Botany - B.Ye. Yakubenko 2017

Part Three. Plant Kingdom (Planthae)
Chapter VII. Systematics of Higher Plants
7.5. Division Lycopodiophyta

Lycophytes represent an independent microphyllous branch in the evolution of vascular spore plants. They reached their greatest diversity and Abundance in the Late Paleozoic, whereas the modern flora is represented by only a modest number of genera and species. Extant lycophytes are perennial, typically evergreen herbaceous plants, whereas some extinct forms were massive, tree-like organisms.

A characteristic feature of lycophytes is the dichotomous branching of their shoots, which bear spirally arranged microphylls. Their subterranean Organs vary: in some species, they appear as a rhizome with modified leaves and adventitious roots; in others, they form a rhizophore—a specialized organ bearing spirally arranged roots.

Lycophytes were the first plants in the course of evolution to develop true roots, and certain fossil forms exhibited secondary thickening thanks to a vascular cambium. Their life cycle is dominated by the sporophyte generation, with spores developing within sporangia located on modified leaves known as sporophylls, which are aggregated into specialized cones called strobili. Compared to bryophytes, lycophytes display a more complex sporophyte Structure that has assumed all principal vital Functions, whereas the gametophyte is markedly reduced, adapted solely to sexual reproduction.

Lycophytes include both homosporous and heterosporous plants. In heterosporous taxa, a small outgrowth known as a ligule is present near the Base of the spore-bearing leaf (sporophyll).

The gametophytes of homosporous and heterosporous forms differ dramatically. Those of extant homosporous taxa are subterranean, fleshy, 2–20 mm long, bisexual, saprophytic, and take 1–15 years to mature. Conversely, the gametophytes of heterosporous species are unisexual, develop within a few weeks, are microscopic, remain enclosed within the spore wall, and only emerge upon maturity. This highlights a profound reduction of the sexual generation.

The sex organs are antheridia and archegonia. Biflagellate spermatozoa develop within the antheridia, and egg Cells form within the archegonia. Fertilization requires the Presence of Water. The zygote develops into the next asexual generation—the sporophyte.

The division Lycopodiophyta is divided into two classes: Lycopodiopsida and Isoetopsida. The Class Lycopodiopsida encompasses three orders: Asteroxylales, Protolepidodendrales, and Lycopodiales.

Asteroxylales and Protolepidodendrales are entirely extinct groups. Asteroxylales ranged from the Devonian period of the Paleozoic to the Triassic period of the Mesozoic; these were herbaceous plants with dichotomously branched stems and small microphylls. Protolepidodendrales differed from Asteroxylales in having dichotomously branched leaves. All representatives of this class are homosporous.

The order Lycopodiales comprises 400 extant species belonging to two genera, distributed primarily in tropical and subtropical regions. Eight species grow in Ukraine. These are exclusively herbaceous, homosporous plants often featuring a creeping stem from whose lower side rhizophores emerge, bearing adventitious roots. Branches diverge dichotomously upward from the stem and are covered with spirally arranged, linear-lanceolate microphylls. The microphylls are covered by an epidermis, and the uniform mesophyll consists of isodiametric cells, traversed by a midvein. The stem lacks cambium; its outer surface is covered by an epidermis with well-developed Stomata, beneath which lie the primary cortex and a central cylinder surrounded by an endodermis and sclerenchyma. The xylem consists of tracheids, and the phloem elements are represented by elongated cells with sieve plates on their longitudinal walls.

Certain branches bear strobili; each strobilus consists of an axis bearing modified leaves—sporophylls—in whose axils sporangia are located. Through Meiosis, the sporangia produce identical spores, or isospores. Upon maturation, the sporangium dehisces via a longitudinal slit, releasing the spores. Under favorable conditions, these spores germinate into a subterranean, bisexual, chlorophyll-free, perennial gametophyte resembling a small tuber 5–20 mm in size. It relies on mycorrhizal Nutrition and, after 12–15 years, develops antheridia and archegonia. Fertilization occurs in the presence of water, and the zygote gives rise to an embryo and subsequently to the adult sporophyte plant (Fig. 88).

