What type of protists are thought of as the ancestors of terrestrial plants?
Terrestrial plants, or land plants, are descended from green algae. Green algae contain chlorophyll, which they use to carry out photosynthesis. Similarly, terrestrial plants rely on chlorophyll to synthesize sunlight, water, and carbon dioxide into usable sugars.
Though it can sometimes be difficult to classify protists, the organisms we know as green algae emerged as a distinct lineage around 1,500 million years ago. As these organisms continued to evolve in response to environmental pressures, they developed a variety of traits like motility, flagellae, and a propensity to "colonize" or group together.
Some botanists believe that land plants may have evolved from multiple kinds of algae. Algae can and does grow on land, and species like those in the genus Klebsormidium can offer insight into the transition from aquatic to terrestrial life forms.