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13. Synopsis
The eyespot of the zoospore of Tetracystis
excentrica (a green alga) has been studied by light and electron
microscopy. In Tetracystis the eyespot consists of about
110 osmiophilic granules which form a plate in the anterior third
of the cell. The granules are about 80 A in diameter and are
found in the outermost portion of the chloroplast; they commonly
show hexagonal close packing and a hexagonal shape. The granules
are confined positionally by the chloroplast envelope and an inner
thylakoid. The plasmalemma over the eyespot is thickened and
is separated from the chloroplast envelope by a 50 m space. The
eyespot of Tetracystis is compared with others reported
in the literature and the possible functional significance of
these studies is discussed. The possibility that the eyespot
plate in Tetracystis serves as a shading device rather
than the primary photoreceptor is considered.