(A) Hepaticophyta - Liverworts

This group of plants is composed of two rather distinct lineages. In the thalloid liverworts, the vegetative body is a flat, more or less ribbon-shaped lamina, or thallus, that often shows dichotomous or irregular branching. In the other group, the leafy liverworts, the vegetative body consists of a central stem bearing two opposite rows of leaflike structures. There are no vascular tissues in either liverwort lineage, and little cell specialization.

Plants first colonized land between 450 and 470 million years ago, as evidenced by meiospore tetrad fossils. These early land plants are thought to have been liverwort-like, though the oldest undisputed liverwort fossil, Pallavicinites, is only 350 million years old. Many important adaptations for life on land first appeared in the liverworts.

Examine the gametophyte of Marchantia, a thalloid liverwort. Examine whole pieces with a dissecting scope, then make a water mount of a freehand section of the thallus, cover it with a cover slip, and examine it under higher power.

1. Examine the structure of the thallus (see Rust, fig. 25a; C&V fig. 6.2, 6.5). Do the cells in all layers contain chloroplasts, or are different layers specialized to different functions? Is there a specialized epidermis?
2. Can you observe a cuticle?
3. Are there pores or openings into the thallus? Are there stomata?
4. Are there air spaces inside the thallus? Why might they be necessary?
5. Examine the lower surface of a thallus. What is the function of the clear or whitish rhizoids (C&V fig 6.4)? Describe their location and structure.
6. Examine a leafy liverwort if one is available. How does it differ from a thalloid liverwort or a moss?