Plants


The four kinds of plants


Plants are the link of the evolutionary chain that helped start life as we know it. Some 700 million years ago we had their first ancestors, algae, developed in the seas, 265 million years after that the first simple stalked plants we found outside the seas now but along their coasts and along streams (Starr, C. Taggart, R. pp.153). Through out the years plants have evolved to the ones we know today and can differentiate them by the following categories: Nonvascular, which consists of the bryophytes; and Vascular, also known as tracheophytes, which consists of seedless plants, gymnosperms and angiosperms.


Nonvascular Plants


The bryophytes are small, herbaceous plants that grow close together in mats or cushions on rocks, soil, or as epiphytes on the trunks and leaves of forest trees. Most bryophytes need a moist habitat to grow, even though some may be found in deserts or at flat terrains in Antarctica. The taller they get is less than 20 centimeters and even though they dry out with the absence of moisture they come back to life if afterwards moisture is present(Starr, C. Taggart, R. pp.155). Two important characters make bryophytes different from vascular plants:


1. In all bryophytes the ecologically persistent, the photosynthetic phase of their life cycle is the haploid, gametophyte generation instead of the diploid sporophyte. Bryophyte sporophytes live for a very short period of time, are attached to their gametophytes and they depend on them nutritionally and consist of only an unbranched stalk, and a single, terminal sporangium.


2. Even though bryophytes have parts that look like leafs, stems and roots, they never form xylem tissue, the special lignin-containing, water-conducting tissue that is found in the sporophytes of all vascular plants.


Life Cycle of Bryophytes (Figure 1.1)


By mitosis, the diploid embryo will develop into a mature diploid sporophyte, consisting of a capsule (sporangium) and stalk. The sporophyte is dependent on the gametophyte nutritionally. The diploid embryo by mitosis will develop into a mature diploid sporophyte, consisting of a capsule (sporangium) and stalk. The sporophyte is dependent on the gametophyte nutritimally. Meiosis always produces 4 non-identical cells with half the ploidy of the parent cell. It occurs in the production of gametes in animals and spores in the plant sporophyte. Haploid spores are produced by meiosis in the capsule. Hygroscopic movement (due to alternately wetting and drying) of teeth at the top of the capsule aid in the gradual discharge of the spores. The wind carries them away from the sporophyte and parent gametophyte. A moss spore divides mitotically to produce a long highly branched photosynthetic haploid filament known as a protonema. After several weeks, buds develop into the leafy green haploid moss gametophyte plant. Mitosis always produces 2 identical cells with the same ploidy as the parent cell. ( x indicates the ploidy of the cell which can be diploid in animal cells and plant sporophytes, or haploid in plant gametophytes). When mature the leafy gametophyte produces male or female reproductive structures at the top of the plant. When mature the leafy gametophyte produces female reproductive structures at the top of the plant. The flask-shaped female reproductive structure, the archegonium, consists of a sterile jacket of cells surrounding a single enclosed haploid egg at its base. When mature the leafy gametophyte produces male reproductive structures at the top of the plant. The male structure, the antheridium, consists of a cylindrical jacket of cells enclosing a mass of bi-flagellate haploid sperm . When wet by raindrops, the sperm are released from the antheridium into the splash cup and are then splashed onto female gametophytes. When near an archegonium the sperm are chemically attracted into it and one sperm fertilises the egg. The diploid zygote divides mitotically to form an embryo By mitosis the diploid embryo will develop into a mature diploid sporophyte, consisting of a capsule (sporangium) and stalk. The sporophyte is dependent on the gametophyte nutritionally; and then we go back to the start (http://fybio.bio.usyd.edu.au).


Bryophytes consist of three separate evolutionary lineages: mosses, liverworts and hornworts (Figure 1.2 a, b, c). The features of each of these lineages are shown in the table 1.1 bellow.


Table 1.1


Character


Mosses


Liverworts


Hornworts


Protonema


Filamentous, forming many buds


Globose, forming one bud


Globose, forming one bud


Gametophyte form


Leafy shoot


Leafy shoot or thallus; thallus simple or with air chambers


Simple thallus


Leaf arrangement


Leaves