Isoetes, commonly known as the quillworts, is a genus of Lycopodiopsida. It is the only living genus in the family Isoetaceae and order Isoetales. , there were about 200 recognized species,
The name of the genus may also be spelled Isoëtes. The diaeresis (two dots over the e) indicates that the o and the e are to be pronounced in two distinct syllables. Including this in print is optional; either spelling ( Isoetes or Isoëtes) is correct.[ International Code of Nomenclature for algae, fungi, and plants (Melbourne Code) see section 60.6: "The diaeresis, indicating that a vowel is to be pronounced separately from the preceding vowel (as in Cephaëlis, Isoëtes), is a phonetic device that is not considered to alter the spelling; as such, its use is optional."]
Description
Quillworts are mostly aquatic or semi-aquatic in clear ponds and slow-moving streams, though several (e.g.
Isoetes butleri,
Isoetes histrix and
I. nuttallii) grow on wet ground that dries out in the summer. The quillworts are spore-producing plants and highly reliant on water dispersion. Quillworts have different ways to spread their spores based on the environment. Quillwort
leaf are hollow and quill-like, with a minute
ligule at the base of the upper surface.
arising from a central
corm. The sporangia are sunk deeply in the leaf bases. Each leaf will either have many small spores or fewer large spores. Both types of leaf are found on each plant.
Each leaf is narrow, long (exceptionally up to ) and wide; they can be either
evergreen, winter
deciduous, or dry-season deciduous. Only 4% of total biomass, the tips of the leaves, is chlorophyllous.
[ Tropical Alpine Environments: Plant Form and Function]
The roots broaden to a swollen base up to wide where they attach in clusters to a bulb-like, underground rhizome characteristic of most quillwort species, though a few (e.g. I. tegetiformans) form spreading mats. This swollen base also contains male and female sporangia, protected by a thin, transparent covering ( velum), which is used diagnostically to help identify quillwort species. They are spore. Quillwort species are very difficult to distinguish by general appearance. The best way to identify them is by examining their megaspores under a microscope. Moreover, habitat, texture, spore size, and velum provide features that distinguish Isoëtes taxa.[Isoëtes Linnaeus, Sp. Pl. 2: 1100. 1753; Gen. Pl. ed. 5, 486, 1754.] They also possess a vestigial form of secondary growth in the basal portions of its cormlike stem, an indication that they evolved from larger ancestors.[ The Formation of Wood in Forest Trees: The Second Symposium Held under the Auspices of the Maria Moors Cabot Foundation for Botanical Research]
Biochemistry and genetics
Quillworts use crassulacean acid metabolism (CAM) for carbon fixation. Some aquatic species do not have
and the leaves have a thick cuticle which prevents CO
2 uptake, a task that is performed by their hollow roots instead, which absorb CO
2 from the sediment.
This has been studied extensively in
Isoetes andicola.
CAM is normally considered an adaptation to life in arid environments to prevent water loss with the plants opening their stomata at night rather than in the heat of the day. This allows CO
2 to enter and minimises water loss. As mostly submerged aquatic plants, quillworts do not lack water and the use of CAM is considered to avoid competition with other aquatic plants for CO
2 during daytime.
The first detailed quillwort genome sequence, of I. taiwanensis,[ Secrets of quillwort photosynthesis could boost crop efficiency] These fundamental differences in biochemistry suggest that CAM in quillworts is probably another example of convergent evolution of CAM during the more than 300 million years since the genus diverged from other plants. However, they may also be because of differences between life in water and in the air.
There are species that switch from CAM to C3 photosynthesis when they go from being submerged in water to living terrestrially, and develop stomata on their leaves. Some species ( I. palmeri, I. lechleri and I. karsteni), even under aerial conditions, rarely form stomata, and in some cases ( I. triquetra and I. andina) appear to have completely lost the ability to produce stomata.[ Permian Extinctions]
Reproduction
Overview
Like all land plants,
Isoetes undergoes an alternation of generations between a diploid sporophyte stage and a sexual haploid gametophyte stage. However, the dominance of one stage over the other has shifted over time. The development of vascular tissue and subsequent diversification of land plants coincides with the increased dominance of the sporophyte and reduction of the gametophyte.
Isoetes, as members of the
Lycopodiopsida class, are part of the oldest extant lineage that reflects this shift to a sporophyte dominant lifecycle. In closely related lineages, such as the extinct
Lepidodendron, spores were dispersed by the sporophyte through large collections of sporangia called strobili for wind-based spore dispersal.
However,
Isoetes are small heterosporous semi-aquatic plants, with different reproductive needs and challenges than large tree-like land plants.
Description
Like the rest of the Lycopodiopsida class,
Isoetes reproduces with spores.
