=301. Sexual reproduction in vaucheria.=—The organs which are concerned in sexual reproduction in vaucheria are very readily obtained for study if one collects the material at the right season. They are found quite readily during the spring and autumn, and may be preserved in formalin for study at any season, if the material cannot be collected fresh at the time it is desired for study. Fine material for study often occurs on the soil of pots in greenhouses during the winter. While the zoogonidia are more apt to be found in material which is quite green and freshly growing, the sexual organs are usually more abundant when the threads appear somewhat yellowish, or yellow green. =302. Vaucheria sessilis; the sessile vaucheria.=—In this plant the sexual organs are sessile, that is they are not borne on a stalk as in some other species. The sexual organs usually occur several in a group. Fig. 139 represents a portion of a fruiting plant. =303. Sexual organs of vaucheria. Antheridium.=—The antheridia are short, slender, curved branches from a main thread. A septum is formed which separates an end portion from the stalk. This end cell is the _antheridium_. Frequently it is collapsed or empty as shown in fig. 140. The protoplasm in the antheridium forms numerous small oval bodies each with two slender lashes, the cilia. When these are formed the antheridium opens at the end and they escape. It is after the escape of these spermatozoids that the antheridium is collapsed. Each spermatozoid is a male gamete. [Illustration: Fig. 140. Vaucheria sessilis, one antheridium between two oogonia.] [Illustration: Fig. 141. Vaucheria sessilis; oogonium opening and emitting a bit of protoplasm; spermatozoids; spermatozoids entering oogonium. (After Pringsheim and Goebel.)] =304. Oogonium.=—The oogonia are short branches also, but they become large and somewhat oval. The septum which separates the protoplasm from that of the main thread is as we see near the junction of the branch with the main thread. The oogonium, as shown in the figure, is usually turned somewhat to one side. When mature the pointed end opens and a bit of the protoplasm escapes. The remaining protoplasm forms the large rounded egg-cell which fills the wall of the oogonium. In some of the oogonia which we examine this egg is surrounded by a thick brown wall, with starchy and oily contents. This is the fertilized egg (sometimes called here the oospore). It is freed from the oogonium by the disintegration of the latter, sinks into the mud, and remains here until the following autumn or spring, when it grows directly into a new plant. [Illustration: Fig. 142. Fertilization in vaucheria, _mn_, male nucleus; _fn_, female nucleus. Male nucleus entering the egg and approaching the female nucleus. (After Oltmans.)]