DIVERSITY AND LIFE CYCLES
Algae are classified as plants or protists and consist of several unicellular divisions and three main multicellular divisions: brown algae, red algae and green algae. There are two divisions of land plants, the bryophytes (mosses and liverworts) and the tracheophytes or vascular plants, including spore bearing groups such as ferns, and the seed plants.
All plants have alternating diploid and haploid generations. Diploid sporophytes produce haploid spores. These germinate to produce gametophytes. Gametes from these fuse to form a diploid cell that can grow into a new sporophyte. There is great variation among the algae with different groups showing reduction of either the sporophyte or the gametophyte. In bryophytes the gametophyte is the main plant. In the tracheophytes the sporophyte is dominant, with gametophytes free-living in most ferns, horsetails and some clubmosses, but much reduced in other vascular plants, the female gametophytes being retained on the sporophyte.
A basic classification of plants is given in Table 1. The dividing line between plants and protists is arbitrary and the algae are considered in either kingdom. The algae contain chlorophyll and are the main primary producers in the sea. They have some other features in common with other plants. Three groups contain large multicellular species, though they are probably not closely related to each other. They and the major unicellular and colonial divisions are considered briefly. The two main divisions among the land plants, bryophytes and tracheophytes, may have arisen independently from marine algae related to the present-day Chlorophyta (green algae), the bryophytes possibly more than once, although DNA evidence suggests one origin of land plants.
Fundamental to the life cycle of all plants and most plant-like algae is an alternation between a sporophyte generation and a gametophyte generation (Fig. 1).The sporophyte is diploid. To reproduce, cells of the sporophyte divide bymeiosis to produce haploid spores. The spores germinate without any fertilizationto form a haploid generation, the gametophyte. This produces gametes bymitosis. Male and female gametes fuse to form a diploid cell that can germinateand grow into a new sporophyte. In most multicellular groups the sporophyte isthe main plant, but in some it is small and dependent on the gametophyte. Thegametophyte may be multicellular and free-living, or much reduced and dependenton the sporophyte. The gametophytes can be hermaphrodite, producinggametes of both sexes, or dioecious, producing either only female or only male
gametes. Divisions in plant classification are based on whether the sporophyte or gametophyte is the main plant, the degree of reduction of the subsidiary stage and the structures involved with reproduction. In algae, fundamental differences in pigment structure and storage compounds are also used.
Among multicellular algae, most green algae (Chlorophyta) have the gametophyte as the main plant, the sporophyte being represented by a single resting spore or zygote, although some have two multicellular generations looking alike. In the brown algae (Phaeophyta), the sporophyte is the dominant generation and the gametophyte much reduced in size. In a few, such as Fucus species (the wracks), the gametophyte is reduced to a gamete, so reproduction resembles that of a vertebrate. The red algae (Rhodophyta) are variable.
Among land plants the two groups differ in which is the dominant generation. In the bryophytes it is the haploid gametophyte, the sporophyte being a multicellular stalk and capsule that remains attached to the living gametophyte and dependent on it. In the tracheophytes (vascular plants) the diploid sporophyte forms the main plant body. The gametophyte of most ferns, horsetails and some clubmosses, is up to 1 cm across, multicellular and separated from the parent sporophyte but short-lived. It is normally hermaphrodite. In the other vascular plants the gametophyte is always dioecious and is reduced to a few cells retained within the spore wall. In seed plants the male gametophyte is reduced to the three nuclei of the pollen grain and the female gametophyte to an embryo sac, normally of eight cells, retained on the parent sporophyte (Topics D2 and D3).
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