Auxospore Structure, Initial Valves and the Development of Populations of Stephanodiscus in Farmoor Reservoir

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Abstract
A population of the planktonic diatom Stephanodiscus growing in Farmoor Reservoir has been investigated with regard to auxospore formation and the subsequent change in cell size. This is the first account of auxospores in Stephanodiscus. The auxospore wall is believed to be organic without any siliceous components. Germination results in two large hemispherical initial valves which are rather crudely formed and lack the ring of spines characteristic of the vegetative cells which are figured for comparison. The division of the initial cell to form hemispherical cells is recorded. These then form a normal vegetative cell and a hemispherical cell. The hemisperhical cells acted as markers in the natural population and their longevity was determined. The natural populations did not exhibit a continuous decrease in cell diameter, at times remaining almost constant and at other times decreasing. The origin and ratio of occurrence of biconcave, biconvex and concavo-convex cells in the population is discussed.

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