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)
Diploid life style Backup system in place Tracheids More cellulose, lignin Benefits outweigh costs Tracheophytes
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 Organizationhttp://www.botany.hawaii.edu/faculty/webb/BOT311/Transition/invasion_landStele.htm • 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.