bio1120_plant_evolutionary_trends 3/21/07
BIOL 1120 REEDER
PLANT EVOLUTIONARY TRENDS
I. Evolutionary Trends Among Plants
A. Fossil Record
1. Multicellular green algae were well developed 700 million years ago.
a. Green algae share a number of biochemical and metabolic traits with plants
1) Both contain same photosynthetic pigments: chlorophylls a & b, carotenes (orange),
xanthophylls (yellow)
2) Both store carbohydrates as starch
3) Both have cellulose in their cell walls
4) Both share details of mitosis
2. Simple green shoots established on moist soil 400 million years ago possessing rudimentary
water-conducting cells assisted by mycorrhizal fungi associated with their underground stems for water
and nutrient absorption; over 90% of plant species today still depend on mycorrhizae.
3. Species with large fern-like fronds, tall woody stems and complex seeds formed lush green forests by
345 million years ago.
B. The Major Trends: primarily in respect to a terrestrial existence.
1. From nonvascular to vascular plants:
a. Significant relationship between plant requirements (dissolved mineral ions, water and sunlight for
photosynthesis) and their influence on the morphological changes that were required for the
transition to land and the eventual evolution of complex land plants.
b. Aquatic algae: water provides bodily support and is readily available containing dissolved
minerals surround the organism; occupy upper layer of water which is best penetrated by light.
c. Increased size and complexity of land plants required root and shoot systems for support
containing vascular tissue (conducting tissue) for transport of substances from one part of the
multicellular plant to another.
1) Vascular tissue: xylem (transports water and mineral ions from roots up through the plant)
and phloem (transports sugars and other photosynthetically produced nutrients)
2) Strengthening material and cells: supporting tissue (lignin in the cell walls; fibers,
sclerenchyma, and collenchyma cells) that allows plants to grow taller and escape the shade
created by others as well as turgor pressure of supporting parenchyma tissue
3) Leaf
: photosynthetic, water and gas regulatory organ.
4) Root: absorptive and anchoring organ
5) Cuticle: waxy covering to restrict water loss and provides UV radiation protection
2. Toward a dominant sporophyte:
a. Principal reproductive mode found in plant life cycles is sexual: haploid cells produced meiotically
(sporulation) ultimately leading to a diploid zygote forming as a result of fertilization = characterizes
the most advanced plants (GAMETIC LIFE CYCLE)
1) Instead of meiotically produced gametes as in animals, plants produce spores meiotically and
gametes mitotically.
b. Main variations in the life cycles: 1) length of time in the haploid (gametophyte) and diploid
(sporophyte) stages: independent or dependent stages (dominance) and 2) how large and complex
the plant body becomes during each stage.
1) Simple algae, mosses, liverworts and hornworts: gametophyte is the dominant, independent
conspicuous stage with the sporophyte dependent on it.
2) Ferns and fern allies: both gametophyte and sporophyte become independent with the
sporophyte being the most conspicuous stage (achieving dominance) during the life cycle.
3) Gymnosperms and angiosperms (flowering plants): sporophyte is the conspicuous,
independent multicellular dominant stage and the gametophyte is tiny and dependent on it.
c. In regard to plant body complexity, variations exist regarding multicellularity in the sporophyte
and gametophyte generations.
1) True alternation of generations: a multicellular haploid stage (gametophyte) alternates with a
multicellular diploid stage (sporophyte) as in mosses, ferns, liverworts, and some algae (as the
green algae Ulva) = SPORIC LIFE CYCLE.
a) The gametophyte produces haploid gametes mitotically which fuse to form the
sporophyte.
b) The sporophyte produces haploid spores meiotically which after germinating, develop
into gametophytes.
