As indicated earlier, the green algae constitute a very diverse group, with an extraordinary variety of forms and chloro-plast shapes. Obviously, each species has to reproduce in order to perpetuate itself. Most undergo both sexual and asexual reproduction, but a few do not. For example, the globally distributed algae that make parts of some tree trunks appear as though they had received a light brushing or spattering of green paint usually are unicellular or colonial forms that reproduce only asexually.
These and Chlorella, another widespread green alga composed of tiny spherical cells, reproduce by forming either daughter cells or autospores through mitosis. The daughter cells often remain together in packets, while the autospores of Chlorella formed inside the parent cells grow to full size as the parent cell wall breaks down.
Chlorella is very easy to culture and is a favorite organism of research scientists. It has been used in many major investigations of photosynthesis and respiration, and in the future, it may become important in human nutrition. (See the section entitled "Human and Ecological Relevance of the Algae," which begins on page 344.)
Chlorella could also play a key role in long-range space exploration. Because present exploration is severely limited by the weight of oxygen tanks and food supplies needed on a spacecraft, scientists have turned to Chlorella and similar algae as portable oxygen generators and food sources. Future spacecraft may be equipped with tanks of such algae. These would carry on photosynthesis, using available light and carbon dioxide given off by the astronauts, while furnishing them with oxygen. As the algae multiplied, the excess could either be eaten or fed to freshwater shrimp that could, in turn, become food for the astronauts. Still other algae and bacteria could recycle other human wastes. Such a self-perpetuating closed system, as it is called, has already been successfully tested with mice and other animals. Many algae, however, are known to produce traces of carbon monoxide gas, which is deadly to most animal life, and until this problem is resolved, humans will not be subjected to such research.
Desmids (Fig. 18.8; see also Fig. 18.2B), whose 2,500 species of crescent-shaped, elliptical, and star-shaped cells are mostly free-floating and unicellular, reproduce by conjugation. Their chloroplasts are among the most beautiful seen in the green algae.
The striking water nets (Hydrodictyon) (Fig. 18.9) form netlike, tubular colonies with hexagonal or polygonal meshes. A daughter net may be asexually produced inside a parent cell and eventually released. Sexual reproduction is isogamous, with two flagellated gametes forming a zygote as they unite. The zygotes become zoospores that produce large angular cells, each new cell becoming a new netlike colony that is released from the parent cell as it breaks down.
Sexual reproduction is also isogamous in the mermaid's wineglass (Acetabularia), a marine alga consisting of a single, huge cell shaped like a delicate mushroom (Fig. 18.10). Each cell is up to 5 centimeters (2 inches) long. This alga has been used in classic experiments demonstrating the influence of the nucleus on the form of the cell. If the cap of an alga is removed and the nucleus is replaced with a nucleus taken from another species, the base regenerates a cap identical to the previous one. If this new cap is also removed, however, the next cap that develops shows form characteristics of both species. If the intermediate cap is then removed, the next cap that develops is identical to that of the species from which the nucleus originally came. Clearly, the original nucleus directs development of cytoplasmic substances regulating cap form, and when these are gone, the replacement nucleus exerts its own influence.
Volvox (see Fig. 18.2A) is representative of a line of green algae that forms colonies, apparently by means of single cells similar to those of Chlamydomonas, held together in a secretion of gelatinous material. In some colonies, the cells
are actually connected to one another by cytoplasmic strands. The flagella of individual cells beat separately but pull the whole colony along. A Volvox colony may consist of several hundred to many thousands of cells that resemble a hollow ball spinning on its axis as it moves. Reproduction may be either asexual or sexual, with smaller daughter colonies being formed inside the parent colony. The daughter colonies are released when the parent colony breaks apart.
Sea lettuce (Ulva) is a multicellular seaweed with flattened, crinkly-edged green blades that may be up to 1 meter (3 feet) or more long (Fig. 18.11). A basal holdfast anchors the blades, which may be either haploid or diploid, to rocks. Diploid blades produce spores that develop into haploid blades bearing gametangia. The gametes from the haploid blades fuse in pairs, forming zygotes, that can potentially grow into new diploid blades. Except for the reproductive structures, the hap-loid and diploid blades of sea lettuce are indistinguishable from one another, a feature known as isomorphism.
Cladophora is a branched, filamentous green alga whose species are represented in both fresh and marine waters. Unlike the cells of other green algae, those of Cladophora and its relatives are mostly multinucleate.
PHYLUM CHROMOPHYTA— THE YELLOW-GREEN ALGAE, GOLDEN-BROWN ALGAE, DIATOMS, AND BROWN ALGAE
About three quarters of the roughly 7,600 members of Phylum Chromophyta are primarily microscopic, but kelps and other brown seaweeds are the best-known representatives. If the microscopic forms were larger, many surely would become collectors' items in the art and antique shops of the world because of their exquisite form and ornamentation (Fig. 18.12).
The algae in this phylum can be grouped into several classes, including the yellow-green algae (Xanthophyceae), golden-brown algae (Chrysophyceae), diatoms (Bacillario-phyceae), and brown algae (Phaeophyceae). Superficially, the organisms of each class may appear unrelated to each other, but they do share several features, including food reserves, specialized pigments, and other cell characteristics. Some members of the first three classes produce a unique resting cell called a statospore (Fig. 18.13). These cells resemble miniature glass apothecary bottles, complete with plugs that dissolve or "uncork," releasing the protoplast inside. Many statospores are striking in form, with finely sculptured ornamentations on the surface.
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