Plants appeared on land about 425 million years ago and the evolutionary history of the plant kingdom reflects increasing adaptation to the terrestrial environment.
General Characteristics of Plants:
1. Multicellular eukaryotes that are photosynthetic autotrophs.
2. Contain chloroplasts with the photosynthetic pigments chlorophyll a, and b and carotenoids.
3. Cell walls containing cellulose.
4. Food reserve is starch that is stored in plastids.
5. Aerial parts are coated with a waxy cuticle that helps prevent desiccation.
6. Gas exchange cannot occur across the waxy cuticle so specialized openings on the under surface of the leaf, called stomata, allow gas exchange.
7. Reproductive adaptations necessary as plants moved from and aquatic to a terrestrial environment
a) Gametes must be dispersed in a nonaquatic environment. Plants produce gametes within gametangia, organs with protective jackets of nonreproductive cells that prevent the gametes from drying out. The ovum is fertilized within the female organ.
b) Embryos must be protected against desiccation. The zygote develops into an embryo that is retained for awhile within the female gametangia’s jacket.
8. All plant life cycles have “Alternation of Generation”.
a) A haploid gametophyte generation produces and alternates with a diploid sporophyte generation. The sporophyte in turn produces the haploid gametophyte.
b) The life cycles are heteromorphic; sporophytes and gametophytes differ in morphology.
c) The sporophyte is larger more conspicuous and more dominate in all except the Bryophytes and their relatives.
Plants evolved 425 million years ago during the Silurian period. They are believed to have evolved from green alga probably charophytes. Evidence includes:
a) These algae have a cell wall composition similar to plants.
b) The structure and function of the chloroplasts is homologous in these two groups. Both have chloroplasts with thylekoid membranes stacked as grana.
c) Plant chloroplast DNA most closely matches that of charophytes.
d) Green algae and plants contain cellulose in their cell walls
e) Similarity in mitosis and meiosis
f) Charophyte sperm are more similar to certain plants than to other green algae
g) DNA and rRNA are similar in charophytes and plants.
1. Lack vascular tissue. Imbibe water though diffusion and capillary action.
2. Require environmental water to reproduce. Their flagellated sperm must swim from the antheridium to the archegonium to fertilize the ovum.
3. With a waxy cuticle to prevent desiccation.
4. With gametangia that protect developing gametes.
a) Antheridium, male gametangia produces flagellated sperm.
b) Archegonium, female gametangia, produces a single ovum and fertilization occurs within the gametangium and the zygote develops into an embryo within the protective jacket if the archegonium.
5. Lack woody tissue and cannot support tall plants.
6. Each plant grips the substrate with rhizoids.
7. The gametophyte is the dominant, conspicuous generation.
Division Hepatophyta, Liverworts
1. Bodies divided into lobes (resembling the lobes of the liver).
2. Life cycle similar to mosses.
3. Lack vascular tissue.
4. Require environmental water for fertilization.
5. Can reproduce asexually from gemmae.
1. Lack vascular tissue.
2. Require environmental water for fertilization.
3. Sporophytes are horn shaped elongated capsules that grow from the mat-like gametophyte.
TRACHEOPHYTES-Vascular plants, with vascular tissue.
Division Sphenophyta, horsetails
Division Pterophyta, ferns
Division Coniferophyta (Conifers), largest division of gymnosperms. Most are evergreens and include pines, firs, spruces, junipers, cedars, cypresses, and redwoods.
Division Cycadophyta Sago Palm, Leather leaf or Cardboard palm.
1. Broad, fan-shaped leaves. Profusely branched and deciduous.
2. The strobili are of two kinds and borne on separate trees.
Division Gnetophyta Mormon tea, Ephedra
Angiosperms-Flowering plants. Most widespread and diverse of all plants.