Plants are multicellular 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. Many evolutionary biologists believe that the green algae gave rise to the land plants.
Plants occur in two major groups: nonvascular plants and 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 unprotected seeds (for example, pines), and the vascular plants with protected seeds (for example, flowering plants). While animals are classified in phyla, 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 an alternation 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 called gametophytes) form gametes, or sex cells. Gametes are haploid cells (they contain one set of chromosomes). Haploid gametes fuse in fertilization. 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.