THE LIVING WORLD
Unit Seven. Plant Life
32. Evolution of Plants
The earliest vascular plants lacked seeds, and two of the seven phyla of modem-day vascular plants do not have them. One phyla of living seedless vascular plants is the ferns, phylum Pterophyta. This phylum includes the typical ferns seen growing on forest floors, like those shown in figure 32.8a,b. It also includes the whisk ferns (figure 32.8c) and the horsetails (figure 32.8d). The other phylum, Lycophyta, contains the club mosses (figure 32.8c). These phyla have free-swimming sperm that require the presence of free water for fertilization.
Figure 32.8. Seedless vascular plants.
(a) A tree fern in the forests of Malaysia (phylum Pterophyta). The ferns are by far the largest group of spore-producing vascular plants. (b) Ferns on the floor of a redwood forest. (c) A whisk fern. Whisk ferns have no roots or leaves. (d) A horsetail, Equisetum telmateia. This species forms two kinds of erect stems; one is green and photosynthetic, and the other, which terminates in a spore-producing "cone," is mostly light brown. (e) The club moss Lycopodium lucidulum, recently renamed Huperzia lucidula. Although superficially similar to the gametophytes of mosses, the conspicuous club moss plants shown here are sporophytes.
By far the most abundant of seedless vascular plants are the ferns, with about 11,000 living species. Ferns are found throughout the world, although they are much more abundant in the tropics than elsewhere. Many are small, only a few centimeters in diameter, but some of the largest plants that live today are also ferns. Descendants of ancient tree ferns, they can have trunks more than 24 meters (79 ft) tall and leaves up to 5 meters (16 ft) long!
The Life of a Fern
In ferns, the life cycle of plants begins a revolutionary change that culminates later with seed plants. Nonvascular plants like mosses are made largely of gametophyte (haploid) tissue. Vascular seedless plants like ferns have both gametophyte and sporophyte individuals, each independent and selfsufficient. The gametophyte (the heart-shaped plant at the top of figure 32.9) produces eggs and sperm. After sperm swim through water and fertilize the egg, the zygote grows into a sporophyte. The sporophyte bears haploid spores on the underside of its leaves, in brown clusters called sori (singular, sorus). The spores are released from the sorus and float to the ground where they germinate, growing into haploid gametophytes. The fern gametophytes are small, thin, heart-shaped photosynthetic plants that live in moist places. The fern sporophytes are much larger and more complex, with long, vertical leaves called fronds. When you see a fern, you are almost always looking at the sporophyte.
Figure 32.9. Fern life cycle.
The haploid gametophytes grow in moist places. Rhizoids (anchoring structures) project from their lower surface. Eggs develop in an archegonium, and sperm develop in an antheridium, both located on the gametophyte's lower surface. The sperm, when released, swim through free water to the mouth of the archegonium, entering and fertilizing the single egg. Following the fusion of egg and sperm to form a zygote—the first cell of the diploid sporophyte generation— the zygote starts to grow within the archegonium. Eventually, the sporophyte, the fern plant, becomes much larger than the gametophyte. Most ferns have more or less horizontal stems, called rhizomes, that creep along below ground. On the sporophyte's leaves, called fronds, occur clusters (called sori; singular, sorus) of sporangia, within which meiosis occurs and spores are formed. The release of these spores, which is explosive in many ferns, and their germination lead to the development of new gametophytes.
Key Learning Outcome 32.5. Ferns are among the vascular plants that lack seeds, reproducing with spores as nonvascular plants do.