188 EVOLUTION OF THE CHLOROPLAST

Fig. 5.51 Dichotomosiphon tuberosus. (a) Vegetative thallus. (b) Fertile plants with oogonia (O) and antheridia (A). (c) Spermatozoid. (d) Portion of thallus with akinetes. ((b) after Ernst, 1902; (c)

after Moestrup and Hoffman, 1975; (d) after Smith, 1955.)

in the cytoplasm (Moestrup and Hoffman, 1973). Siphonein is present, but the related pigment siphonoxanthin is absent (Kleinig, 1969). The cell wall has a -1,3 linked xylan as the main structural wall component (Maeda et al., 1966).

There are two species of Dichotomosiphon: the marine D. pusillus and the freshwater D. tuberosus (Fig. 5.51). The latter grows in lakes with an organic silty bottom, most of the thallus buried, and only the tips of the branches above the silt. In water deeper than 2 m only asexual reproduction occurs, by the formation of akinetes in series at the ends of branches. The akinetes germinate directly to form new thalli (Ernst, 1902). In shallow water, reproduction is sexual, homothallic, and oogamous. Conical antheridia are produced at the tips of branches and separated from the rest of the thallus by a septum. The antheridia burst open explosively at the apex, releasing the biflagellate spermatozoids, which have a single reduced chloroplast but lack an eyespot (Moestrup and Hoffman, 1975). An oogonium is spherical, and just before fertilization develops a small beak-like opening at its apex.

Siphonocladales

These organisms have multicellular thalli, are wholly marine, and are usually tropical. The cells are multinucleate, with reticulate chloroplasts, and divide in a distinct manner known as

segregative cell division. Most of the organisms have siphonoxanthin (Fig. 5.1) (except Dictyosphaeria) in addition to the normal pigments of the Chlorophyta (Kleinig, 1969). Sexual reproduction appears to be isogamous in most cases.

Siphonocladus tropicus initially has an undivided single-celled primary vesicle (Fig. 5.52). In segregative cell division, the continuous protoplast of the primary vesicle breaks into spherical masses of varying size that soon become surrounded by a wall and enlarge to fill the area within the expanding parent vesicle. After each segment has become firmly pressed against adjacent ones, it sends out a lateral protuberance, which constitutes a branch initial. The mature thallus consists of an erect axis with lateral branches (Egerod, 1952).

A young plant of Valonia (Fig. 5.52(h)) consists of a bladder-like multinucleate primary cell. Small holdfast cells are formed by segregative cell division. A mature cell has a conspicuous central vacuole surrounded by a layer of protoplasm. Any vegetative cell can divide into biflagellate swarmers. Although zygotes have been seen, fusion of gametes has not, but it is presumed that meiosis occurs in the cells immediately before the formation of swarmers.

Valonia is a common tool for investigators interested in the relationship between the vacuole and protoplasm because it is relatively