The most widespread lycopod species in Ukraine include Lycopodium clavatum, Lycopodium annotinum, Diphasiastrum complanatum, and Huperzia selago, the last two of which are listed in the Red Data Book of Ukraine.

The spores of Lycopodium clavatum were formerly used as baby powder, in metallurgy for dusting casting Molds, and in pyrotechnics.

The class Isoetopsida includes heterosporous species that played a vital role in forming the forest canopy during the Late Paleozoic. This class unites the orders Lepidodendrales, Selaginellales, and Isoetales.

Lepidodendrales, or scale trees, were extinct arboreal plants with columnar trunks and dichotomously branched crowns. Their trunks were covered with long, awl-shaped leaves up to 1 m in length. As these leaves gradually shed, leaf cushions with ligules at their upper ends persisted on the upper portion of the trunk. The subterranean axes bore spirally arranged roots known as rhizophores or stigmaria. The strobili of lepidodendrons, situated at the branch tips, reached up to 50 cm in length and contained microsporangia with microspores and megasporangia with megaspores. They went extinct at the end of the Paleozoic and contributed to extensive coal deposits.

The order Selaginellales comprises modern perennial herbaceous plants with stems capable of secondary thickening via a cambium that produces secondary phloem toward the outside and secondary xylem toward the center. Adventitious aerial roots arise from the stem. The microphylls of selaginellas are ovate, forming a sheath at the base with membranous ligules on the inner side. Certain shoots terminate in strobili, where sporangia develop on sporophylls. There are two types of sporophylls: microsporophylls and megasporophylls. The former are situated on one side of the axis, while the latter occupy the other side or the base of the strobilus. Each sporophyll consists of a modified microphyll bearing an oval sporangia on a short stalk. Numerous microspores develop within the microsporangia, whereas megasporangia typically produce four megaspores. Spores are released upon The breakdown of the sporangial wall. Microspores germinate internally into a microscopic male gametophyte consisting of just 8 cells. Once antheridia and sperm cells form, the spore wall ruptures, releasing the spermatozoa. In the presence of water, they swim to the female gametophytes developing inside the megaspores. The female gametophytes are multicellular; after 2–4 archegonia form on them, the spore wall splits, and fertilization takes place. A zygote develops into an embryo and ultimately a new plant (Fig. 89).

This order includes the genus Selaginella. Only two species occur in Ukraine—Selaginella selaginoides and Selaginella helvetica—both found in the high-altitude zone of the Carpathians.

The order Isoetales includes a single family and a single genus, Isoetes, comprising 62 species, of which only one is found in Ukraine—Isoetes lacustris (lake quillwort). It possesses a shortened, corm-like stem from the base of which adventitious roots radiate. The microphylls are awl-shaped and clustered in a tuft, in the center of which lie the spore-bearing leaves, or sporophylls. They form a sheath at the base, bearing membranous ligules on their inner surface. Special depressions at the base of the sporophylls house megasporangia or microsporangia on their floors. Numerous microspores develop in the microsporangium, and four megaspores in the megasporangia. Megasporangia and microsporangia detach from the parent plant and float on the water's surface, releasing their spores as their walls decay. Germinating spores produce highly reduced unisexual gametophytes (prothallia). The male gametophyte bears 1–2 antheridia, each producing four multiflagellate spermatozoa, while the female gametophyte bears several archegonia containing egg cells. Following fertilization, embryogeny begins, giving rise to a new quillwort sporophyte.

Quillworts are aquatic plants inhabiting lakes and bogs. Isoetes lacustris is rarely encountered in the lakes of the Ukrainian Polissya region.

The Practical significance of lycophytes is minor, but their evolutionary role in The history of the plant kingdom is profound. The life cycle of heterosporous lycophytes exhibits advanced evolutionary traits, namely a reduction in the number of megaspores, simplification of the gametophytes, and their partial development within the spore walls. In certain species, the life cycle even demonstrates The Development of the female gametophyte directly on the megasporophyll and fertilization occurring within the archegonium while still attached to the parent plant, a consequence of microsporophylls being positioned above megasporophylls in the strobilus. The young sporophyte develops for a time under the Protection of the parent plant—a feature characteristic of seed plants.