Editing Equisetum (section)

==Description==
[[File:Equisetum arvense 001.JPG|right|thumb|''[[Equisetum arvense]]'' (field horsetail)]]
''Equisetum'' [[Leaf|leaves]] are greatly reduced and usually non-[[Photosynthesis|photosynthetic]]. They contain a single, non-branching [[vascular bundle|vascular trace]], which is the defining feature of [[microphyll]]s. However, it has recently been recognised that horsetail microphylls are probably not ancestral as in [[lycophyte]]s (clubmosses and relatives), but rather derived [[adaptation]]s, evolved by reduction of [[wikt:megaphyll|megaphylls]].<ref>{{cite journal |last=Rutishauser |first=R |s2cid=4658142 |title=Polymerous leaf whorls in vascular plants: Developmental morphology and fuzziness of organ identities |journal=International Journal of Plant Sciences |volume=160 |issue=S6 |pages=S81–S103 |date=November 1999 |pmid=10572024 |doi=10.1086/314221|bibcode=1999IJPlS.160S..81R }}</ref>

The leaves of horsetails are arranged in [[Whorl (botany)|whorls]] fused into [[node (botany)|nodal]] sheaths. The stems are usually green and photosynthetic, and are distinctive in being hollow, jointed and ridged (with sometimes 3 but usually 6–40&nbsp;ridges). There may or may not be whorls of branches at the nodes.<ref name = Streeter>Streeter D, Hart-Davies C, Hardcastle A, Cole F, Harper L. 2009. ''Collins Flower Guide''. Harper Collins {{ISBN|9-78-000718389-0}}</ref> Unusually, the branches often emerge below the leaves in an internode, and grow from buds between their bases.

[[Image:Horsetail vegeative stem.JPG|120px|thumb|Vegetative stem:<br/>B = branch in whorl<br/>I = internode<br/>L = leaves<br/>N = node]]
[[Image:Equisetum braunii (strobilus), Portland, Oregon.jpg|thumb|Strobilus of ''[[Equisetum braunii]]'', terminal on an unbranched stem]]
[[Image:Microscopic view of Equisetum in Japan one 20thmm graduation.jpg|thumb|Microscopic view of ''[[Equisetum hyemale]]'' (rough horsetail) (2-1-0-1-2 is one [[millimetre]] with {{frac|1|20}}th [[graduation (scale)|graduation]]).<br/>The small white protuberances are accumulated [[silicate]]s on [[Cell (biology)|cell]]s.]]

===Spores===
The [[spore]]s are borne under [[sporangiophore]]s in [[strobilus|strobili]], cone-like structures at the tips of some of the stems. In many species the cone-bearing shoots are unbranched, and in some (e.g. ''[[Equisetum arvense|E.&nbsp;arvense]]'', field horsetail) they are non-photosynthetic, produced early in spring. In some other species (e.g. ''[[Equisetum palustre|E.&nbsp;palustre]]'', marsh horsetail) they are very similar to sterile shoots, photosynthetic and with whorls of branches.<ref name=Stace>{{cite book|last=Stace|first=C. A.|author-link = Stace, C. A.|year=2019|title=New Flora of the British Isles|edition=Fourth|publisher=C & M Floristics|location = Middlewood Green, Suffolk, U.K.| isbn=978-1-5272-2630-2}}</ref>{{rp|12–15}}

Horsetails are mostly [[Spore|homosporous]], though in the field horsetail, smaller spores give rise to male [[Prothallus|prothalli]]. The spores have four [[elater]]s that act as moisture-sensitive springs, assisting spore dispersal through crawling and hopping motions after the [[Sporangium|sporangia]] have split open longitudinally.<ref>{{cite news |last1=Gill |first1=Victoria |title=Horsetail plant spores use 'legs' to walk and jump |url=https://www.bbc.com/news/science-environment-24025365 |work=[[BBC News]] |date=11 September 2013}}</ref> They are photosynthetic and have a lifespan that is usually two weeks at most, but will germinate immediately under humid conditions and develop into a [[gametophyte]].<ref>{{cite journal | pmc=4469821 | date=2015 | last1=Zhao | first1=Q. | last2=Gao | first2=J. | last3=Suo | first3=J. | last4=Chen | first4=S. | last5=Wang | first5=T. | last6=Dai | first6=S. | title=Cytological and proteomic analyses of horsetail (Equisetum arvense L.) spore germination | journal=Frontiers in Plant Science | volume=6 | page=441 | doi=10.3389/fpls.2015.00441 | doi-access=free | pmid=26136760 | bibcode=2015FrPS....6..441Z }}</ref>

