Life Science: Session 4
Alternation of Generations
How do plant life cycles compare to animal life cycles?
At first glance, a comparison of plants and animals suggests that members of these groups have very little in common. A closer look at their life cycles, however, reveals similarities that often surprise people. Just like animals, plants are characterized by sexual reproduction, with new individuals being formed by the union of sex cells. While plants cannot be said to have separate sexes in the same way that animals do, they form sperm and eggs in male and female structures, just like animals. And, just like the sex cells of animals, the sperm and eggs of plants carry half of the hereditary material in the parent’s genome. Beyond this, however, plant life cycles are unique in that they exhibit a process found in no other group of organisms — alternation of generations.
Alternation of generations in a flowering plant
What is alternation of generations?
In an animal life cycle, male and female parents each create sex cells (sperm and eggs) that unite to form a fertilized egg and develop into an offspring organism. Plants, likewise, have sperm and eggs in their life cycles, but these are produced by an intermediate stage between the adult and the offspring.
These stages, which were explained by Dr. Judith Sumner in the video, can be thought of as different "generations" within the same life cycle. The adult generation produces spores, while the spore generation produces sex cells. The scientific terms for these generations are sporophyte (sporo = spore; phyte = plant; therefore, spore-producing plant) and gametophyte (gameto = sex cell; phyte = plant; therefore, sex-cell-producing plant).
To understand the differences between these two generations, it may help to revisit ideas explored in Session 3: Animal Life Cycles. In the body cells of animals, chromosomes exist in pairs — a condition we’ll call doubles. Animal sex cells have half as many chromosomes as their body cells — a condition we’ll call singles. In an animal life cycle, the sex cells are the only cells where chromosomes exist as singles. The rest of the life cycle involves body cells that carry chromosome doubles. This is where plants differ. In plants, one entire generation carries its chromosomes as singles, while the other carries its chromosomes as doubles. Let’s take a closer look.
When we look at a plant, we’re almost certain to be looking at the sporophyte generation. The sporophyte generation carries its chromosomes as doubles. In this sense, it is analogous to the adult animal, which also carries its chromosomes as doubles. The sporophyte generation, as its name indicates, produces spores. Spores carry chromosomes as singles. The spores then develop into the gametophyte generation.
In most plants, the gametophyte is tiny compared to the sporophyte. As its name implies, the gametophyte generation produces sex cells — sperm and eggs. Like the gametophyte itself — and like the sex cells of animals — the sex cells carry chromosomes as singles. Fertilization brings chromosome doubles back together in the fertilized egg. The life cycle is completed with the development of the sporophyte, which carries chromosomes as doubles.
What is the significance of this?
Life scientists have developed several theories to account for the evolution of alternation of generations in plants. One theory has to do with having the “best of both worlds” in terms of variation in a population (a topic explored in Session 5: Variation, Adaptation, and Natural Selection). In the formation of spores, only one parent contributes the hereditary material. This could be beneficial if that parent exists in a stable environment — it creates offspring with the same characteristics that allowed it to survive and reproduce. With sex cells, two parents are involved, and a mixing of hereditary material occurs. This results in offspring that vary from both parents and from one another. This could be beneficial in a changing environment where some variants are likely to be suited to that environment while others may not be.
How can alternation of generations be observed in the plant kingdom?
It’s actually easiest to observe alternation of generations in the most primitive group of plants: the mosses. If you’ve ever looked closely at a moss, you may have noticed a tiny leafy green mat from which a stalk protrudes at certain times of the year. The stalk is the sporophyte. From its cap, spores are cast that land on the ground and develop into the gametophyte—the leafy green mat. Special structures within the mat produce sperm and egg. The sperm swim to the eggs and fertilize them. A stalk, which remains attached to the mat, results from each fertilized egg. The moss life cycle thus requires ground water in order to be completed—this is why mosses are always found in moist environments.
Another major plant group includes the ferns. In ferns, the different generations exist as distinct individuals. The graceful fronds, or leaves, that we see adorn the sporophytes. If you look under the fronds of a mature plant, you’ll see structures where the spores are produced. The spores are cast from these structures onto the ground, where they develop into gametophytes. The gametophytes are tiny heart-shaped structures that are nearly invisible to the naked eye. They require a moist environment to develop and, once mature, produce sperm and egg. Like the mosses, the sperm require water to swim to the eggs, with each fertilized egg developing into the familiar, frond-bearing sporophyte.
Ferns with gametophye and sporophyte sections
Christmas fern with sporangia
In the conifers, the stately needle- and cone-bearing trees are the sporophytes. Conifers actually have two different types of cones. The female cone is probably what you are familiar with, bearing hard, woody scales. In a structure on top of each scale of the female cone, female spores are produced, which develop into the microscopic female gametophyte — a plant that consists of only one cell for most of its existence. The gametophyte remains inside the structure that produced it, which itself remains attached to the scale.
The male cones are much smaller than the female cones and are the structures that produce copious amounts of yellow “dust” in the Spring. On the underside of each tiny scale are structures that produce numerous male spores, which develop into gametophytes that consist of just four cells. The gamteophyte and its covering are the pollen, which is carried by wind to the female cone. Pollination occurs when pollen lands at the sticky base of the scale and the sperm grows to and fertilizes an egg, which eventually forms a papery seed on top of the scale. Note that, unlike mosses and ferns, water is not required to bring sex cells together and that the embryo develops in a seed, where it is protected from drying-out and is supplied with food.
In the video, flowering plants were used to introduce alternation of generations.
Alternation of generations in flowering plants is essentially the same as in the conifers (and just as complicated), except that flowers represent the sporophyte. Female structures, called ovaries, contain structures that produce the female spores. These develop into a seven-celled gametophyte inside the ovary — you can think of it as a tiny plant inside a plant. The male stuctures, called stamens, produce the pollen. As in the conifers, the male gametophyte develops inside the pollen grain.
Pollen from the male parts of one flower is delivered to the female parts of another flower in various ways: wind, insects, birds, bats, etc. When pollination occurs, sperm form and grow to the ovaries, where they fertilize eggs. A fertilized egg develops into a seed inside the ovary. Again, notice that this process does not require water to bring sex cells together, and that a seed protects the developing embryo. The difference between conifers and flowering plants is that the seeds develop within an ovary (the fruit) rather than on top of a cone scale.
Though teaching life cycles to a K-6 classroom doesn’t require this much detail, once you are armed with this knowledge, you can look at plants and their life cycles in a much more informed way. The main message is this: plant life cycles are unique from animals because of alternation of generations. And even though there are differences between groups of plants, the pattern
is the same: spore-producing stage alternating with sex cell-producing stage.
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