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These algae are easily introduced by looking at the organization of Chlamydomonas reinhardi. The genetics, physiology, and fine structure of this member of the Chlorophycophyta have been intensively studied. What we find is a complex cell with well-defined cytoplasmic organelles. It reproduces asexually, undergoes sexual processes, and has various other complicated processes in its life cycle. Asexual reproduction ensures a relatively rapid increase in numbers under suitable conditions, namely, the availability of simple nutrients and energy (adequate sunlight). The sexual process, in which whole cells fuse into a nonmotile zygote, leads to meiotic cell division and the segregation of new genotypes in the progeny cells. So we find that this species takes advantage of asexual reproduction for rapid increase in number of haploid cells, parallel to the situation in bacteria, and also uses sexuality tohlorophyll a, b         paramylon absent  new genotypes. Interestingly, fusion of complementary mating types is brought about by mild starvation, again paralleling the case in bacteria in which sexual reproduction most commonly occurs when food resources are depleted. Last, we observe that a parallel to bacterial spores is also found here in that chlamydomonad cells can encapsulate in a jelly-like secretion in which nonmotile, thick-walled spores are found. This occurs under adverse growth conditions.

Major protophytan phyla and their evolutionary trends. Thus far we have mentioned two phyla among the protophyta, the Chlorophycophyta and the Chrysophycophyta. Two more deserve mention. (We speak of phyla here, but botanists also refer to them as divisions. The terms are synonymous.) These are the Pyrrophycophyta and the Euglenophycophyta. They differ in their cell structure, photosynthetic pigments, and storage products resulting from photosynthesis. Figure 11-5 provides examples of the Pyrrophycophyta and the Euglenophycophyta.

Two evolutionary trends should be considered: (1) colony formation and (2) loss of photosynthesis or the appearance of colorless forms. Colony formation is common in the Chlorophycophyta and the Chrysophycophyta. It can take the form of spherical aggregates of cells and of chains of cells forming filaments, plates of cells, branching colonies, net-like aggregates, and even other variations. For our purposes the most interesting examples are those found in the Chlorophycophyta.