What develops from a spore in bryophytes?

In bryophytes, the spore germinates to form a protonema, which is a filamentous structure that often resembles algae. The protonema is an early stage in the lifecycle of bryophytes and serves as the initial growth phase after a spore's germination. Following this stage, the protonema can develop into the more complex gametophyte structure, which is the dominant phase in the lifecycle of bryophytes and is responsible for producing gametes. Thus, the protonema plays a crucial role as the first step in developing the gametophyte.

The subsequent structures, such as sporophytes and rhizoids, arise in relation to the established gametophyte rather than directly from the spore. The sporophyte grows from the gametophyte and produces spores, while rhizoids may develop from the gametophyte as well, providing anchorage but not directly arising from the spore itself. Understanding the sequence of development in bryophytes clarifies why the correct answer is protonema.