Taxonomy of Elodea Michx in the British Isles

Three species of Elodea are now known to occur in the British Isles: E. canadensis Michx, E. nuttallii (Planch.) H. St John and E. callitrichoides (Rich.) Casp. The taxonomic histories of the genus and three species are reviewed. Keys and descriptions of the species are given, together with details of their habitats and distribution. Elodea Michx is a New World genus, of which several species are known to be adventive in the Old World. A recent investigation (Simpson 1983) has shown three species to be adventive in the British Isles, namely E. canadensis Michx, E. nuttallii (Planch.) H. St John and E. callitrichoides (Rich.) Casp. (following Cook & Urmi-Konig's (1984) recommendation that plants previously referred to as E. ernstiae H. St John should now be assigned to E. callitrichoides). Simpson (1984) outlined the introduction and spread of these species in the British Isles. This paper presents a brief taxonomic history of the genus together with the British and Irish species, and summarizes the taxonomic conclusions drawn from the work in the form of a key and brief systematic accounts. A further paper in preparation will give a more detailed account of intraspecific variation in E. canadensis and E. nuttallii. The vegetative structure of Elodea is comparatively simple, consisting of a series of axillary, branched, terete stems with narrow, sessile and minutely serrate leaves. Three decussate pairs of leaves occur on the lowest part of the stem, above which all the leaves are in whorls. The decussate leaves are also distinctly smaller than the whorled ones. A midrib is present in all leaves but no other venation is apparent. Epidermal hairs are absent from both stems and leaves. A pair of minute, entire, nodal scales (or squamulae intravaginales) are attached between the stem and leaf base on the adaxial side of the leaf. Adventitious roots are produced at the nodes, coinciding with the growth of a new stem. One root is produced with each stem, and root hairs occur only when the root is in sediment. Despite its simplicity, the vegetative morphology of Elodea is, in common with other aquatic macrophytes, highly plastic, and this has caused much of the taxonomic confusion. Such variation appears to be brought about by a variety of environmental factors, particularly in relation to light and temperature (Simpson 1983). The main growing season is between mid-April and mid-September. At other times growth is very much reduced or ceases completely. The plant overwinters by means of short, unbranched stems, or by the production of turions (hibernacula). The latter arise in the upper leaf axils during early autumn and may remain in situ, or break free and fall to the substrate, where they germinate the following spring. The vegetative anatomy of Elodea reflects the plant's morphological simplicity. The stem consists of a single epidermal layer, a cortex consisting of parenchymatous and aerenchymatous tissues, and a simple stele with a central protoxylem lacuna. The leaf consists of only two cell layers. Stomata are absent and there is only a thin cuticle. Moreover there are no distinct strengthening tissues in the stem, and only 2-3 rows of sclerenchymatous cells along the leaf margins. All the British and Irish species are dioecious. The female flowers are sessile and solitary in the leaf axils, and they usually occur within the 3-5 cm of the stem apex. Each consists of a perianth surmounting an elongated thread-like tube connected to the ovary. The latter is unilocular, containing three to ten ovules with either basal or parietal placentation. The ovary and lower part