Final answer:
Spirogyra turns brown after zygospore formation because the cells that produced the zygospore die off, while the zygospore itself enters a dormant state with a thick protective wall awaiting favorable conditions to germinate.
Step-by-step explanation:
Spirogyra turns brown after the formation of a zygospore because the cells that created the zygospore undergo a series of changes. The zygospore is typically a dormant structure that can withstand various environmental conditions. In the zygomycetes, which includes Spirogyra, the formation of the zygospore is part of the sexual reproduction process, where two compatible hyphae come together and their nuclei fuse. This leads to the development of a thick-walled zygospore that enters dormancy. The cells around the zygospore may die, resulting in a brown appearance. Once conditions are favorable, the zygospore germinates, producing a sporangiophore that bears a sporangium at its tip. Within this sporangium, the diploid nuclei undergo meiosis to form haploid spores, which are then dispersed when the sporangium disintegrates.