BOTANY WITH BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

IX. LOWER VASCULAR PLANTS (PTERIDOPHYTES)

Division Equisetophyta (Horsetails)

Equisetophyta are lower vascular plants in which the sporophyte is dominant over the gametophyte; both sporophytes and gametophytes are independent organisms.

The vegetative body of the sporophyte in modern horsetails is represented by herbaceous plants differentiated into stems, leaves, and roots. Among fossil horsetails, woody forms are also known.

The leaves are simple, single-veined, wedge-shaped, either well-developed or reduced, and arranged in whorls along the stem. By fusing at their bases, they form annular sheaths. The shoots feature clearly defined nodes and internodes. Due to leaf shape and arrangement, horsetails are often referred to as sphenopsids or articulate plants.

The stem of horsetails is ribbed, externally covered by an epidermis with Stomata. The epidermal Cell walls are thickened and impregnated with silica, which provides rigidity to the stem. Beneath the epidermis lies the sclerenchyma, a mechanical tissue that enhances stem strength. Internal to the sclerenchyma is the primary cortex, whose outer Cells form a robust chlorenchyma ensuring the photosynthetic function of the stem. A pericycle underlies the cortex, marking the boundary of the central vascular cylinder. Vascular bundles are collateral, arranged in a circle, and consist of phloem and weakly developed xylem formed by tracheids. Unlike fossil calamites, modern horsetails lack a cambium.

The stems of modern horsetails are hollow at the internodes. Three types of air cavities can be distinguished: a large central cavity, small cavities located within the vascular bundles opposite the Ribs, and cavities situated in the cortical parenchyma opposite the furrows.

Whorled branching is characteristic of both the stems and roots of Equisetophyta.

Sporangia (5–10) are located on the underside of sporophylls, which are grouped into strobili at the tips of ordinary or specialized spore-bearing shoots. Each sporophyll consists of a stalk attaching it to the strobilus axis and a peltate shield bearing sporangia on its lower surface.

In addition to the exine and intine, spores possess a third layer—the epispore—formed by four ribbon-like elaters. Their tapered ends are attached to the spore, while their expanded ends are free. In humid weather, the elaters coil around the spore, whereas in dry weather, they spring straight. This mechanism helps discharge spores from the sporangium, entangle them together, and disperse them in clumps by the wind.

All spores are morphologically identical, but they germinate into unisexual, dioecious gametophytes.

The vegetative body of the gametophyte is a green thallus up to 1 cm in diameter, bearing rhizoids and sex Organs: antheridia on male plants and archegonia on female plants. Spermatozoids are multiflagellate. Fertilization requires Water. A sporophyte embryo develops from the zygote. Among fossil horsetails, heterosporous plants are known.

Horsetails possess a high capacity for Vegetative Reproduction.

Fossil remains of Equisetophyta date back to the Upper Devonian. They flourished in the early and middle Carboniferous period, but began to decline by its end. The ancestors of Equisetophyta were most likely rhyniophytes.

The Division Equisetophyta is divided into three classes: Hyeniopsida, Sphenophyllopsida, and Equisetopsida.