EVOLUTION

                                 Permission granted by Michael Farabee                                                                                    Evolution of Plants from Ancestral Green Algae

Evolution and Phylogenetic relationships

            Fossils and other biological evidences indicate that plants are descended from an ancestor green algae, Coleochaete. The oldest known fossil record of vascular plants was the Cooksonia which emerge 420 billion years ago. The early plant evolved and branched into many different phyla of plants. One of these groups are the vascular(tracheophyte), seedless plants. These include ferns, some mosses, and other distinct species, that produce through spores instead of seeds.

            Vascular seedless plants were the first vascular plants on the planet. Therefore, many of these plants have to develop adaptations to land. The waxy cuticle evolved to protect plants from dessication-the tendency of organisms to lose water to the air (Raven & Johnson). The stomata allows gas diffusion in and out of plants. Evolution of leaves allows plants to have increased photosynthetic surface area. The vascular evolution also allowed these vascular, seedless plants to have better structural support, allowing plants to grow to greater heights.     

            The history of plant evolution is pretty straightforward compared to that of animals. First, the sporophyte (diploid) stage developed and dominated over the gametophyte (haploid) stage*. The early sporophyte was dichotomously branched, having rhizomes instead of roots and leaves. Vascular plants developed, as plants developed true roots, leaves, and stems. Along with these outer physiological developments grew the xylem and the phloem, in order to transport the nutrients and water. Seedless vascular plants developed before seeded plants. Seedless plant sporophyte are still dominant over the gametophyte, but have not developed an outer protective shell. The sperm are flagellated and require water to reach the egg. The life cycle of seedless plants are similar to that of nonvascular plants:

Archegonium - egg, antheridium - sperm.

*this will be explained further on in the next slides.

                            Permission granted by Ravi Rao                                       Evolutionary table, showing appearance of different phyla of plants