Differential interference contrast (DIC) light micrograph of Spirogyra sp. green alga filaments. These are two alga filaments 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 (green spheres). Five conjugation tubes are visible here together with six zygotes, three cells could not develop a tube. When conditions are favourable the zygospore will germinate, producing new filaments. Magnification: x100 when printed at 10 centimetres wide.