Section P – Plant diversity
P1 Diversity and life cycles
Classification A basic classification of plants is given in Table 1. The dividing line between
plants and protists is arbitrary (Section A) and the algaeare 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.
Life cycles Fundamental to the life cycle of all plants and most plant-like algae is an alter-
nationbetween a sporophytegeneration and a gametophytegeneration (Fig. 1).
The sporophyte is diploid. To reproduce, cells of the sporophyte divide by
meiosis to produce haploid spores. The spores germinate without any fertiliza-
tion to form a haploid generation, the gametophyte. This produces gametes by
mitosis. Male and female gametes fuse to form a diploid cell that can germinate
and grow into a new sporophyte. In most multicellular groups the sporophyte is
the main plant, but in some it is small and dependent on the gametophyte. The
gametophyte may be multicellular and free-living, or much reduced and depen-
dent on the sporophyte. The gametophytes can be hermaphrodite, producing
gametes of both sexes, or dioecious, producing either only female or only male
Key Notes
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.Related topics The algae (P2) Early evolution of vascular plants
The bryophytes (P3) (Q1)ClassificationLife cycles