Abstract: A population of Meridion circulare var. circulare (Greville) C.A. Agardh from Inner Mongolia was found to produce Innenschalen or internal spores. Examination of this population with light and scanning electron microscopy showed morphological differentiation between vegetative and spore morphologies. Vegetative valves typically bear costae and one rimoportula at the headpole. Spores lack costae and have two rimoportulae, one at the headpole and the other at the footpole. There is plasticity in the production of valve morphologies, and a variety of vegetative valve and spore combinations are evident. This population of M. circulare var. circulare has initial valves of over 90 μm in length, and all of the initial cells encountered are acostate and bear two rimoportulae. These observations suggest that either spores are the product of sexual reproduction, or that initial valves may be produced parthenogenetically in Meridion. Spores as products of the sexual process have not been reported in diatoms previously, and parthenogenesis in Meridion was reported previously but discounted in other published reports. The plasticity of valve morphologies expressed in M. circulare var. ciculare, between vegetative valves and spores (and back) across a short temporal period suggests that diatoms can alter their cell wall structure dramatically and quickly in response to external variables.

Key words: Bacillariophyceae, diatom, Innenschalen, internal valves, Meridion, systematics.