What does the term 'thallus' refer to in the context of algae or plants?

The term 'thallus' refers to a plant or algal body that does not have the differentiated structures such as true roots, leaves, or stems that are typically found in higher plants. In the context of algae, the thallus can be thought of as the entire body of the organism, which often takes on a variety of forms such as sheets, filaments, or branched structures. This body plan allows algae to carry out essential functions such as photosynthesis and nutrient absorption directly through their surface, facilitating their adaptation to aquatic environments.

This definition highlights the simplicity and diversity of algal anatomy compared to vascular plants, which have evolved more specialized structures. In contrast, the other options describe different aspects of plant anatomy or reproduction, not directly related to the general body form of algae or non-vascular plants.