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There was a research carried out on Stratiotes aloides L. in Lake Ilmoilanselkä situated in southern Finland. The aim of the research was to establish the reproductive ability of the emergent specimens of S. aloides in Finland, compare the Finnish and Polish populations of S. aloides and examine the quality of water in Lake Ilmoilanselkä. The Finnish population of S. aloides is characterized by large vegetative size, small numbers of generative shoots and large numbers of turions. The dry mass of turions in Finnish specimens exceeds the dry mass of all generative organs in these specimens. Significant differences in the following characteristics were found between the Finnish and Polish populations of S. aloides: the plant’s diameter, the number of generative organs, buds, flowers, turions, total dry mass of the specimens, their generative and vegetative mass as well as turion mass. Variance analysis for leaves confirmed significant differences between the Polish and Finnish populations of S. aloides in the length of their leaves and width of their leaves measured 3 cm from their base. The discussion involved data concerning different populations of S. aloides from Poland and Sweden as well as the influence of climatic and habitat factors on reproductive abilities of that species in the northern area of its occurrence. In the light of the obtained results of varied reproduction of S. aloides, the habitat factor is dominant while the climatic differences in the areas where the lakes are distributed in Poland and Finland are insignificant.
Key words: Stratiotes, ecology, population, reproduction