CHLOROPHYTA 189

Fig. 5.52 Segregative cell division in Siphonocladus tropicus: (a) germling; (b) cytoplasm in spherical masses; (c) expansion of cytoplasmic masses; (d) lateral branches forming; (e) mature thallus. Dictyosphaeria cavernosa: (f) young aseptate vesicle; (g) secondary vesicle attached to primary vesicle. (h) Ventricaria ventricosa (Valonia ventricosa). (p) Primary vesicle cell; (r) rhizoidal cell. ((f),(g) after Egerod, 1952;

(h) after Taylor, 1960.)

easy to remove the vacuolar contents from the cell. The osmotic values of the vacuolar sap are 1 to 3 atm higher than seawater, with the concentration of potassium in the vacuolar sap being 66 times higher than in seawater (Mimietz et al., 2003).

Damage to the thallus of algae in this order results in the formation of aplanospores through a modified process of segregative cell division.

Ventricaria ventricosa (Valonia ventricosa) lives in coral rubble in tropical reef environments such as the Great Barrier Reef. The alga has a large central vacuole with peripheral protoplasm under the cell wall (Fig. 5.53) (Shepard et al., 2004). The protoplasm has chloroplasts to the outside and nuclei to the inside. These organelles are surrounded by the highly convoluted tonoplast of the central vacuole. The organelles are essentially in islands in the vacuole held together by cytoplasmic strands containing microtubules. The tonoplast is “multifolded” by a factor of nine, giving the protoplasm a spongy appearance. Each nucleus is associated with a certain number of chloroplasts and other organelles,

which together can be considered as a fundamental protoplasmic domain. Damage to the thallus results in cytoplasm aggregating around the nuclei to produce aplanospores, with the tonoplast becoming the plasma membrane of the aplanospores.

Chlorophyceae

The distinguishing characteristics of the Chlorophyceae are the theca outside of the cells and a collapsing telophase spindle that brings the daughter cells close together, followed by cell division by a phycoplast. The flagellar root system is cruciate.

Some of the flagellates in the class do not have a theca, but these are assumed to have lost the theca in evolution because the cells have the other characteristics of the class. The Chlorophyceae are predominantly freshwater. The few unicellular, planktonic species that occur in coastal seawater are members of genera that have a much greater number of freshwater species,

190 EVOLUTION OF THE CHLOROPLAST

Fig. 5.53 The coenocyte Ventricaria ventricosa (Valonia ventricosa) has a large central vacuole with a thin layer of protoplasm under the cell wall. The nuclei in the protoplasm, and associated chloroplasts, are held apart by microtubules. The tonoplast is highly convoluted, resulting in the vacuole weaving between the organelles and giving a spongy appearance to the protoplasm. Damage to the thallus results in aggregation of chloroplasts around the nuclei. Aplanospores are formed with the tonoplast of the original cell becoming the plasmalemma of the aplanospores. (Adapted from Shepherd et al., 2004.)

such as Chlamydomonas. The Chlorophyceae whose sexual reproduction is known produce a dormant zygote, with meiosis usually occurring when the zygote germinates.

The other characteristics of the Chlorophyceae were described earlier in this chapter and include motile cells with radial or near-radial external symmetry, flagella attached at the anterior end of the cell, the possibility of a rhizoplast, no multilayered structure, eyespots common, glycolate breakdown by glycolate dehydrogenase, and urea breakdown by urea amidolyase.