[Illustration: Fig. 21. Conjugation of Zygnema quininum:—A. two filaments in the first stage of conjugation; B, completion of the act of conjugation.]
Some of the spores are quiescent, others have cilia and are motile, but both after a time become attached at one end by two or three root-like fibres, and grow into filaments by repeated bisection. According to the observations of M. Itzigsohn, the endochrome in certain filaments of Spirogyra breaks up before conjugation into little spherical aggregations, which are gradually converted into nearly colourless spiral filaments, having an active spontaneous motion, and therefore corresponding precisely to antherozoids. With the exception of South America, the Conjugatæ are widely dispersed in warm and temperate climates.
The genus Vaucheria may be assumed as a type of the Siphoneæ, whose essential character is, that the plant consists of one single tubular cell, however branched and complicated its form may be. The Vaucherias form tufted masses of branching tubes, filled with bright green granular matter, on mud and damp soil; they abound in fresh-water pools, and some grow in the sea. When about to produce fruit, the extremities of some of the tubes swell out in the shape of a club, in which a portion of the green matter collects, takes a darker hue, and is separated from the rest by a transparent space and a new envelope. After various changes, the darker green matter forms itself into a zoospore, which is so active that it breaks open the top of its club-shaped cell, and comes into the water; sometimes several come, one after another. They are egg-shaped, with a colourless beak, and as their whole body is bristled with cilia, they leave a long current in their wake when they swim, which they do with such impetus that they are flattened against any obstacle they meet with, even to the discharge of their green endochrome. They escape from their cell about eight in the morning, move for two hours, then come to rest, and begin to grow into a new plant.
M. Pringsheim discovered another mode of reproduction in the Vaucherias, which are monœcious plants, that is to say, the same plant produces snake-like fertilizing spermatozoids and female germ cells. For example, the Vaucheria sessilis consists of one long branched cell; on the same side of it two swellings appear near to each other, one of which elongates, curls round like a horn, and is soon filled with snake-shaped filaments having long cilia at their thin end, with which they move rapidly both within the horn, and after they come out of it into the water. They are perfectly colourless, and correspond to the pollen of flowering plants. The other protrusion which swells into a globose germ cell, and which corresponds to the pistil of a flower, contains a mass of green endochrome, which, after being fertilized by the snake-like filaments, becomes a primordial cell which has no motion, but after having secreted a strong coating of cellulose, it sinks to the bottom of the water, becomes a winter or resting spore, and lays the foundation for a new generation of plants. The resting spores produce new forms, while the zoospores, like buds, only multiply the type of the individual plant with all its peculiarities.