Biology 106 Lecture Topic # 19 EVOLUTION OF PLANTS

I. Ancestry - Invaded land more than 400 mybp, - Descended from multicellular green algae

II. Importance

-Only 3 groups of organisms responsible for almost all the fixation of carbon in the world, thus providing the substance for all living organisms. 1. algae 2. photosynthetic bacteria 3. land plants

III. General Characteristics

1. multicellular eukaryotes 3. chloroplasts with chlorophyll a and b, and carotenoids

2. cellulose rich cell wall 4. starch is primary carbohydrate food reserve

IV. Problems confronting plants invasion onto land

A. Keeping cells supplied with water by developing:

1. dessication resistant spores 3. stomata in leaf surface to permit fine control of water exchange

2. waxy cuticle layer on outer surface 4. vascular tissue to transport water around the plant

B. Develop specialized organs

1. roots - absorptive system and anchor; 2. leaves - house photosynthesis apparatus

3. stems - structure containing conductive system and provides physical support for the plant

4. vascular tissue to transport water and minerals (xylem) and sugars, amino acids, and other nutrients (phloem).

C. Physiological Adaptations

1. enough water on plant surfaces to promote efficient gas exchange of O2 and C02

2. withstand extreme temperatures (evaporative cooling)

3. withstand UltraViolet radiation via absorption by pigments for flower color

4. withstand strong winds etc., 5. withstand the pull of gravity

D. Carry on reproduction outside of watery medium

1. eventually develop into pollen (male gametes) and ovules (female gametes) and the formation of seeds

V. Two major groups of land plants

1. Nonvascular:

- ex., mosses, liverworts, hornworts,> 24,000 species, nonvascular plants, thus need free water to thrive, small

- need free water to reproduce sexually (motile sperm swim to eggs)

- gametophytes are green, relatively large and conspicuous (principal vegetative stage)

-sporophytes are small and are dependent on gametophytes for nutrients

2. Vascular:

- ex. horse tails, ferns, all seed plants (angiosperms and gymnosperms)

- true vascular tissue to conduct food and water

- specialized organs (i.e., leaves, stems, roots, cuticles, and stomata)

- decrease prominence of gametophyte stage, now microscopic and transient

- large, dominant and nutritionally independent sporophyte stage

- Heterospory: microspore = male gametophyte; megaspore = female gametophyte

- evolution of seeds only occurs in heterosporous species

5 Divisions of Vascular Plants: Seedless : Psilotophyta, Lycopdoiophyta, Sphenophyta, Pteridophyta

Seed: Spermophyta

VI. Seed Plants -2 types

1. Gymnosperms - Non-flowering. Ovules and seeds are exposed on the surface of sporophylls or analogous structures without protective ovarian walls. ex. cycads, 1 spp ginko, and Conifers.

A. Conifers, 450 spp living, woody perennials, mostly trees, some shrubs, narrow, needle like or scale like lvs., diploid sporophyte, a large tree with often 2 types of cones female and male on the same plant.

 

2. Angiosperms - Flowering Plants. Ovules are enclosed within the tissue of the parent plant (sporophyte) in structures called carpels. Carpels eventually mature into fruit. Ancestry: could not have directly evolved from a living division of gymnosperms. However, presence of seed suggests ancestor may have been an extinct gymnosperm. > 250,000 spp living, ex., herbaceous plants, shrubs, trees, nearly all our food is derived directly or indirectly from flowering plants.

A. Angiosperms are divided into 2 major groups.

Monocots: 60,000 spp, ex. grasses, corn wheat, lilies. Dicots: 200,000 spp, ex. peas, azaleas, dogwoods

Double Fertilization - unique to angiosperms, involves multiple pollen grain nuclei, (1) pollen tube, (2) pollen grain nuclei fuses with 2 polar nuclei forming the endosperm (3n), and (3) pollen grain nucleus + ovule nucleus -> zygote.

Take Home Messages:

1. Alternation of generations from more primitive to advanced plants is documented by a shift in prominence from the gametophytic to the sporophytic stage. 2. Plants confronted many problems upon invading land. 3. Know major groups of plants in evolutionary history and what traits evolved in our major groups today.

Terms to Know: alternation of generations, bryophyte, tracheophyte, heterospory, angiosperm, gymnosperm, double fertilization