VALVE AND BAND MORPHOLOGY OF SOME FRESHWATER DIATOMS. III. PRE‐ AND POST‐AUXOSPORE FRUSTULES AND THE INITIAL CELL OFMELOSIRA ROESEANA1

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ABSTRACTFrustules of a clonal culture ofMelosira roeseanaRabenh. were examined with light and scanning electron microscopy. Vegetative valves in the post‐auxospore (full size) stage exhibit a larger width/length ratio than those in the pre‐auxospore (size‐reduced) stage. Cells form chains by linking spines of adjacent valves which occur at the periphery of the valve face‐mantle junction. Three or jour large pores occur at the center of the valve face, with the diameter of each pore tapering from the inner to the outer valve surface; these pores are often occluded by siliceous processes. Features ofM. roeseana,not shown previously forMelosira,include a “stepped” mantle, on only one of the two valves resulting from the same cell division, flattened processes attached to short siliceous stalks on the valve face, disk‐like processes on the mantle, and an open girdle band with up to eight antiligulae. Siliceous scales on the surface of the initial cell are remnants of the auxospore wall. The epivalve of the initial cell is larger in diameter than the hypovalve, and both valves lack linking spines and a step on the valve surface. The initial, cell epicingulum consists of only two bands; the hypocingulum has up to seven. Initial cells with four or more hypocingular bands divide to form new post‐auxospore filaments.Melosira roeseanashould not be included in the genusMelosiraas it is presently defined by the type species,M. nurnmuloidesC. Ag. Major differences include irregular linking spines, a closed pseudoloculate valve construction, and labiate processes on the valve face and mantle ofM. nummuloides,compared with well‐defined linking spines, a valve constructed of a basal siliceous layer perforated by poroid areolae, and labiate processes lacking on the valve ofM. roeseana.