Spirogyra conjugating. Polarised light micrograph showing conjugating filaments of Spirogyra sp. green alga. Spirogyra is named for its spiral chloroplasts (green) found within each cell. Conjugation, a form of sexual reproduction, occurs when adjacent filaments put out papillae which meet to form conjugation tubes. Cytoplasm from the cells of one of the threads migrates through the tubes to fuse with the cytoplasm in the cells of the other thread, forming zygospores which lie dormant until the next spring. Magnification: x186 when printed 10 centimetres wide.