Page:Popular Science Monthly Volume 60.djvu/77

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or frequently 4 cilia at the pointed end. The illustrations show these and other characters clearly. A portion of the protoplasm is differentiated as a green body (chloroplast), which only partially fills the rounded end of the zoospore, leaving the rest of the cell quite clear. Sometimes the chloroplast contains a central body called the pyrenoid, which is associated with the starch-forming activities of the chloroplast. This structure must not be confused with the nucleus of the cell, the latter being almost always invisible in the living zoospore. Finally, one may always expect to find in the colorless pointed end, near the cilia, a small bright red body called the pigment spot. The pigment spot is generally believed to have a relation to the sensitiveness displayed by zoospores towards light, not in any sense, however, as an organ of vision, as might be judged by the unfortunate term 'eye-spot' that is sometimes applied to it.

Such is the structure of the zoospores. Now let us consider their habits. As we have said before, several are likely to be developed in a single cell, but there is no rule as to number. Sometimes the entire contents of a cell will slip out as a single zoospore, but more frequently 8, 16 or 32 will be formed, a variable number even in the same plant, and in certain cases the parent cell will give rise to hundreds. The zoospores escape from the mother cell or sporange usually through some opening in the wall and immediately swim off. They may be developed so numerously that the water is actually colored greenish and the field of the microscope shows hundreds of these organisms moving rapidly in various directions. Such appearances have given them the appropriate name of swarm-spores. The swarming of zoospores is best shown under certain conditions of illumination. The zoospores are very sensitive to light and usually arrange themselves with reference to its source so that the long axes are parallel with the incoming rays. If a vessel of water be placed so that the light rays come from one direction, as from a window, the zoospores will move in parallel lines towards or away from the source of the illumination. They will thus collect in clouds in various parts of the vessel, the exact position being somewhat modified by the currents of water that slowly circulate through every brightly illuminated vessel.

Generally speaking the swarm-spores that one is most likely to see will be asexual. Their activities cease after a few hours or perhaps minutes and they then attach themselves in some suitable position, germinate and develop young plants called sporelings. Sporelings of the alga, Ulothrix, are shown in Figure 2, d, they having developed from a zoospore like c.

But frequently and under conditions that have been in part determined swarm-spores will behave quite differently. They will swim at first very actively, approaching one another and then darting away,