=305. Fertilization.=—Fertilization is accomplished by the spermatozoids swimming in at the open end of the oogonium, when one of them makes its way down into the egg and fuses with the nucleus of the egg. [Illustration: Fig. 143. Fertilization of vaucheria. _fn_, female nucleus; _mn_, male nucleus. The different figures show various stages in the fusion of the nuclei.] =306. The twin vaucheria (V. geminata).=—Another species of vaucheria is the twin vaucheria. This is also a common one, and may be used for study instead of the sessile vaucheria if the latter cannot be obtained. The sexual organs are borne at the end of a club-shaped branch. There are usually two oogonia, and one antheridium between them which terminates the branch. In a closely related species, instead of the two oogonia there is a whorl of them with the antheridium in the center. =307. Vaucheria compared with spirogyra.=—In vaucheria we have a plant which is very interesting to compare with spirogyra in several respects. Growth takes place, not in all parts of the thread, but is localized at the ends of the thread and its branches. This represents a distinct advance on such a plant as spirogyra. Again, only specialized parts of the plant in vaucheria form the sexual organs. These are short branches. Farther there is a great difference in the size of the two organs, and especially in the size of the gametes, the supplying gametes (spermatozoids) being very minute, while the receptive gamete is large and contains all the nutriment for the fertilized egg. In spirogyra, on the other hand, there is usually no difference in size of the gametes, as we have seen, and each contributes equally in the matter of nutriment for the fertilized egg. Vaucheria, therefore, represents a distinct advance, not only in the vegetative condition of the plant, but in the specialization of the sexual organs. Vaucheria, with other related algæ, belongs to a group known as the _Siphoneæ_, so called because the plants are tube-like or _siphon_-like. [Illustration: Fig. 143_a_. Botrydium granulatum. _A_, the whole plant; _B_, swarm spore; _C_, planogametes; _a_, a single gamete; _b_-_e_, two gametes in process of fusion; _f_, zygote.]