Among the lycophytes, both
Isoetes and the
Selaginellaceae (spikemosses) are
Heterospory, while the remaining lycophyte family
Lycopodiaceae (clubmosses) is
homosporous.
As heterosporous plants, fertile Isoetes sporophytes produce megaspores and microspores, which develop in the megasporangia and microsporangia.
These spores are highly ornate and are the primary way by which species are identified, although no one functional purpose of the intricate surface patterns is agreed upon.
The megasporangia occur within the outermost microphylls (single-veined leaves) of the plant while the microsporangia are found in the innermost microphylls.
This pattern of development is hypothesized to improve the dispersal of the heavier megaspore.
These spores then germinate and divide into mega- and micro- gametophytes.
The microgametophytes have antheridia, which in turn produce sperm.
The megagametophytes have archegonia, which produce egg cells.
Fertilization takes place when the motile sperm from a microgametophyte locates the archegonia of a megagametophyte and swims inside to fertilize the egg.
Outside of heterospory, a distinguishing feature of Isoetes (and Selaginella) from other , is that their gametophytes grow inside the spores.
Dispersal
Spore dispersal occurs primarily in water (
hydrochory) but may also occur via adherence to animals (
Seed dispersal) and as a result of ingestion (
Seed dispersal).
These are among the reasons suggested for the ornamentations of the spore, with some authors demonstrating that certain patterns seem well-adapted for sticking to relevant animals like waterfowl.
Another critical element of dispersal is the observation that in some species of
Isoetes, the outer coat of megaspores have pockets that trap microspores, a condition known as synaptospory.
Typically, heterospory means that colonization and long-dispersal are more difficult due to the fact that a single spore cannot grow a bisexual gametophyte and thus cannot establish a new population from a single spore as can happen in homosporous ferns.
may mitigate this issue via microspores stuck to megaspores, greatly increasing the possibility of successful fertilization upon dispersal.
Taxonomy
Compared to other genera,
Isoetes is poorly known. The first critical monograph on their taxonomy, written by Norma Etta Pfeiffer, was published in 1922 and remained a standard reference into the twenty-first century.
Even after studies with cytology, scanning electron microscopy, and chromatography, species are difficult to identify and their phylogeny is disputed. Vegetative characteristics commonly used to distinguish other genera, such as leaf length, rigidity, color, or shape are variable and depend on the habitat. Most classification systems for
Isoetes rely on spore characteristics, which make species identification nearly impossible without microscopy.
Some
split the genus, separating two South American species into the genus
Stylites, although molecular data place these species among other species of
Isoetes, so that
Stylites does not warrant taxonomic recognition.
Evolution
The earliest fossil that has been assigned to the genus is
Isoetes beestonii from the latest Permian
of New South Wales, Australia, around 252 million years ago.
However, the relationships of pre-Jurassic isoetaleans to modern
Isotetes have been regarded as unclear by other authors.
from the Late Jurassic of North America has been described as the "earliest clear example of a isoetalean lycopsid containing all the major features uniting modern
Isoetes", including the loss of the elongated stem and vegetative leaves. Based on this, it has been stated that "the overall morphology of
Isoetes appears to have persisted virtually unchanged since at least the Jurassic".
The timing of the origin of the
crown group is uncertain. Wood
et al (2020) asserted there to be no morphological features that define the major clades within
Isoetes, and no fossils are known that can be definitively assigned to the crown group.
While Wood
et al. suggested a young origin dating to the early
Cenozoic based on
molecular clock estimates
, the results were questioned by Wikström
et al. (2023) who regarded the molecular clock as providing no firm evidence for the origin time of the genus, which could date to the
Mesozoic or even the late
Paleozoic, depending on the calibration method used.
Extant species
,
Plants of the World Online accepted the following extant species:
Many species, such as the Louisiana quillwort and the mat-forming quillwort, are endangered species. Several species of Isoetes are commonly called Merlin's grass, especially I. lacustris, but also the endangered species I. tegetiformans.
Hybrids
-
I. × altonharvillii
-
I. × brittonii
-
I. × bruntonii
-
I. × carltaylorii
-
I. × dodgei
-
I. × eatonii – Eaton's quillwort
-
I. × echtuckerii
-
I. × fairbrothersii
-
I. × foveolata
-
I. × gopalkrishnae
-
I. × harveyi (syn. I. × heterospora )
-
I. × herb-wagneri
-
I. × hickeyi
-
I. × jeffreyi
-
I. × marensis
-
I. × michinokuana
-
I. × novae-angliae
-
I. × paratunica
-
I. × pseudotruncata
Fossil species
-
† Isoetes beestonii (Permian, Australia)
-
† Isoetes bulbiformis (Cretaceous, Australia)
-
† lsoetes ermayinensis (Triassic, China)
-
† Isoetes gramineoides (Triassic, US)
-
† Isoetes hillii (Miocene, Tasmania)
External links
