Formation and ultrastructure of a complex, multilayered wall around the oospore ofCharaandLamprothamnium(Characeae)

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Abstract
The formation and ultrastructural features of a complex, multilayered wall around the oospore of Chara and Lamprothamnium were examined using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). This multilayered wall, called here the compound oosporangial wall (COW) and formerly known as the “oospore membrane”, is composed of eight distinct layers and is partly derived from the oospore and partly derived from the ensheathing cells around it (i.e. the sterile cell and the five spiral cells). The primary walls of the oospore and the ensheathing cells are in intimate contact and remain so whilst post-fertilization development proceeds by the deposition of three secondary wall layers onto the inside of the oospore wall and three secondary wall layers onto the inside of the walls of the ensheathing cells. This forms the COW which, therefore, has six secondary walls and two primary walls. The secondary wall layers deposited by the oospore are first the amorphous layer, then the oospore helicoidal layer and finally the microfibrillar layer. The secondary wall layers deposited by the ensheathing cells are first a crystalline layer (the crystine), then the pigmented layer and finally the ornamentation layer. In some species the ornamentation layer varies in thickness resulting in the formation of lumps, knobs or tubercles. These are the ornaments seen on the surface of the COW.