Fig. 19.4 Ophiocytium majus. (a) Vegetative cell.

(b) Semidiagrammatic drawing of the fine structure of a vegetative cell. (B) Basal tubular portion of wall; (C) cap of wall; (Ch) chloroplast; (G) Golgi; (N) nucleus; (V) vesicle. (Adapted from Hibberd and Leedale, 1971.)

Tribonema (Figs. 19.2, 19.3), have a wall composed of H-shaped pieces. These alternately overlap each other so that each protoplast is enclosed by halves of two successive H-pieces (Lokhorst and Star, 1988).

Chloroplasts and food reserves

Two membranes of chloroplast E.R. surround the chloroplasts, the outer membrane of chloroplast E.R. being continuous with the outer membrane of the nuclear envelope (Fig. 19.1) (Hibberd and Leedale, 1971). The thylakoids are grouped into bands of three and in many genera there is a pyrenoid in the chloroplast (Fig. 19.5) (Marchant, 1972). The eyespot consists of globules beneath the chloroplast envelope at the anterior end of the chloroplast (Figs. 19.1, 19.5). Where the short flagellum passes over the eyespot, the flagellar sheath is dilated into the flagellar swelling, which is closely applied to the plasmalemma in the area of the eyespot.

Chlorophylls a and c are present in the chloroplasts (Sullivan et al., 1990), with the major carotenoids being diadinoxanthin, heteroxanthin and vaucheriaxanthin ester.

Mannitol and glucose accumulate during photosynthesis in the plastids (Cleare and Percival, 1973). The principal storage product is probably a-1,3 linked glucan similar to paramylon, although lipids have been suggested as also being important.

Asexual reproduction

Xanthophycean organisms multiply asexually by fragmentation, zoospores, and aplanospores. In addition, they have the ability to form specialized resting spores. Fragmentation is limited to the tetrasporine and filamentous colonies, and is due to the breaking of the colony into parts.

Zoospores are formed by a majority of the genera. The zoospores are biflagellate, with the forward tinsel flagellum usually being four to six times longer than the shorter whiplash flagellum (Figs. 19.1, 19.5). The zoospores are naked and usually pyriform (pear-shaped). Zoospore production has been studied at the fine-structural level in

Pseudobumilleriopsis pyrenoidosa by Deason (1971) (Fig. 19.5). This alga has rod-shaped cells with several nuclei and laminate chloroplasts. Vegetative cells prior to zoosporogenesis have the nuclei and vacuoles in the center of the cell, whereas the chloroplasts are flattened against the plasmalemma. The first indication of cleavage in zoosporogenesis is the appearance of vacuoles between the ends of adjacent chloroplasts. The chloroplasts move away from the plasmalemma, and each becomes associated with a nucleus. The vacuoles then coalesce and separate the nucleus–chloroplast pairs, each of which becomes a zoospore. Basal bodies are present near the nuclei of the vegetative cells; the basal bodies migrate to one end of the chloroplast as cleavage begins and produce flagella early in zoosporogenesis. One to 16 zoospores are produced, which are released by dissolution and/or separation of the sporangial walls where they overlap. In the zoospore, the chloroplast is massive and has a pyrenoid. The nucleus is elongate, and there are two or more contractile vacuoles present.