Off the Trails: Plant evolution and life cycles

Occasionally, friends have asked me about the life cycles of plants, so here I will attempt to summarize them, in the context of plant evolution. I set the stage by describing the basic pattern of life cycles in animals, with which we are more familiar, in order to make the contrast with plants.

Most animal life cycles are relatively simple, compared to most plants. During sexual reproduction, eggs from a female-functioning animal are fertilized by sperm from a male-functioning individual, creating a zygote. Egg and sperm each have one complement of chromosomes —one set of DNA, so when they join, the zygote has two sets of chromosomes with DNA from both parents. Having one set of chromosomes is called “haploid” and having two sets is called “diploid” — or 1N and 2N for short.

The 2N animal zygote develops into an adult, in some cases passing through an intermediate juvenile stage or several such stages, which may look different and behave differently from the adult form. For instance, caterpillars are juvenile moths and butterflies. But the development is more or less continuous, from fertilized egg to adult, and generally only the 2N adult form is capable of reproduction. The 2N adult produces eggs or sperm that are 1N by the process of meiosis.

In the plant world, things are quite different. Plant life cycles (in plants consisting of more than one cell) in general consist of two phases or two generations. A haploid or 1N phase is typically known as a gametophyte — a plant that produces 1N gametes (eggs or sperm or both). When a 1N sperm fertilizes a 1N egg, the resulting 2N zygote grows into a 2N sporophyte—a plant that by meiosis produces 1N spores that grow into 1N gametophytes. Thus, the life cycle alternates between a 1N gametophyte plant and a 2N sporophyte plant (this is known as alternation of generations).

However, the relative sizes of the gametophyte and sporophyte vary tremendously in the plant kingdom. All plants were derived originally — hundreds of millions of years ago—from algae (probably green algae), so I’ll start there. Among the thousands of species of multicellular algae, there are many in which the gametophyte is tiny, microscopically small. A zygote — the future and much larger sporophyte — typically develops on the gametophyte, which may then disintegrate. In some algae, however, the gametophytes are large and persistent, easily visible, and about the same size as the sporophytes.

Somewhere along the line, over 400 million years ago, aquatic green algae found a way to colonize land (possibly by means of symbiosis with fungi). These ancestral algae led to two quite different evolutionary lineages: the mosses, which are small and absorb water and nutrients from air and soil but do not transport them very far internally, and all the other plants (ferns and seed plants), which have internal vascular systems for transporting water and nutrients throughout the plant (and so they are known as the vascular plants).

In the mosses and their relatives, the eggs and zygotes began to be protected in jackets of sterile cells. The visible mosses that we see throughout our forests and muskegs here are gametophytes (1N). Sperm swim in water to reach eggs on female gametophytes. A fertilized egg (in its protective jacket) on a female gametophyte produces a sporophyte (2N), which we see as a little stalked capsule growing atop a frond of visible moss. The 2N sporophyte reduces the chromosome set to 1N by meiosis, producing 1N spores, which are dispersed and grow into gametophytes. Both of the alternating generations of moss plants are readily visible.

The other evolutionary lineage led to a huge diversity of vascular plants: ferns and their relatives and all the seed plants (conifers, wildflowers, our familiar trees, etc.). Having a vascular system for transporting water and nutrients allowed the plants to grow taller, sometimes much taller, than mosses. Ferns continue the pattern of alternating generations, with a small, typically microscopic, 1N gametophyte and a much larger 2N sporophyte, which is what we see. Some ancient forms of club-mosses, which are distantly related to ferns, had sporophytes as large as our present-day trees; and even today, there are tree-sized ferns in some parts of the world. Spores from the sporophyte disperse and germinate into the tiny gametophytes in the soil; as in the mosses, water is necessary for sperm to swim to the eggs.

Next to evolve were the seed plants, in which it is the 2N sporophyte that we see. Freely dispersing spores (which would have made independent gametophytes) are not produced. Instead, the 1N gametophytes are now reduced to tiny things on the sporophyte: pollen grains enclosing the male gametophytes and sperm, and miniscule female gametophytes containing eggs (or ovules), inside an ovary. The ovules are enclosed in protective layers of tissue and, after fertilization, will become the seeds, which germinate and grow. Ecologically, then, the life cycle of seed plants resembles that of animals, with each individual developing from seed to reproductive adult, and the evolutionary history, with the alternating generations, is hidden from sight.

What the seed plants accomplished was finding a way for sperm to reach eggs through the air, rather than depending on water for sperm to swim or float to the eggs. Pollen grains containing sperm are transported by air currents or animals to receptive surfaces that capture the pollen. Sperm then move down a tube produced by the pollen grain to an ovule, resulting in a 2N zygote. The ovules are contained in several layers of tissue, some of which are derived from the tiny female gametophyte and some of which are derived from the large sporophyte. We call these “seeds” and they are often wrapped up in additional fleshy or protective tissue (from the sporophyte) that we call fruits, pods or cones. The seeds of most plants also contain a supply of nutrients to support the growth of a seedling.

Thus, over the millennia, the life cycles of multicellular plants on land have taken several directions. The mosses have gametophytes and sporophytes that are fairly similar in size. The ferns and their allies have tiny gametophytes alternating with much larger sporophytes. The seed plants have reduced the gametophytes to tiny things dependent on large sporophytes, and the alternation of generations is no longer apparent. The evolutionary reasons for all these variations are a subject of scholarly debate.

It is easy to think of the seed plants as being dominant on our landscapes. Indeed, they are the largest land plants today. But the mosses and ferns are still with us, doing things in their own ways; in Southeast, they are important and visible components of the land-plant communities. So they cannot be viewed as merely primitive or evolutionary failures in any way — they are just smaller.

(I have neglected the fungi here. Historically, taxonomists have sometimes classified them as plants and sometimes not. Their life cycles are varied, complex, and quite different from those described here.)

• Mary F. Willson is a retired professor of ecology.

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