Phylum Chlorophyta includes about 7,500 species of organisms commonly known as the green algae. They occur in a rich variety of forms and occur in very diverse and widespread habitats; they possess some of the most beautiful chloroplasts of all photosynthetic organisms. Some are unicellular and microscopic; in fact, the green alga Micromonas is only 1 |m in diameter—the smallest eukaryotic cell known. Most unicellular green algae are, however, considerably larger. Other green algae form threadlike filaments, platelike colonies, netlike tubes, or hollow spheres (Fig. 18.2).
Some green algae are seaweeds, resembling lettuce leaves or long green ropes. Several unicellular species grow in greenish patches or streaks on the bark of trees, whereas others grow in large numbers on the fur of sloths and other jungle animals. The green fur camouflages the animals in their natural habitats. Still others thrive in snowbanks, live in flatworms and sponges, on rocks, or are found on the backs of turtles. They are the most common member in lichen "partnerships" (discussed in Chapter 19). The greatest variety, however, is found in freshwater ponds, lakes, and streams. Ocean forms are also varied; there they are an important part of the plankton (free-floating, mostly microscopic organisms) and, therefore, of food chains.
The chlorophylls (a and b) and other pigments of green algae are similar to those of higher plants. The green algae also are believed to have been the ancestors of higher plants and, like the higher plants, they store their food in the form of starch within the chloroplasts. With the exception of bry-opsids, which are multinucleate, most green algae have a single nucleus in each cell. Most green algae reproduce both asexually and sexually. How they do so illustrates the forms of reproduction of most of the organisms discussed in the chapters to follow, and so we'll examine several different representative green algae in some detail.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.