Fungi, mosses, lichens, and ferns are among the plants that produce spores rather than seeds as their vehicles of reproduction.
Reproductive Evolution
Intermediate between the simple cell division of the algae and the complex machinery of flowers and seeds, a form of reproduction evolved wherein plants produce spores, which serve a function similar to that of seeds but with some crucial differences, as we shall see. Fungi, mosses, lichens, and ferns are all spore-bearing plants, more primitive in form than the flowering species but equally important to the ecosystem.
A primary difference between spore-based and seed-based reproduction is that in flowering (seed-producing) plants, the act of fertilization (union of the male and female cells) takes place before the seed leaves the parent plant, while in spore-producing plants, fertilization takes place after the spore leaves the parent.
Another difference is that self-fertilization is more likely in the process of spore-based reproduction than in the pollination of a flowering plant. The seed-based process, therefore, has more possibilities for genetic adaptation and evolution.
Germination of Spores
Spore development varies slightly according to the class of plant. We’ll use ferns as our example. Many ferns bear fruitdots, or sori (singular: sorus), containing microscopic spore cases, on the undersides of some of their leaves. Other fern species produce spores on separate stems or specialized leaves. When the single-celled spores are ripe, and the air is dry, the walls of the spore cases burst open, casting the spores into the air, ideally on a breeze that will carry them away from the parent plant.
When a spore lands at a suitable warm, moist, and shaded location, it begins to develop into a gametophyte (reproductive phase of plant), putting out first a rhizoid, tiny, rootlike hair that secures it in the soil. Through cell division, the body enlarges into a prothallus, a flat, green, heart-shaped structure about ¼ inch in size. Clusters of microscopic male and female organs develop on the underside of this structure.
Fertilization of Gametophytes
When mature, the male organs burst open in the presence of moisture, and the sperm swim to the neck-like openings of the female organs, heading toward the egg chambers within. (The ferns are thus often self-fertilizing, although the sperm may also swim to an adjacent gametophyte, which may have originated from a separate plant.) Once an egg is fertilized, the necks of all that gametophyte’s female organs are believed to close, so each gametophyte produces only one plantlet.
Nourished by the cells of the gametophyte, the fertilized egg puts out a root and stem, which emerge from the notch in the heart-shaped prothallus. Once developed, the leaves and root take over the process of feeding the new fern plant, leaves with new sori develop, and the next round of the cycle begins.
See Plants: An Introduction to Modern Botany by Victor A. Greulach and J. Edison Adams (New York: John Wiley & Sons), 1962.
