Plant Diversity and Evolution

Main notes

Plants conquered land about 470 million years ago and have since diversified into four major groups. This chapter follows that journey from water-loving mosses to flowering plants, the group that dominates nearly every landscape today. The same life-cycle logic connects all four groups, and the pattern sets up the next chapter, which examines how plant bodies work from the inside. Plants also matter to almost every other unit in this course, because they sit at the base of nearly every food web.

Alternation of Generations

All plants share a life cycle built on alternation of generations: two multicellular generations trade places in every life cycle. The gametophyte is haploid (n) and makes gametes by mitosis. The sporophyte is diploid (2n) and makes haploid spores by meiosis. A spore germinates and grows into a new gametophyte without fusing with any other cell. Fertilization joins a sperm and an egg to form a diploid zygote, which grows into the next sporophyte.

So the full cycle is: the sporophyte makes spores, spores grow into gametophytes, gametophytes make gametes, and gametes fuse to make a new sporophyte. The two generations differ in chromosome number, size, and lifespan, and the balance shifts dramatically across plant groups. In mosses the gametophyte is the familiar green plant, while in pines the sporophyte is the tree and the gametophyte is microscopic. This trend, called gametophyte reduction, is one of the biggest patterns in plant evolution.

Common Mistake: Gametes in plants are produced by mitosis, not meiosis. Meiosis happens in the sporophyte and produces spores. A haploid gametophyte cannot undergo meiosis to make gametes because it has only one copy of each chromosome.

ELI-10

Think of the plant life cycle as a relay race with two very different runners. The small runner grows from a spore and makes eggs and sperm. The big runner grows from a fertilized egg and makes spores. Each runner hands the race to the other, and the family line always passes through both.

Bryophytes

Bryophytes are the nonvascular plants: mosses, liverworts, and hornworts, about 20,000 species in all. They were among the first plants to colonize land, but they never escaped the need for water. They have no true roots, stems, or leaves; instead, thin filaments called rhizoids anchor them to the ground. Water and nutrients move from cell to cell, which keeps them small and low to the ground. The gametophyte is the dominant, photosynthetic generation, and the sporophyte is a small stalk that grows out of it and depends on it for food.

Mosses are the largest group, with more than 12,000 species, and peat mosses build enormous bog deposits that lock up carbon for thousands of years. Fertilization still requires a film of water, because each sperm has two flagella and must swim to the egg. A single rain shower can trigger a whole moss colony to reproduce at once.

Common Mistake: The green moss mat you see is the gametophyte, not the sporophyte. The sporophyte is the small brown stalk with a capsule that sticks up from the mat. Sperm must swim through water to fertilize the egg, so a dry moss colony cannot reproduce.

1. A spore lands on damp soil and grows into a green gametophyte mat.
2. The mat produces eggs and flagellated swimming sperm.
3. A sperm swims through a water film and fuses with an egg.
4. The zygote grows into a sporophyte stalk with a spore capsule.
5. The capsule releases spores, and the cycle begins again.

ELI-10

Picture a low green carpet that lies flat on the ground and only stays healthy where the ground is always damp. That carpet is a moss. The carpet itself is the main plant, and the tiny stalk with a cap that pops up from it is a second plant that grows for just a short time. The carpet makes eggs and sperm, and the sperm have to swim through a puddle to reach an egg, just like tiny tadpoles.

Seedless Vascular Plants

Seedless vascular plants, the ferns, horsetails, and club mosses, solved the height problem. Their vascular tissue lets them grow far taller than any moss, because every part of the body can stay supplied with water and nutrients. There are about 12,000 species alive today, and ferns make up the bulk of them with roughly 10,500 species. They still reproduce without seeds: mature sporophytes shed dust-like spores from sporangia, and each spore grows into a small free-living gametophyte that must find water for its swimming sperm.

Horsetails have jointed stems armored with silica, and club mosses carpet forest floors, but the giants of the group are gone. About 300 million years ago, tree-sized relatives of today's ferns and horsetails formed swamp forests that later turned into the coal we burn today. Fern fronds unroll from fiddleheads, and most ferns grow in shade and damp habitats because their gametophytes dry out quickly.

Common Mistake: Ferns do not make seeds or flowers. The brown dots on the underside of a frond are sori, clusters of spore cases. Like mosses, ferns still need liquid water for their sperm to swim, no matter how tall the adult plant grows.

ELI-10

Imagine a tall plant that has small pipes running through its body, the way a building has water pipes running through every floor. Those pipes let the plant stand tall and feed every part of itself. Even so, the tiny plant that makes eggs and sperm still needs a puddle of water for the sperm to swim through. The tall plant can grow for many years, but the small one lives for only a short time.

Gymnosperms

Gymnosperms are seed plants with naked seeds: the seed lies exposed on a cone scale instead of inside a fruit. The group first appeared about 360 million years ago, long before any flower existed, and today it includes the conifers (pines, firs, spruces, and their relatives), plus cycads, ginkgo, and gnetophytes, about 1,000 species in total. Conifers are by far the largest group, and ginkgo survives as a single species.

The seed is the great invention of this group. A seed packages a dormant embryo, a food supply, and a protective coat, so the next generation can wait out droughts and winters and travel without water. Seeds replace spores as the dispersal unit, and that freed seed plants from the last tie to watery reproduction.

Fertilization no longer needs a water film because of pollen: a wind-carried grain that is the male gametophyte. A pollen grain lands near the ovule, grows a tube, and delivers sperm directly to the egg. Female gametophytes in gymnosperms live inside ovules on the cone, and after fertilization the ovule matures into the seed. Conifers are evergreen trees with needle-shaped leaves and woody cones, and their pollen blows through forests in clouds during spring.

