| Lycophyte | |
|---|---|
 | |
| Collage of modern lycophytes. Upper left:Lycopodium clavatum (Lycopodiales,Lycopodioideae) Lower left:Huperzia serrata (Lycopodiales,Huperzioideae) Top right:Isoetes japonica (Isoetales) Right centre:Selaginella tamariscina Lower right:Selaginella remotifoliaSelaginellales | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Lycophytes |
| Classes | |
| |
Thelycophytes, when broadlycircumscribed, are a group ofvascular plants that include theclubmosses. They are sometimes placed in a divisionLycopodiophyta orLycophyta or in a subdivisionLycopodiophytina. They are one of the oldest lineages ofextant (living) vascular plants; the group contains extinct plants that have been dated from theSilurian (ca. 425 million years ago).[2][3] Lycophytes were some of the dominating plant species of theCarboniferous period, and included the tree-likeLepidodendrales, some of which grew over 40 metres (130 ft) in height, although extant lycophytes are relatively small plants.[4]
The scientific names and the informal English names used for this group of plants are ambiguous. For example, "Lycopodiophyta" and the shorter "Lycophyta" as well as the informal "lycophyte" may be used to include the extinctzosterophylls or to exclude them.
Lycophytes reproduce byspores and havealternation of generations in which (like other vascular plants) thesporophyte generation is dominant. Some lycophytes arehomosporous while others areheterosporous.[5] When broadlycircumscribed, the lycophytes represent a line of evolution distinct from that leading to all othervascular plants, theeuphyllophytes, such asferns,gymnosperms andflowering plants. They are defined by twosynapomorphies: lateral rather than terminalsporangia (often kidney-shaped or reniform), andexarchprotosteles, in which the protoxylem is outside the metaxylem rather than vice versa. The extinctzosterophylls have at most only flap-like extensions of the stem ("enations") rather than leaves, whereas extant lycophyte species havemicrophylls, leaves that have only a single vascular trace (vein), rather than the much more complexmegaphylls of other vascular plants. The extinct genusAsteroxylon represents a transition between these two groups: it has a vascular trace leaving the central protostele, but this extends only to the base of the enation.[6] See§ Evolution of microphylls.
Zosterophylls and extant lycophytes are all relatively small plants, but some extinct species, such as theLepidodendrales, were tree-like, and formed extensive forests that dominated the landscape and contributed to the formation ofcoal.[6]
In the broadestcircumscription of the lycophytes, the group includes the extinctzosterophylls as well as the extant (living) lycophytes and their closest extinct relatives. The names and ranks used for this group vary considerably. Some sources use the names "Lycopodiophyta" or the shorter "Lycophyta" to include zosterophylls as well as extant lycophytes and their closest extinct relatives,[7] while others use these names to exclude zosterophylls.[8][6] The name "Lycopodiophytina" has also been used in the inclusive sense.[9][10] English names, such as "lycophyte", "lycopodiophyte" or "lycopod", are similarly ambiguous, and may refer to the broadly defined group or only to the extant lycophytes and their closest extinct relatives.
The consensus classification produced by thePteridophyte Phylogeny Group classification in 2016 (PPG I) places all extant (living) lycophytes in the classLycopodiopsida.[11] There are around 1,290 to 1,340 such species.[12][13][11] For more information on the classification of extant lycophytes, seeLycopodiopsida § Classification.
A majorcladistic study of land plants was published in 1997 by Kenrick and Crane.[1] In 2004, Crane et al. published some simplifiedcladograms, based on a number of figures in Kenrick and Crane (1997). Their cladogram for the lycophytes is reproduced below (with some branches collapsed into 'basal groups' to reduce the size of the diagram).[14]
| panlycophyte |
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In this view, the "zosterophylls" comprise aparaphyletic group, ranging from forms likeHicklingia, which had bare stems,[15] to forms likeSawdonia andNothia, whose stems are covered with unvascularized spines or enations.[16][17] The genusRenalia illustrates the problems in classifying early land plants. It has characteristics both of the non-lycophyterhyniophytes – terminal rather than lateral sporangia – and of the zosterophylls – kidney-shaped sporangia opening along the distal margin.[18]
A rather different view is presented in a 2013 analysis by Hao and Xue. Their preferred cladogram shows the zosterophylls and associated genera basal to both the lycopodiopsids and the euphyllophytes, so that there is no clade corresponding to the broadly defined group of lycophytes used by other authors.[19]
| "lycophytes" of other authors |
Some extinct orders of lycophytes fall into the same group as the extant orders. Different sources use varying numbers and names of the extinct orders. The following phylogram shows a likely relationship between some of the proposed Lycopodiopsida orders.[citation needed]
Within the broadly defined lycophyte group, species placed in the classLycopodiopsida are distinguished from species placed in theZosterophyllopsida by the possession ofmicrophylls. Some zosterophylls, such as theDevonianZosterophyllum myretonianum, had smooth stems (axes). Others, such asSawdonia ornata, had flap-like extensions on the stems ("enations"), but without any vascular tissue.Asteroxylon, identified as an early lycopodiopsid, had vascular traces that extended to the base of the enations. Species in the genusLeclercqia had fully vascularized microphylls. These are considered to be stages in the evolution of microphylls.[20]
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