Light micrograph showing conjugating filaments of Spirogyra, a green alga which forms slimy threads 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 conjugation tubes, seen here. Cytoplasmic mass from the cells of one of the threads (termed male) migrates through the tubes to fuse with the cytoplasm in the (female) cells of the second thread. This produces a zygospore which either germinates at once or lies dormant until the following spring. Bright field illumination. Magnification: x200 at 35mm size.