Spirogyra algae. Differential interference contrast micrograph of Spirogyra sp. algae after conjugation, a form of sexual reproduction. During conjugation, conjugation tubes (bridge-like structures) are formed between two algal filaments and the contents of one cell passes through the tube. The gametes (sex cells) of the two cells fuse forming a zygospore (black spheres). When conditions are favourable the zygospore will germinate, producing new filaments. Magnification: x75 when printed at 10 centimetres wide.