A small, actively moving little alga, Chlamydomonas (Fig. 18.3) is a common inhabitant of quiet freshwater pools. It has an ancient history among eukaryotic organisms, with
Stern-Jansky-Bidlack: Introductory Plant Biology, Ninth Edition
18. Kingdom Protista
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Figure 18.2 Representative green algae. A. Volvox. The cells form hollow, spherical colonies that spin on their axes as the flagella of each cell beat in such a way that the motion is coordinated. New colonies are produced within the older ones, x 800. B. Micrasterias, a desmid. These algae consist of single cells that often have a constriction in the center, x1,200. C. Pediastrum. A colonial alga that forms flat plates, x1,200. D. Part of a filament of Ulothrix, whose cells each have at their periphery a chloroplast in the form of a curved plate, x 800. E. Scenedesmus, a green alga that typically occurs in colonies of four cells, x2,000.
flagella contractile . , vacuoles-V
eyespot nucleus chloroplast— pyrenoid-
fossil relatives occurring in rock formations reported to be nearly 1 billion years old. Chlamydomonas is unicellular, with a slightly oval cell surrounded by a complex multi-layered wall that is partially composed of glycoproteins. A pair of whiplike flagella at one end pull the cell very rapidly through the water. The flagella are, however, difficult to see with ordinary light microscopes. The cell itself is usually less than 25 micrometers (one ten-thousandth of an inch) long, which is, however, more than three times larger than a human red blood cell. Near the base of the flagella there are two or more vacuoles that contract and expand. They apparently regulate the water content of the cell.
A dominant feature of each Chlamydomonas is a single, usually cup-shaped chloroplast that at least partially hides the centrally located nucleus. One or two roundish pyrenoids are located in each chloroplast. Pyrenoids are proteinaceous structures thought to contain enzymes associated with the synthesis of starch. Most species also have a red eyespot on the chloroplast near the base of the flagella. The eyespot is sensitive to light; it is, however, merely part of an organelle within a single cell and is in no way complex and multicellular like an eye.
Before a Chlamydomonas reproduces asexually, the cell's flagella degenerate and drop off or are reabsorbed. Then the nucleus divides by mitosis, and the entire cell contents become two cells within the cellulose wall. The two daughter cells develop flagella, escape, and swim away as the parent cell wall breaks down. Once they have grown to their full size, they may repeat the process. Sometimes mitosis occurs more than once, so that 4, 8, or up to 32 little cells with flagella are produced inside the parent cell. Occasionally, flagella do not develop, and the cells remain together in a colony. When growth conditions change, however, each cell of the colony may develop flagella and swim away. This type of reproduction brings about no changes in the number of chromosomes present in the nucleus, and all the cells remain haploid.
Under certain combinations of light, temperature, and additional unknown environmental forces, many cells in a population of Chlamydomonas may congregate together. Careful study of such events has revealed that pairs of cells appear to be attracted to each other by their flagella and function as gametes that are sometimes of two types. The cell walls break down as the protoplasts slowly emerge and mate, fusing together and forming zygotes. A new wall, often relatively thick and ornamented with little bumps, forms around each zygote. This may remain dormant for several days, weeks, or even months, but under favorable conditions, a dramatic change occurs. The cell contents, now diploid, undergo meiosis, producing four haploid zoospores (motile cells that do not unite with other cells; many different kinds of algae produce zoospores). When the old zygote wall breaks down, the zoospores swim away and grow to full-sized Chlamydomonas cells (Fig. 18.4).
Often, an examination of dead twigs, rocks, and other debris in cold freshwater ponds, lakes, and streams (and a few marine habitats) reveals a threadlike alga called Ulothrix (Fig. 18.5), whose name is derived from the Greek words oulos (woolly) and thrix (hair). Each alga consists of a single row of cylindrical cells attached end to end and forming a thread, or filament. The basal cell of each filament is slightly longer than the other cells and functions as an attachment cell, or holdfast. Around the periphery of each cell is a wide, curved, somewhat flattened chloroplast. Each chloroplast contains one to several pyrenoids. Any of the cells, except the holdfast, may divide, and as they do so, the filaments grow longer.
The cell contents of any cell except the holdfast appear to clump and condense inside the rigid cell wall, divide by mitosis, and become zoospores. The zoospores of Ulothrix are quite similar to Chlamydomonas cells in that they have contractile vacuoles and an eyespot, but they have four, rather than two, flagella.
Frequently, the cell contents divide one to several times before becoming zoospores, but after zoospores are formed, they usually escape from the parent cell through a pore in the wall. After swimming about for a few hours to several days, they settle on submerged objects, the flagella are shed, and the cells divide. One of the first two daughter cells becomes a holdfast, while the other continues to divide, becoming a new filament. In some instances, the protoplasts do not produce flagella after they have condensed and divided like developing zoospores, but they are otherwise capable of germinating and producing new filaments. Such cells, called autospores,
lack the capacity to become motile and are released when the parent cell wall breaks down. Similar cells that do have the capacity to become motile are called aplanospores.
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