===Cell walls ===
The crude cell extracts of all ''Equisetum'' species tested contain [[mixed-linkage glucan : xyloglucan endotransglucosylase]] (MXE) activity.<ref>{{Cite journal | last1 = Fry | first1 = S. C. | last2 = Mohler | first2 = K. E. | last3 = Nesselrode | first3 = B. H. W. A. | last4 = Frankov | first4 = L. | title = Mixed-linkage -glucan:xyloglucan endotransglucosylase, a novel wall-remodelling enzyme from ''Equisetum'' (horsetails) and charophytic algae | doi = 10.1111/j.1365-313X.2008.03504.x | journal = The Plant Journal | volume = 55 | issue = 2 | pages = 240–252 | year = 2008 | pmid = 18397375| doi-access = free }}</ref> This is a novel enzyme and is not known to occur in any other plants. In addition, the cell walls of all ''Equisetum'' species tested contain [[mixed-linkage glucan]] (MLG), a [[polysaccharide]] which, until recently, was thought to be confined to the [[Poales]].<ref>{{cite journal|pmid=18393951 | doi=10.1111/j.1469-8137.2008.02435.x|title=Mixed-linkage (1→3,1→4)-β-d-glucan is a major hemicellulose of ''Equisetum'' (horsetail) cell walls|year=2008|last1=Fry|first1=Stephen C.|last2=Nesselrode|first2=Bertram H. W. A.|last3=Miller|first3=Janice G.|last4=Mewburn|first4=Ben R.|journal=New Phytologist|volume=179|pages=104–15|issue=1|doi-access=free| bibcode=2008NewPh.179..104F}}</ref><ref>{{cite journal|pmid=18284587 | doi=10.1111/j.1365-313X.2008.03453.x|title=Mixed-linkage (1→3),(1→4)-β-d-glucan is not unique to the Poales and is an abundant component of ''Equisetum arvense'' cell walls|year=2008|last1=Sørensen|first1=Iben|last2=Pettolino|first2=Filomena A.|last3=Wilson|first3=Sarah M.|last4=Doblin|first4=Monika S.|last5=Johansen|first5=Bo|last6=Bacic|first6=Antony|last7=Willats|first7=William G. T.|journal=The Plant Journal|volume=54|issue=3|pages=510–21|doi-access=free}}</ref> The evolutionary distance between ''Equisetum'' and the Poales suggests that each evolved MLG independently. The presence of MXE activity in ''Equisetum'' suggests that they have evolved MLG along with some mechanism of cell wall modification. Non-''Equisetum'' land plants tested lack detectable MXE activity. An observed negative correlation between [[Xyloglucan endotransglucosylase|XET]] activity and cell age led to the suggestion that XET is catalysing endotransglycosylation in controlled wall-loosening during cell expansion.<ref>{{cite journal|last1=Simmons|first1=Thomas J.|last2=Fry|first2=Stephen C. |url=http://www.biochemj.org/content/474/7/1055|title=Bonds broken & formed during the mixed-linkage glucan: xyloglucan endotransglucosylase reaction catalysed by Equisetum hetero-trans-β-glucanase|date=2017|journal= Biochemical Journal|doi=10.1042/BCJ20160935|pmc=5341106|pmid=28108640|volume=474|issue=7|pages=1055–1070|access-date=2019-07-17}}</ref> The lack of MXE in the Poales suggests that there it must play some other, currently unknown, role. Due to the correlation between MXE activity and cell age, MXE has been proposed to promote the cessation of cell expansion.{{citation needed|date=August 2016}}