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

IX. VASCULAR CRYPTOGAMS

Class Quillworts (Isoetopsida)

The Class is represented exclusively by heterosporous plants (about 800 species) grouped into the orders Selaginellales, Lepidodendrales, and Isoëtales.

Order Isoëtales comprises heterosporous perennial herbaceous plants characterized by leaves bearing a ligule. Gametophytes are unisexual and dioecious. Spermatozoids are multiflagellate. Known since the Cretaceous period of the Paleozoic era. The order includes a single family, Isoëtaceae, and a single genus, Isoëtas, encompassing about 70 species. In Ukraine, it is represented by a single species, lake quillwort (Isoëtas lacustris), which grows in the lakes of Western Polissia within Rivne Region.

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Fig. 97. Lake quillwort

Lake quillwort (Isoëta lacustris) is a perennial herbaceous plant 5-20 cm high (Fig. 97). The stem is shortened, corm-like, and possesses cambium. A cluster of numerous dichotomously branched rootlets extends downward from the stem, while a rosette of dark green, awl-shaped, ligulate leaves projects upward. The leaves feature a single vein and are differentiated into trophophylls and sporophylls. Megasporangia are situated in depressions on the inner surface of the lower portion of megasporophylls, whereas microsporangia occupy the same position on microsporophylls. Trophophylls and sporophylls alternate within the rosette as follows: megasporophylls are located externally, microsporophylls closer to the center, and sterile trophophyllous leaves in the very center.

In August, the sporophylls shed into the water, their sporangial walls disintegrate, and the spores are released, float to the surface, and germinate. The microspore develops into a male gametophyte consisting of an antheridium. The antheridium features a multi-layered wall and two spermatogenous Cells, each of which undergoes mitotic division to form two multiflagellate spermatozoids.

A multicellular female gametophyte with a single archegonium develops within the megaspore. Following Fertilization, a sporophyte emerges from the zygote.

Quillwort can occasionally form true underwater meadows on sandy, less frequently peaty or muddy substrates in the littoral zone of lakes, providing a shelter for juvenile fish from predators.

Sporangia and spores floating on the water surface in autumn may be consumed by fish. The leaf tips are grazed by muskrats.