Presentation on theme: "Plants Land plants retain derived features they share with green algae: – Chlorophyll a and b. – Starch as a storage product. – Cellulose in cell walls."— Presentation transcript:
Plants Land plants retain derived features they share with green algae: – Chlorophyll a and b. – Starch as a storage product. – Cellulose in cell walls.
2 Major Divisions Vascular plants, or tracheophytes (seven clades)—all have conducting cells called tracheids; vessels for transport of H 2 O, nutrients, minerals, etc. [xylem & phloem] Nonvascular plants: the remaining three clades—liverworts, hornworts, and mosses; no vessels.
All land plants have a life cycle with alternation of generations. Includes multicellular diploid (2n)(sporophyte) and multicellular haploid (1n) (gametophyte) individuals.
Nonvascular Plants The plant you ‘see’ is the gametophyte (1n); they are photosynthetic and nutritionally independent from the next generation. The sporophyte (2n) is dependent & attached to the gametophyte-cannot live without it (may or may not be photosynthetic)
Nonvascular Plants Female: archegonium; produces one egg. Male: antheridium; produces many sperm with two flagella each. Water splashes sperm from antheridium to archegonia where they unite with egg to form an embryo (2n). Embryo develops into sporophyte (2n).
Liverworts Some have leafy gametophytes; some have thalloid gametophytes. Sporophytes very short—a few mm. A stalk raises the simple sporangium above ground level to allow spores to be dispersed. Liverworts also reproduce asexually: by simple fragmentation of the gametophyte; by gemmae—lens-shaped clumps of cells in gemmae cups. Gemmae are dispersed by raindrops. p. 623
Hornworts Gametophytes are flat plates of cells. Have stomata, which do not close. Hornwort cells have a single, large chloroplast. The sporophyte has no stalk; but has a basal region capable of infinite cell division. Sporophytes can grow up to 20 cm. Hornworts have internal cavities filled with nitrogen-fixing cyanobacteria.
Mosses (Bryophytes) Gametophyte begins as a branched, filamentous structure—the protonema. Some filaments are photosynthetic, others are rhizoids that anchor the protonema. Tips of photosynthetic filaments form buds which produce the leafy moss shoots. Moss sporophytes and vascular plants grow by apical cell division—a region at the growing tip provides an organized pattern of division, elongation, and differentiation.
Mosses Some moss gametophytes are too large to depend on diffusion for water transport. Cells called hydroids die, and provide channels in which water can travel. Hydroids may be the progenitor of tracheids.
Vascular Plants Developed Heterospory: produce two types of spores: Megaspore develops into female gametophyte—megagametophyte: produces small numbers of spores. Microspore develops into male gametophyte—microgametophyte: produces large numbers of spores. Does not rely on H 2 O for fertilization!!
Horsetails & Whisk Ferns Horsetails: Silica in cell walls—“scouring rushes.” Have true roots; sporangia are on short stalks called sporangiophores. Leaves are reduced megaphylls in whorls. Each stem segment grows from the base. Whisk ferns: No roots but well-developed vascular system. Psilotum flaccidum has scales instead of leaves. Tmesipteris has flattened, reduced megaphylls.
Ferns Ferns: Sporophytes have true roots, stems, and leaves. Starts development as a coiled “fiddlehead.” (p. 626)
Fern Life Cycle Spore mother cells in sporangia form haploid spores by meiosis. Spores can be blown by wind and develop into gametophyte far from parent plant. Fern gametophytes produce antheridia and archegonia, not always at the same time or on the same gametophyte. Sperm swim through water to archegonium to fertilize egg. Zygote develops into independent sporophyte.