Plant Life Cycles All land plants have a common sexual reproductive life cycle called alteration of generations Gametophyte – haploid (n) generation Sporophyte – diploid (2n) generation One generation is always dominant (more conspicuous) – In the Bryophytes (mosses and liverworts) the gametophyte is dominant – In all other land plants the sporophyte is dominant
Alternation of Generations (also look at figure 2 in lab manual)
Alternation of Generations Vocab Spores – (n) produced by the sporophyte generation through meiosis – germinate (undergo mitosis) to produce the gametophyte (n) Sporangium – a protective nonreproductive jacket that contains the spores Gametes – produced inside the gametangia located on the gametophyte – Eggs (n) are produced inside the archegonia through mitosis – Sperm (n) are produced inside the antheridium through mitosis Zygote – (2n) formed by fusion of the gametes, this is the first stage of the sporophyte generation Note: the archegonia and antheridium may occur simultaneously in the same gametophyte, but on difference branches.
Non-vascular Plants 3 Phyla: – Bryophyta (mosses) – Hepatophyta (liverworts) – Anthocerophyta (hornworts) Small plants, lacking vascular tissue (specialized cells for transport of material) The gametophyte generation is dominant and conspicuous plant. Restricted to moist habitats – Because they lack vascular tissue – And because this enables their mobile sperm to swim and fertilize the egg Have a cuticle, but lack stomata on the surface of the thallus (plant body)
Bryophyta (mosses) Most common group Occurs in moist environments, but also found in dry habitats that are periodically moist Peat moss (Sphagnum)
Hepatophyta (liverworts) Flattened and lobed thallus (plant body) Early herbalists believed that these plants were could treat liver disorders. (based on the doctrine of signatures) Found along streams on moist rocks Very small
Liverwort body form Rhizoids – root-like extensions on the lower surface of the thallus There are pores on the leaf-like thallus that function in gas exchange, but lack guard cells so are always open. Gemmae cups are located on the upper surface of the thallus, they are circular cups that contain flat disks of green tissue called gemmae. The gemmae are washed out of the cups when it rains, and they grow into new, genetically identical liverworts. (asexual reproduction!)
Seedless Vascular Plants Two phyla: – Lycophyta (club mosses) – Pterophyta (ferns, horsetails, whisk ferns). Depend on water for external fertilization and development of the unprotected, free-living embryo. Lived in vast swampy areas during the Carboniferous period, but declined as Earth became drier. The fossilized remains of the swamp forests are the coal deposits we mine and use today.
Seedless Vascular Plants All have vascular tissue (specialized for conducting water, nutrients, and photosynthetic products). Alternation of generations where the sporophyte is dominant and the gametophyte is usually independent of the sporophyte. These plants have stomata and structural support tissue. Still retain primitive feature of motile sperm that requires water for fertilization, thus the gametophyte is small and only in moist habitats
Lycophyta (club mosses) Found in moist habiats (i.e. bogs, streamsides) One species of Selaginella, the resurrection plant, lives in deserts! Produce two kinds of spores (heterospory). – Megaspores – large spores that produce female gametophytes – Microspores – small spores that produce male gametophyte
Pterophyta (ferns, horsetails, whisk ferns) Until recently these three groups of seedless vascular plants were placed in separate divisions: Pterophyta (ferns), Sphenophyta (horsetails), and Psilophyta (whisk ferns). New molecular evidence has shown that they are all closely related, thus they are all now in the division Pterophyta.
Psilophytes (whisk ferns) Small, dichotomously branched (repeated Y-branches), photosynthetic stems that reproduce by aerial spores. Found today in some areas of Florida and in the tropics.
Sphenophytes (horsetails) Have green jointed stems with occasional clusters of leaves or branches. Their cell walls contain silica that give the stem a rough texture. Occasionally used by pioneers to scrub dishes – thus they were commonly called the scouring rushes. In cooler regions of North America they grow as weeds along roadsides.
Pterophyta (ferns) Most successful group of seedless vascular plants. Occupy habitats from the desert to tropical rain forests. Most are small plants that lack woody tissue, except the tree ferns found in tropical regions.