Introduction to Plants
Plants are multi-cellular eukaryotic organisms with the ability to produce their own food by the process of photosynthesis. (They are autotrophs.) Algae have historically been included with the plants, but they are now classified with the protists. The modern definition of plants includes organisms that live primarily on land (and sometimes in water), excluding algae that live primarily in water.
Another distinguishing characteristic of plants is their type of chlorophyll. Chlorophyll is used to absorb energy from the sun during the process of photosynthesis. Plants have chlorophyll a and chlorophyll b, while many species of algae do not have chlorophyll b.
Green algae have both types of chlorophyll, but they are single-celled
forms. Many evolutionary biologists believe that the green algae gave
rise to the land plants.
Plants occur in two major groups: the nonvascular plants and the vascular plants.Nonvascular plants do not have specialized tissues to transport fluids, while vascular plants do
have specialized tissues. The bryophytes (the mosses and liverworts)
are the only major group of nonvascular plants. There are three large
groups of vascular plants: the seedless vascular plants (for example,
ferns), the vascular plants with naked seeds (for example, pines), and
the vascular plants with protected seeds (for example, flowering
plants). While animals are classified in phyla, the plants are
classified in divisions.
The
life cycle of plants has both a multicellular haploid and multicellular
diploid phase. Because both phases of the life cycle are multicellular,
this type of life cycle is analternation of generations. In contrast, animal life cycles have a multicellular diploid phase and a unicellular haploid phase.
The
alternating generations of plants are the sporophyte generation and the
gametophyte generation. Individuals in the gametophyte generation
(often calledgametophytes)
form gametes, or sex cells. Gametes are haploid cells (they contain one
set of chromosomes). Haploid gametes fuse in fertilization, a process
that takes place in water. This fusion produces fertilized eggs, which
are diploid cells (they have two sets of chromosomes). The plants that
develop are diploid plants of the sporophyte generation. Individuals in
the sporophyte generation ( sporophytes) undergo meiosis to produce haploid spores.
Plants
produce their gametes in specialized structures. In the nonvascular
bryophytes and in the vascular plants, the egg cells are formed in
structures called archegonia (the singular is archegonium). Sperm cells are produced in structures called antheridia (the singular is antheridium).
In some specialized plants, these structures are reduced, and the
sporophyte generation is dominant over the gametophyte generation in the
life cycle.
Food Webs - Page 1
Energy continuously flows through all trophic levels in an ecosystem. Green plants fix energy from the sun during photosynthesis; these plants are consumed by herbivores; and they, in turn, by carnivores.
Plant Reproduction
There are two ways in which plants reproduce: asexually and sexually. Some plants intersperse sexual reproduction with asexual reproduction, so that the recombination of genes occurs, but successful genotypes can be multiplied exactly. This is called alternation of generations.
