Page:Popular Science Monthly Volume 60.djvu/80

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72
THE POPULAR SCIENCE MONTHLY.

with great ease in the laboratory. This plant is a great cell colony, involving usually thousands of elements which are joined to one another to form a net of polygonal meshes. A portion of such a plant is shown in Figure 3, a. These cells after they have reached a certain age produce zoospores that may be either asexual or sexual (gametes). The gametes are smaller and are produced much more numerously than

PSM V60 D080 Hydrodictyon.png

Fig. 3. Hydrodictyon. a, Portion of Net. b, End of Cell showing Young Net in its Interior, c, Gamete. d, e, Conjugation of Gametes.

the asexual elements. They escape from the mother-cell and after swarming in the water conjugate in pairs (see Figure 3, c, d, e). The asexual swarm-spores have the peculiarity of never leaving the mother cell. They swim around in the cavity bounded by the cell wall and shortly come to rest, arranging themselves to form a new net entirely within the parent cell. Thus by their habits one may readily distinguish the asexual zoospores and gametes of Hydrodictyon.

Now let us summarize the factors that will make Hydrodictyon form asexual zoospores at one time and gametes at another. The waternet grows luxuriantly in a culture solution containing a number of inorganic salts. If plants are removed from such a culture solution and placed in fresh water they will develop zoospores in 34 hours. The process takes place most rapidly if the temperature is slightly above the normal; indeed merely warming the water in which plants are living will frequently induce the production of zoospores. Plants refuse to form zoospores at a temperature as low as 8° C. but if such a culture be raised to the warmth of 16° or 20° the process is immediately resumed. Gametes are produced under very different conditions from those stated above. They demand organic food. Cultures of Hydrodictyon in a solution of cane sugar will almost certainly yield gametes after several days.

It frequently happens that the nets of Hydrodictyon will exhibit a well-defined tendency or preference to form either gametes or zoospores. Such a habit may be quite thoroughly broken by cultivating a plant under proper surroundings. Gamete-forming nets will shortly