As we move through the evolutionary history of vascular plants, getting closer and closer to the present, the next group we must discuss is gymnosperms. Gymnosperms are the first of two kinds of true seed-bearing plants called spermatophytes, and the word “gymnosperm” means “naked seeds” because their seeds do not exist within chambers such as fruits or nuts. This group includes the plants known as conifers, as well as the cycads and ginkgo trees. Ginkgo biloba, or the ginkgo tree, is a special example.
These trees first evolved in the early Jurassic period, more than 270 million years ago, and they have changed relatively little since then, so they’re known as “living fossils” and were some of the first gymnosperms on the planet. The ginkgo trees we’re familiar with today are the last extant, or still living species of their kind. As is true for all vascular plants, with gymnosperms, the diploid sporophyte is the dominant generation, or the generation that we’re most aware of seeing. However, the gametophytes in spermatophytes or seed-bearing plants are much smaller and less distinct than all of the other gametophytes we’ve discussed.
When a gymnosperm sporophyte like a pine tree becomes sexually mature, it develops two types of cones: male cones and female cones. Meiosis occurs within the male cones in order to create male gametophytes, which are sometimes referred to as “pollen grains” though the gametophyte is actually inside the grain, protected by sporopollenin in the pollen wall. Meiosis also occurs within the female cones to create female gametophytes within structures called ovules.
Pollination is the process of moving pollen from the male cones to the female cones for fertilization. But there are some novel features to gymnosperm fertilization which gave them an immense evolutionary advantage over other existing plants. With gymnosperms, pollination is sometimes facilitated by water, but more often by wind, which was a brand new mechanism, and this had an important result. Plants no longer required a body of water nearby within which to transfer sperm to achieve fertilization.
Where earlier plants like bryophytes and lycophytes were all relegated to very a small distance from some body of water, suddenly gymnosperms with their incredible seeds were not tethered in the same way, and for the first time in the history of the planet, forests full of large trees could suddenly stretch out over vast expanses of dry terrain. Getting back to pollination, once the pollen has successfully arrived at the female cone, some of it will enter the cone, gaining access to the ovule. Now there are two primary methods of fertilization in gymnosperms.
Cycads and Gingkos have motile sperm that can swim directly to the egg within the ovule. Conifers, however, have sperm with no flagellum. Thus, an outgrowth called a pollen tube will form, through which the generative cell in the pollen will travel to encounter the female gametophyte. The generative cell splits into two sperm nuclei, and one of these will fuse with the egg, while the other degenerates. After fertilization of the egg, we have a diploid zygote, which will eventually become a new sporophyte.
This then divides by mitosis to form the embryo, just like in animals. A mature gymnosperm seed is made up of the embryo, the remains of the female gametophyte which functions as a food supply, and the seed coat. No fruit surrounds the seed, which is why scientists refer to this plant group as having naked seeds. While we outlined this whole process in a rather abbreviated manner, it actually takes around three years for the male and female gametophytes to be produced and brought together, and for mature seeds to then form from fertilized ovules. Once a gymnosperm seed is ready to be dispersed, the female cone will open up to allow for dispersal to occur.
These seeds offer numerous advantages over the spores of predecessors, as seeds offer more protection, nourishment, and can remain dormant for extended periods of time before germinating, which is when it begins to develop into a plant. And of course as we said, with seeded plants, the seeds can be carried very far by the wind, enabling these plants to spread over large distances. The evolution of gymnosperms transformed the face of the Earth, as the land was not covered by forests until these plants came along. Now that we know more about gymnosperms, let’s investigate the other type of seed-bearing plants, called angiosperms.