Spirogyra is a green alga which forms slimy threads called "water-silk" or "mermaid's tresses" in stagnant or slow moving water. Conjugation, a form of sexual reproduction, occurs in spring when the threads come together in pairs. Both filaments put out papillae, which meet to form the conjugation tubes seen here. Normally one of the threads produces male gametes and the other female gametes. The cytoplasmic mass of the male cell migrates through the tube into the female cell, where fusion takes place. This produces a zygospore which either germinates at once or lies dormant until the following spring.