Wondrous Events in Evolution

Notes for Plant Taxonomy

Biology 4420 at Utah State UniversityPrepared 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

microphyllousleaves

megaphyllousleaves

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• Progymnosperm

s (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.