Isoetes

Isoetes, commonly known as the quillworts, is the only extant genus of plants in the family Isoetaceae, which is in the class of lycopods. There are currently 192 recognized species,[1] with a cosmopolitan distribution but with the individual species often scarce to rare. Some botanists 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.[2] Species of Isoetes virtually identical to modern forms have existed since the Jurassic epoch.[3]

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.[4]

Quillworts are mostly aquatic or semi-aquatic in clear ponds and slow-moving streams, though several (e.g. I. butleri, I. 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 leaves are hollow and quill-like, with a minute ligule at the base of the upper surface.[5]: 7  arising from a central corm. Each leaf is narrow, 2–20 centimetres (0.8–8 in) long (exceptionally up to 100 cm or 40 in) and 0.5–3.0 mm (0.02–0.12 in) wide; they can be either evergreen, winter deciduous, or dry-season deciduous. Stomata are absent, yet the leaves have a thick cuticle which prevents CO2 uptake, a task that is performed by their hollow roots instead, which absorbs CO2 from the sediment.[6] Isoetes andicola is unusual in being the only known terrestrial vascular plant that take up all its CO2 through the roots. Only 4% of total biomass, the tips of the leaves, is chlorophyllous.[7] The roots broaden to a swollen base up to 5 mm (0.2 in) 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 heterosporous. 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 will distinguish Isoëtes taxa.[8] 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.[9]

Compared to other genera, Isoetes is poorly known. Even after studies with cytology, scanning electron microscopy, and chromatography, species are difficult to identify and their phylogeny is disputed. Vegetative characters commonly used to distinguish other genera, such as leaf length, rigidity, color, or shape are variable and depend on habitat. Most classification systems for Isoetes rely on spore characteristics, which make species identification nearly impossible without microscopy.[10]

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 reflect 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.[11] However, Isoetes are small heterosporous semi-aquatic plants, with different reproductive needs and challenges than large tree-like land plants.

Like the rest of the Lycopodiopsida class, Isoetes reproduces with spores[12]. Among the lycophytes, both Isoetes and the Selaginellaceae (Spike Mosses) are heterosporous, while the remaining lycophyte family Lycopodiaceae (Club Mosses) is homosporous.[13] As heterosporous plants, fertile Isoetes sporophytes produce megaspores and microspores, which develop in the megasporangia and microsporangia.[14] 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.[15] The megasporangia occur within the outermost microphylls (single-veined leaves) of the plant while the microsporangia are found in the innermost microphylls.[16] This pattern of development is hypothesized to improve the dispersibility of the heavier megaspore[12]. These spores then germinate and divide into mega- and micro- gametophytes.[14][17][18] The microgametophytes have antheridia, which in turn produce sperm.[18] The megagametophytes have archegonia, which grow the egg cells.[18] 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 other lycophytes) from ferns, is that their gametophytes grow inside the spores.[14][18][16] This means that the gametophytes never leave the protection of the spore that disperses them, cracking the perispore (the outer layer of the spore) just enough to allow the passage of gametes. This is fundamentally different from ferns, where the gametophyte is a photosynthetic plant exposed to the elements of its environment. However, containment creates a separate problem for Isoetes, which is that the gametophytes have no way to acquire energy on their own. Isoetes sporophytes solve this problem by provisioning starches and other nutrients to the spores as an energy reserve for the eventual gametophytes.[18][19] Although not a homologous process, this provisioning is somewhat analogous to other modes of offspring resource investment in seed-plants, such as fruits and seeds. The extent to which resources provisioned to the megaspore also support the growth of the new sporophyte is unknown in Isoetes.

Reproductive cycle of Isoetes. The diploid sporophyte (A) produces microsporangia and megasporangia, which are located at the leaf bases. A cross section of the plant (B) shows that the megasporangia are located more towards the outer leaves (2) and the microsporangia are concentrated in the center (1). Via meiosis, the sporangia produce haploid spores (C). The megasporangia produce megaspores (3) which become female gametophytes and the microsporangia produce microspores (4) which become male gametophytes. The gametophytes germinate inside the spore, cracking the outer layer known as the perispore (5) as they grow via mitosis to expose the reproductive organs (6). Sperm from the male gametophytes locate the archegonia neck cells on the female gametophyte (6) and swim down to fertilize the egg. A diploid embryo is formed and a young sporophyte (D) is rapidly created through mitosis, eventually growing into another adult sporophyte.

Spore dispersal occurs primarily in water (hydrochory) but may also occur via adherence to animals (zoochory) and as a result of ingestion (endozoochory).[12][20] 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.[20] 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.[20][21] 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.[22] Isoetes may mitigate this issue via microspores stuck to megaspores, greatly increasing the possibility of successful fertilization upon dispersal.[20][21]

As of November 2019, Plants of the World Online accepted the following extant species:[23]

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.

Fossilised specimens of I. beestonii have been found in rocks dating to the latest Permian.[25][26] Quillworts are considered to be the closest extant relatives of the fossil tree Lepidodendron, with which they share some unusual features including the development of both wood and bark, a modified shoot system acting as roots, bipolar growth, and an upright stance.[citation needed]