Wondrous Events in Evolution Notes for Plant Taxonomy Biology 4420 at Utah State University Prepared by M.E. Barkworth

Factors important to success Aspects of life to consider: Nutrition Reproduction Competition Cost/benefits of structures and strategies For dominance, it is not enough to be good – one must be better than one’s competitors For survival, it is enough to be better than or as good as one’s competitors in some situations.

Land plants 450 MYBP First evidence of land plants Cutin, stomates,?mycorrhizal partnerships Bryophytes and tracheophytes more or less simultaneous (give or take a million years or so)

Tracheophytes Diploid life style Backup system in place Tracheids More cellulose, lignin Benefits outweigh costs

Megaphyllous leaves Microphyllous leaves One vein No axillary branches No leaf gaps Restricts shape, size Megaphyllous leaves Better transportation Diverse shapes Leaf gaps so less expensive Benefits outweigh costs microphyllous leaves megaphyllous leaves

Tracheophytes can be tall … What’s the advantage? Sex at ground level so no still restricted Sex requires water so still restricted A film of water is adequate Chemical guidance system works well Gametophytes independent, need moisture and nutrition source (soil) so still restricted

The solution is …. Ovules and pollen grains Ovule provide food and protection for female gametophyte Pollen grain protects the wandering male gametophyte Still have to get the two together Once fertilized, the ovule starts to become a seed

Seeds 1 Integumented Protective cell layers Integument(s) diploid Indehiscent Does not split Megasporangium Where female spores form and make megagametophyte which makes female gamete, aka egg

Seeds 2 Megasporangium produces Megaspores which form megagametophyte which forms egg (female gamete) Other cells of gametophyte provide nutrition – endosperm Megasporangium wall forms nucellus Megasporangium is surrounded by diploid tissue (integument) with vascular connection to parent Integument displays great diversity (think seed coat)

Seeds 3 Advantages Place genetically vulnerable phase, gametophyte, in protected surrounding Provide great environment for that difficult event – mixing of DNA from parents Great place for children to start life Problem: How does male gamete get there?

Solution - Pollen Grains Armored airship for male gametophyte Armor is sporopollenin Meiosis in microsporangia forms first cell of microgametophyte Male gametes develop through mitotic divisions in microgametophyte Dispersal Initially wind Eventually insect Some water

Seed and Pollen Grain Sex above ground level Adequate moisture from female parent Eventually reduction number of cell divisions and time in gametophyte phase for both males and females

New problems Bringing the male and female gametes together Watch for new developments as plants solve this problem “Educating” wind Educating animals

Back to being tall Wood is energetically expensive Need to minimize cost, maximize benefit Fibers, the strong part of wood, primarily associated with vascular bundles Organization of vascular tissue important

Vascular Tissue Organization Rod Cylinder Scattered Circle, aha! Circular cambium “Good Wood” has circle of vascular bundles that subsequently are united by cambium which form xylem on inside and phloem on outside

Important but Extinct Groups Progymnosperms (370 MYBP) 35 million years 12 m tall Had “good wood”, fernlike foliage and reproduction

Important but Extinct Groups Pteridosperms Seeds Fern like leaves Some had “good wood” Cycads, Pinophytes, Gnetophytes, Angiosperms may have evolved independently from different groups of pteridosperms.