Solution

Isogamous: a condition in which the sexual cells, or gametes, are of the same form and size and are usually indistinguishable from each other. Many algae and some fungi have isogamous gametes. Spirogyra, genus of some 400 species of free-floating green algae (division Chlorophyta), found in freshwater environments around the world. Named for their beautiful spiral chloroplasts, Spirogyra species are filamentous algae that consist of thin, unbranched chains of cylindrical cells. Spirogyra species can reproduce both sexually and asexually. Asexual, or vegetative, reproduction occurs by simple fragmentation of the filaments. Sexual reproduction occurs by a process known as conjugation, in which cells of two filaments lying side by side are joined by outgrowths called conjugation tubes. This allows the contents of one cell to completely pass into and fuse with the contents of the other. The resulting fused cell (zygote) becomes surrounded by a thick wall and overwinters, while the vegetative filaments die.