Common Mistake: A pinecone is not a fruit, and its seeds are not enclosed. Naked seed is exactly what gymnosperm means: the ovule and later the seed sit exposed on the cone scale. A pollen grain is also not the sperm; it is the package that carries the male gametophyte to the ovule.

ELI-10

Think of a seed as a backpack lunch that a baby plant carries on a long trip. The lunch has a snack for later, a blanket, and a hard shell to keep the baby safe. The trip can last through a cold winter or a long dry spell, and the baby only wakes up when conditions are right. Because the baby travels in its own backpack, it no longer needs water to make the trip.

Angiosperms

Angiosperms, the flowering plants, are the youngest and most successful plant group. They appeared about 140 million years ago in the early Cretaceous and exploded into more than 300,000 species, the largest count of any plant group and the foundation of most of Earth's ecosystems today. Two innovations explain the explosion: flowers, which recruit animals to move pollen, and fruits, which recruit animals to move seeds.

The angiosperm life cycle keeps the same alternation of generations, with the sporophyte as the tree, shrub, or herb and both gametophytes tiny and sheltered inside the flower. The pollen grain is still the male gametophyte, but it is delivered by bees, birds, bats, or wind to a sticky stigma. From there a pollen tube grows down into the ovary, where a unique event called double fertilization occurs: one sperm fuses with the egg to form the diploid zygote, and a second sperm fuses with two polar nuclei to form the triploid (3n) endosperm. The endosperm is a nutritious tissue that feeds the embryo as it develops.

Angiosperms fall into two great lineages: monocots, whose embryos carry one seed leaf, and eudicots, whose embryos carry two. Grasses, orchids, and palms are monocots; roses, oaks, and daisies are eudicots. Flowering plants now outnumber all other plant groups combined.

Common Mistake: Double fertilization does not produce two embryos. One fertilization makes the embryo, and the other makes the food tissue, the triploid endosperm. Without that second fusion, the embryo would have nothing to eat.

1. A pollen tube grows into the ovule and releases two sperm cells.
2. The first sperm fuses with the egg, forming the diploid zygote.
3. The second sperm fuses with two polar nuclei, forming triploid endosperm.
4. The zygote becomes the embryo, and the endosperm becomes its food store.

ELI-10

Picture a nursery room where two deliveries arrive at the same moment. One delivery brings the baby itself, and the other brings a full pantry of food for the baby. Every flowering plant makes these two deliveries together, which is why the process is called double fertilization. The baby never waits for a meal because the pantry is stocked at the same instant the baby appears.

Flower and Fruit Evolution

Flowers are modified reproductive shoots, and every part of a flower is a modified leaf. From the outside in, a typical flower has green sepals that protect the bud, colorful petals that attract pollinators, stamens that produce pollen from their anthers, and one or more carpels that hold the ovules in an ovary at their base. The evolution of flowers is a story of coevolution with animals: petals advertise with color and scent, and nectar rewards the visitor, while pollen rides from flower to flower on the pollinator's body. Wind-pollinated flowers instead stay small, green, and scentless.

Fruits are mature ovaries, or clusters of ovaries, that enclose and protect the seeds. A fruit can be fleshy and sweet, like a peach, or dry and papery, like a dandelion's. The key job of the fruit is seed dispersal: an animal eats a berry and drops or deposits the seeds far from the parent plant, while hooked and winged fruits hitch rides on fur or in the wind. Dispersal matters because seedlings that land near the parent must compete with it for light, water, and soil.

Flower and fruit structures did not appear together overnight. Early flowers were small, simple, and pollinated by beetles and flies, and nectar-drinking insects evolved alongside them. Fruits spread the same logic to the next generation: the showy, sugary package evolved to move seeds, which is why so many fruits are designed to be eaten.

Common Mistake: Botanically, a fruit is the ripened ovary of a flower, not something that must be sweet. Tomatoes, peppers, cucumbers, and squash are fruits. A potato is not a fruit; it is an underground stem, and lettuce leaves are not fruit either.

ELI-10

Think of a fruit as a gift box that a plant hands to an animal. The box is bright and tasty so the animal wants to carry it away, and hidden inside is the seed. The animal eats the box, walks far away, and leaves the seed behind with its own little pile of fertilizer. The plant gets its baby carried to a new neighborhood without spending any energy on legs.

High-Yield:

• The four plant groups in order: bryophytes, seedless vascular plants, gymnosperms, angiosperms.
• The gametophyte shrinks and the sporophyte grows more dominant across the four groups.
• Pollen freed seed plants from the need for liquid water during fertilization.
• A seed is a packaged embryo, and a fruit is a mature ovary built for dispersal.
• Angiosperms outnumber every other plant group, with more than 300,000 species and triploid endosperm made by double fertilization.

Quick Review

• All plants alternate between a haploid gametophyte and a diploid sporophyte; gametes come from mitosis, spores from meiosis.
• Gametophyte dominance falls across the groups: dominant in bryophytes, reduced in vascular plants, and tiny inside seed plants.
• Bryophytes (about 20,000 mosses, liverworts, and hornworts) lack true roots and still need water for swimming sperm.
• Seedless vascular plants (about 12,000 ferns, horsetails, and club mosses) grow tall with vascular tissue but still reproduce with spores and swimming sperm.
• Gymnosperms (about 1,000 conifers, cycads, ginkgo, and gnetophytes) made the first seeds about 360 million years ago and fertilize with wind-carried pollen.
• Angiosperms appeared about 140 million years ago, now number more than 300,000 species, and use flowers, fruits, and double fertilization to dominate.
• A seed is a packaged embryo; a fruit is a mature ovary that carries seeds to new ground.

Key terms

Key terms are emphasized and defined within the main notes.

Important formulas or processes

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Common mistakes

See the labeled common-mistake callouts in the main notes where present.