From spores to seeds

A seed consists of an embryo with a packet of nutrition (usually), all housed in a protective coat. They range in size from the dust-like seeds of orchids to coconuts. And they all trace their origin to spores. 

The first land plants evolved from green algae, perhaps five hundred million years ago or more, and dispersed their offspring as spores; modern mosses and ferns still do so. A long time passed before seeds evolved (and of course they have not stopped evolving). Botanists have deduced that spores evolved to become seeds by several major steps and conjectured about what innovative features made such steps successful, such that the next generations maintained and continued those traits. I thought it might be interesting to visualize those steps, to begin to understand what was involved with the process.

Start with spores: Spores germinate to form tiny organisms, called gametophytes because they produce gametes (sperm and eggs) that have one set of genetic chromosomes (one set is termed 1N). Sperm swim around, looking for eggs to fertilize, sometimes joining with an egg from the same gametophyte. The fertilized egg and then an embryo with two sets of chromosomes (termed 2N) is held by the gametophyte as it grows into a recognizable moss or fern that will mature and produce spores (therefore called a sporophyte). So the life cycle is complete—alternating a sporophyte generation with a gametophyte generation (see diagram).

Life cycles of spore-producing plant (left) and seed-producing plant (right). Illustration by K. Hocker

In most spore-bearing species, the spores are all alike, making gametophytes that produce both sperm and eggs. However, in a number of lineages, the sexes became separated such that distinct male and female spores were produced, and these became much smaller. Botanists have suggested that, in one of these lineages, the separation of sexes was one of the important early steps on the evolutionary way that led to seed plants. What might have been the advantages to this arrangement? The two sexes would produce two kinds of gametophytes that could now evolve different characteristics and exploit their habitats in different ways. Females could invest all their energies in producing eggs and rearing embryos, no longer investing in producing sperm, and males could invest all of their energies in sperm. Separation of the sexes also helped reduce the rate of self-fertilization, thus reducing the risks of inbreeding.

The next major innovation was the retention of female gametophytes with their embryos on the sporophyte. These gametophytes became even smaller; fossils show step-by-step examples of how maternal tissues eventually formed integuments surrounding a gametophyte and embryo, thus providing a protective covering that we call a seed coat. The whole package is called an ovule, which matures into a seed. That happened about three hundred and fifty million years ago or so.

Presumably those primitive seeds were simply shed, as spores were, to be dispersed on breezes. Having a protective coat may have allowed seeds to remain dormant until conditions were right for germination—something that most spores cannot do.

Then, about a hundred and sixty million years ago, a major division occurred. Some plants, called gymnosperms (naked seed), kept their ovules and seeds exposed on the surface of the spore-bearing structure. Another set of plants, called angiosperms (enclosed seed), began to put additional layers of maternal tissue around the seed, probably by folding a leaf-like ovule-bearing structure to enclose and protect the maturing seeds from desiccation and consumers such as beetles. (The gymnosperms would have had to solve such problems differently). This structure evolved in many different directions in various lineages of angiosperms, forming the pistils in our familiar flowers. The lower part of the pistil became the ovary, housing the seeds, while the upper part became the receptive surface (the stigma) for pollen.

The evolution of pollen is complex, but it broke the sperm cells’ dependence on water. Instead of swimming, wind-or animal-carried pollen became the sperm delivery system. Somehow the many sperm produced by the male gametophytes of seed-plant ancestors were reduced in number, such that each pollen grain contained one very tiny male gametophyte that produced two sperm.

In gymnosperms, one sperm fertilizes the egg, and the other one just degenerates. Nutrition for the embryo and seedling are provided by the female gametophyte, as was true for spores but now the gametophyte and embryo are inside the seed.

A new invention arose, somehow, in the angiosperms: one sperm fertilizes the ovule and the non-fertilizing sperm unites with certain other nuclei in the ovule to create essential food material (commonly starches or oils) for the developing embryo and eventually the early seedling. (The angiosperm ancestors of orchids did this too, but orchid seeds have lost that stored nutrition and depend on mycorrhizal fungi for food.)

After those major steps, natural selection in angiosperms led to the evolution of flowers and a great variety of pollination techniques and fruit types. Much of that diversification was related to interactions with animals.

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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—many, many tens 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 four hundred 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).

Moss sporophytes. Photo by Katherine Hocker

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. 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 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 egg (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 or 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.)

Spores and seeds

All plants need to produce offspring and those offspring need to be sent out into the world to get established and grow up to make offspring of their own. Although the mother plant that produces the offspring clearly landed in a suitable site, it often happens that plant eaters concentrate their foraging on areas of higher density, so that is one good reason to for the offspring to colonize a new place. Competition among offspring can also favor spreading out to new sites, where competition with siblings is less.

Dispersal is always a risky business, and by far the great majority of young ones die. Although some plants produce ‘bulblets’ (think of tiger lilies, anderson’s sword fern) on leaves or stems, the bulb-like structures are not generally capable of dispersing much beyond the base of the parent plant, so they seldom reach new sites.

For dispersal farther from the parent plant, plants typically use spores or seeds. What is the distinction?

A spore is defined in my big dictionary as a small entity with a tough covering that’s resistant to environmental extremes; it is often a single cell but in some cases it has several cells. When conditions are right, a spore can germinate, producing a new individual, which may come from sexual reproduction (union of sperm and egg to form a zygote) or more commonly from asexual reproduction (mere cell division). Being tough and resistant means that a spore can sit around for a considerable time, waiting for the right conditions for germination. In effect, this is dispersal in time. However, such very small entities generally also have the capacity to disperse in space—moved about by currents of air or water. (There is at least one exception, of course: for instance, the spores of truffle fungi are dispersed by squirrels that eat the fungus and excrete viable spores.)

Spores are produced by algae, fungi, mosses, and ferns, and their allies. One sees the spore-bearing structures of fungi in the form of mushrooms and conks (such as ‘bear-bread’), of mosses in the tiny, stalked capsules borne by the moss plant, and of ferns in the arrays of small discs on the backs of the fronds (or, in some species, such as deer fern, on a separate frond).

In contrast, all the familiar conifers and flowering plants produce seeds, usually by sexual reproduction. A seed typically contains stored food for the embryo, giving it a head start in growth after the seed germinates. (There’s an exception here too, of course: orchid seeds have no stored food and depend on special, mycorrhizal fungi for germination and initial growth.) The stored food supplies are derived from maternal tissues in the conifers, but in the flowering plants, they derive from the joining of nuclei from male and female parents (one set of nuclei forms the zygote, and others make the storage product). A seed has a protective coat of maternal tissues that enclose both embryo and storage products.

Seeds, like spores, can in some species lie dormant, awaiting the right conditions, thus dispersing in time. Many kinds of seeds have special adaptations for dispersal in space: fleshy fruits (such as blueberries, strawberries) to be eaten by animals that excrete viable seeds, wings or plumes (such as those on fireweed) to carry the seeds on the wind, explosive capsules (such as those on lupine) that shoot the seeds away from the parent, hooks (such as those of buttercups) that stick to fur or socks, and so on.

• Both spores and seeds can accomplish dispersal in space and in time. Spores were around for a long time before seeds evolved and did (as they still do) the job of dispersal for fungi, mosses, and ferns. Yet the seed-producing plants have become the dominant form of plant life over much of the earth’s land surface. We can then ask if the evolution of seeds somehow allowed plants to exploit new ways of life, leading to the great diversity of seed-bearing plants that now inhabit the land. Or did the environment change in some way that made it advantageous for plant offspring to start off with the support of stored food?