Botany - B.Ye. Yakubenko 2017

Part Three. Kingdom Plantae
Chapter VII. Systematics of Higher Plants
7.6. Division Equisetophyta

Equisetophytes (horsetails and their allies) are both extant and extinct herbaceous or woody plants characterized by stems clearly divided into nodes and internodes. At each node, the stem bears a whorl of small, often reduced leaves. Equisetophytes belong to the microphyllous evolutionary Lineage of vascular spore-producing plants and represent a group in which true leaves of telomic origin first appeared during evolution. Horsetails exhibit monopodial branching with whorled arrangement of branches. Due to the reduction of leaves, the Functions of Photosynthesis are carried out by the stems and branches. The sporophyte dominates The life cycle.

Horsetails trace their ancestry back to ancient rhyniophytes. Their primitive forms date back to the Devonian period of the Paleozoic era, and the fossil record includes arborescent forms that became extinct by the end of the Paleozoic.

The division is divided into four classes, three of which are exclusively extinct plants.

Class Hyeniales comprises small shrubs that lived in the early Paleozoic, originating from rhyniophytes and serving as an intermediate evolutionary branch between rhyniophytes and equisetophytes.

Class Sphenophyllales was represented by woody plants that inhabited the forests of the Carboniferous period, including both homosporous and heterosporous species. Despite a relatively uniform external appearance, sphenophylls differed from one another in The Structure of their strobili located at the branch tips. Some of them gave rise to the ancestors of the class Equisetopsida.

Class Equisetopsida is a modern group of plants comprising a single family, a single genus, and 32 species distributed worldwide. All species are perennial herbs growing in Water-logged meadows, fields, bogs, along riverbanks and lake shores, and more rarely in forests.

The sporophyte possesses a robust rhizome on which small tubers form to store nutrients. These tubers overwinter and give rise to new shoots in spring. Stems arise from the rhizome (Fig. 90); in some species, these are of two types—fertile and vegetative.

Fertile shoots appear early in spring, lacking lateral branches and chlorophyll. At their apex, a strobilus develops, consisting of an axis bearing modified leaves known as sporophylls. Each sporophyll features a stalk—the sporangiophore—and a shield-like apex beneath which cylindrical sporangia are attached. Within these sporangia, morphologically identical spores are formed. A spore possesses three walls; the outer layer, the epispore, splits to form two ribbon-like elaters. Thanks to the elaters, spores tend to cluster together into small groups. This has profound biological significance: since the spores are physiologically differentiated, both male and female gametophytes germinate in close proximity, thereby increasing the probability of successful Fertilization and The Emergence of a new plant.

The gametophytes of horsetails are terrestrial, green, anchored to the soil by rhizoids, and survive for several weeks. Male gametophytes appear as slightly lobed thalli bearing a few antheridia, which produce multiflagellated spermatozoa. Female gametophytes are more dissected into multi-lobed plates, between the lobes of which flask-shaped archegonia develop, each containing an egg Cell within its venter. Fertilization is dependent on water, resulting in The formation of a zygote that subsequently develops into a sporophyte embryo. The Life Cycle of Equisetum arvense is illustrated in Fig. 90.

Following spore shedding, the fertile SHOOT withers, while green, photosynthetic summer shoots develop in the meantime. These shoots are ribbed and hollow on the inside, with epidermal Cells impregnated with silica. Their sizes vary considerably; certain tropical species attain stem lengths of up to 6 m. Beneath the epidermis lies the primary cortex, composed of mechanical tissue and chlorenchyma. The vascular cylinder consists of small vascular bundles, each surrounded by an endodermis, with intervening parenchymal tissue. A pericycle and endodermis are located just outside the vascular cylinder. Leaves are reduced to scales fused at the base into a sheath; consequently, photosynthesis is performed by the stem and its whorled branches.

The Practical significance of extant horsetails is minor: Equisetum arvense is used in herbal medicine, whereas Equisetum palustre and Equisetum fluviatile are toxic to livestock. Animals generally avoid them when fresh, but if ingested within hay, they can cause intestinal injury in small animals. Equisetum arvense finds application in medicine. Furthermore, like other horsetail species, it serves as an indicator of mildly acidic soils. Most horsetails function as weeds and undesirable components of meadow phytocenoses. Having survived since the Paleozoic era, horsetails are true relics of bygone geological epochs.