|10: The Land Plants An Evolutionary Overview||Introduction|
The evolution of terrestrial plants from their aquatic ancestors began over 450 million years ago. In the next several labs, we will investigate several aspects of the evolving adaptations of plants to the terrestrial environment. The appearance of differentiated organs and tissues, as exemplified in leaves, stems,roots, and vascular systems, is a feature of obvious importance. We will later spend some time studying the evolution of the leaf and adaptations of leaves to different environmental conditions. Major changes also took place in the reproductive systems of plants, largely because sperm could no longer swim freely from one plant to another.With each new group of plants, the problem of transferring gametes was solved in a different and increasingly more efficient way.
Certain green algae (Charophytes and Coleochaete), with similar photosynthetic pigments, cell wall chemistry, storage products, mitotic characteristics, and distinctive enzyme systems, are considered to have an immediate common ancestor with land plants. While there is some variety of life cycles in the green algae, the direct immediate aquatic ancestor of the land plants was probably characterized by sporic meiosis with an isomorphic alternation of generations. This type of life cycle probably gave way to heteromorphic alternation of generations in land plants. Thus all land plants show alternation of generations between a diploid stage (the sporophyte, which produces haploid spores by meiosis) and a haploid stage (the gametophyte, which produces gametes by mitosis), but one stage or the other is dominant in the life cycle (that is, one stage is larger and better developed than the other). In one line of land plants (the mosses and their relatives), the gametophyte is dominant, while in the remaining land plants the sporophyte is dominant. Be sure to learn the differences between gametophytes and sporophytes, since much of the evolution of reproductive systems in land plants involved changes in the relative sizes of these stages.
The objective of this laboratory is to survey the extant (i.e. non-extinct) groups of terrestrial plants. This will provide an introduction to the general characteristics, life cycles, and overall size and structural complexity of the major groups of living plants. In later labs we will examine in greater detail the evolution of some structural and physiological characteristics of vegetative organs. In this lab, then, do not be overly concerned with details of reproduction or structure. Rather, use this time as an opportunity to: