myriochaetum (Moyroud, 2001, personal communication).  Equisetum myriochaetum appears to be by far the most common species in cultivation in the temperate northern hemisphere.  Many of the plants labeled E. giganteum in U.S. botanical gardens appear to be divisions of a clone of E. myriochaetum that was collected in Ecuador in 1987 and brought to the University of California Botanic Garden . Unfortunately, until recently, this clone was labeled as E. giganteum .  In early 2001, Dr. Alan R. Smith determined that the clone is actually E. myriochaetum. (Holly Forbes, 2002, personal communication).  Furthermore, after receivieng a division of a clone labeled E. giganteum (originally collected in Peru) from the Royal Botanic Gardens Edinburgh I determined that this clone is most likely E. x schaffneri (the plant has stomata in bands of 1-2 and irregular branch ridge tubercles). Confusion regarding the identities of cultivated plants has lead to uncertainty about the implications of the results of scientific studies utilizing those plants (see " Gametophyte sexuality controversy " and "Capability of stems to remain free standing ").  Because accurate identification of giant horsetails generally requires microscopic examination, it might be wise for field researchers to bring portable microscopes (and possibly staining materials for examining endodermal patters) for on-site determination of specific identity.


Gametophyte sexuality controversy

    Hauke (1969b ; 1963 ) studied gametophyte development in the giant horsetails in laboratory culture.  He observed that Equisetum giganteum gametophytes, unlike all other known species, are "normally bisexual" (i.e. produce anteridia and archegonia simultaneously). All other Equisetum species, including E. myriochaetum, are normally unisexual (i.e. producing only antheridia or archegonia at a given time, but not simultaneously) ( Hauke, 1969b ; 1980 ).  However, Duckett and Pang ( 1984 ), challenged Hauke's (1969b ; 1963 ) findings with experimental observations that gametophytes of E. giganteum are unisexual like those of other Equisetum species.  Duckett and Pang ( 1984 ) attributed the differences between their findings and Hauke's to their practice of growing single gametophytes in individual culture vessels, in contrast to Hauke's mass culture technique of growing multiple gametophytes in the same container.  Duckett and Pang ( 1984 ) hypothesized that Hauke's ( 1969b ; 1963 ) culture technique stimulated E. giganteum gametophytes to quckly switch from producing antheridia to archegonia and that this rapid shift was then misinterpreted by Hauke as simultaneous production of antheridia and archegonia.  However, it turns out that Duckett and Pang ( 1984 ) had not confirmed the identity of the giant horsetail from which they obtained their spores.  Hauke ( 1985 ) obtained a specimen from that plant from the Royal Botanic Gardens Edinburgh and determined that it was actually E. myriochaetum .  Hence, Duckett and Pang's ( 1984 ) observations were actually in agreement with Hauke's ( 1969b ) observations of E. myriochaetum gametophyte development.  As a result, Hauke's (1969b ; 1963 ) observation still stands that the sexual behavior of E. giganteum gametophytes is unique in the genus Equisetum .  Unfortunately, it appears that no researchers have revisited this question and investigated the behavior of true E. giganteum gametophytes using the single gametophyte culture method favored by Duckett and Pang ( 1984 ).  Such an investigation could lay to rest any lingering doubt as to Hauke's (1969b ; 1963 ) original conclusions.  This unfortunate controversy demonstrates the great importance of accurate identification of giant Equisetum clones used in scientific research.


Giant Equisetum  in the classroom

    Giant horsetails and other Equisetum species seem to have excellent potential as experimental plants for demonstrating botanical phenomena in the classroom.  Schaffner ( 1938 ) discussed the suitability of using stem cuttings of E. hyemale , rooted in water, to demonstrate root hairs.  He mentioned that the hairs are quite long (5-6 mm or longer) and are readily visible without magnification.  Page ( 2002 ) noted that the root hairs of Equisetum species are "unusally persistent" in water culture.  This is certainly the case for giant Equisetum species (Chad Husby, personal observation).  Johnson ( 1937 ) noted the suitability of many Equisetum species for demonstrating the phenomenon of guttation .  She mentions that placing a jar or other clear covering (to increase relative humidity) over  Equisetum plants that are amply supplied with substrate moisture will cause drops of xylem liquid to form around the leaves and sheaths.  This method could be applied to larger plants by using large transparent plastic bags.  As I discussed above, the developing stems of giant horsetails (and other Equisetum sepcies) exhibit marked phototropism and Srinivasan et al. (1979 ) observed that Equisetum stems also exhibit a negative geotropic response.  Hence, giant Equiseta would make an excellent model plant for demonstrating both types of these stem responses to environmental stimuli.  Furthermore, developing stems of giant horsetails would provide an excellent demonstration of the activity of intercalary meristems .


Increasing horticultural interest in Equisetum

    Although Equisetum species have long been perceived as "weeds" by gardeners (this being largely attributable to unfavorable experiences with the only really "weedy" member of the genus, E. arvense ), there appears to be a growing appreciation of the ornamental value of many members of the genus.  This increase in appreciation of Equiseta as desirable plants is especially apparent in the U.K., where several articles have been published in the prominent horticulture magazine, "The Garden" ( Ardle, 2001 ; Lancaster, 1997 ), and in "The Times of London" ( Anderton, 1999 ).  The most recent such article, by Ardle ( 2001 ), actually mentions the giant horsetails as worthy horticultural subjects.  The recent launching of a website for the U.K. National Collection of Equisetum , maintained by Anthony Pigott, will undoubtedly serve to foster further interest in the U.K. and Europe.   Although enthusiasm for horsetails in the United States appears to be much less than than in the U.K., the rise in popularity of water gardening in the U.S. has led to  interest in E. hyemale and E. scirpoides as ornamental pond plants (Husby, personal observation).  I would be interested to learn how Equisetum species are perceived in other parts of the world.


Scientific interest in Equisetum
   
    Unfortunately, scientific interest in Equisetum , appears to be on the wane in the botanical community (Husby, personal observation).  Perhaps this is due to increasingly scarce funding to study such relatively obscure plants.  However, as shown by the recent study of Marsh et al. ( 2000 ) , even though Equisetum species often make up only a small part of a plant community, they can still play an important role in ecosystem functioning.  Futhermore, by learning more about the how the biology of Equisetum species compares to the biology of other pteridophytes and seed plants, we can learn  to better appreciate the functional and structural diversity of the Plant Kingdom.  Hopefully, a few botanists can be found who will continue in the tradition of the Equisetologists Drs. Richard L. Hauke, Julius Milde, and John H. Schaffner,  by helping us better understand this unique and fascinating genus.

1.  The actual specific identity of the clone they studied is uncertain because the authors do not mention making any attempt to verify the Frieburg Botanical Garden's labeling of the clone.  See discussion of " misidentification "
If you have any comments or questions, please contact the author, Chad Husby ( chad.husby@fiu.edu or husby.1@osu.edu )

© Chad E. Husby 2003

Last updated March 19, 2003

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