Land Plants: Vascular Tissue and a Life Cycle That Alternates

When plants first moved onto land, they had to solve some big problems that life in water never asked of them. On land there is no water surrounding you to hold you up, bring you food, or carry your reproductive cells to a partner. Over millions of years, plants evolved a set of shared solutions, and every land plant alive today still shows them.

All land plants share a life cycle called alternation of generations. The plant spends part of its life as a haploid form called the gametophyte, which makes gametes (sperm and eggs). It spends the other part as a diploid form called the sporophyte, which makes spores. A spore grows into a gametophyte, gametes join in fertilization, and the resulting cell grows into a sporophyte, and the cycle turns again. Which of these two forms is the big, obvious plant you notice depends on the group: in mosses the gametophyte is what you see, while in ferns, pines, and flowering plants the sporophyte takes over.

Land plants also share other traits. They protect their embryos, they make spores inside a case called a sporangium, they make gametes inside organs called gametangia, and they grow from tips called apical meristems. Later groups added vascular tissue, a plumbing system of tubes that moves water and sugars around the body, plus true roots and leaves. Each new trait opened up more of the dry land for plants to live on.

More than 300,000 species of plants have been catalogued, from mosses and ferns to conifers and flowering plants, and most of them are seed plants. Yet all of them came from ancestors that lived in water. Moving onto land meant facing four challenges at once.

First, gravity. Water holds things up, air does not, so a land plant needs support to stand. Second, drying out. A plant surrounded by air loses water constantly, so land plants evolved a waxy coating called a cuticle that seals moisture in. Third, moving water around. Once a plant grew tall, it could not rely on soaking water up like a sponge, so vascular tissue evolved: xylem carries water up from the roots, and phloem carries sugars from the leaves to the rest of the plant. Fourth, reproduction. In water, sperm can simply swim to an egg. On dry land that fails, and later plant groups solved it with pollen, which carries the sperm cells through the air, and seeds, which protect and feed the young embryo until it can grow on its own.

The fossil record preserves this story in order. Simple nonvascular plants like mosses appear first, then seedless vascular plants like ferns, then gymnosperms with seeds, and finally the flowering plants. Each layer of rock shows plants with one more solution to life on dry land.

Bryophytes, the group that includes liverworts, hornworts, and mosses, are the closest living relatives of the very first land plants. The first liverworts probably appeared in the Ordovician period, hundreds of millions of years ago.

Mosses show what it took to survive on land before vascular tissue existed. They are small and low to the ground because they have no pipes to move water upward. Instead, every part of the plant must stay close to damp surfaces so water can soak in directly. This is why mosses form soft green carpets in shady, moist places, and why they can even grow on roofs, pulling water and minerals straight from rain.

In the moss life cycle, the generation you actually notice is the gametophyte. The green, leafy carpet is haploid, and it produces sperm and eggs inside special organs. The sperm still must swim through a film of water to reach an egg, which is the main reason mosses never fully escaped wet habitats. After fertilization, a diploid sporophyte grows out of the gametophyte and stays attached to it, depending on it for food. The sporophyte is little more than a thin stalk topped by a capsule, where spores are made and released to blow away and start new gametophytes. In mosses, the sporophyte is a small guest living on the dominant gametophyte, the opposite of the arrangement in ferns and flowering plants.

The vascular plants, also called tracheophytes, are the dominant and most visible group of land plants, with more than 260,000 species. Their key invention was vascular tissue: xylem tubes that carry water and minerals up from the roots, and phloem tubes that carry sugars made in the leaves to every other part of the plant. Strengthened by a tough material called lignin, this plumbing also works as a skeleton, letting plants stand tall against gravity for the first time.

Ferns and their relatives show what vascular tissue made possible before seeds existed. Because water can be lifted up from the soil, ferns can grow true roots, stems, and leaves, and some tree ferns reach several meters in height. This size advantage changed their life cycle too. In ferns, the sporophyte is the generation you notice. The leafy fern plant is diploid, and it makes spores in little cases on the undersides of its fronds.

The gametophyte, by contrast, shrank to a tiny, short lived, heart shaped green flake that sits on damp soil. It makes sperm and eggs, and the sperm must still swim through a film of water to fertilize an egg. So even though ferns conquered height, they still needed moisture for reproduction. Fully escaping water would take two more inventions: pollen and seeds.

Seed plants appeared during the Carboniferous period, and they took over the land thanks to two linked innovations: the seed and pollen. Together, these freed plants from needing liquid water to reproduce.

Pollen solved the delivery problem. In mosses and ferns, sperm must swim through a film of water to reach an egg. A pollen grain is a tiny, protected package that carries the male gametophyte, and with it the sperm cells, through the air on wind or on the bodies of animals. The male cells travel to the egg without a single drop of surface water.

The seed solved the survival problem. A seed holds a young diploid embryo together with a supply of stored food, all wrapped in a tough protective coat. The embryo can wait out cold or dry seasons in dormancy and start growing only when conditions turn friendly. Spores, by comparison, are single cells with no food reserve and little protection.

Seed plants also flipped the balance of the life cycle completely. Their gametophytes shrank to a few cells living inside pollen grains and ovules, while the sporophyte became the whole visible plant: the pine tree, the rose bush, the wheat stalk. Gymnosperms like conifers dominated the landscape in the Triassic, and later the flowering plants, which enclose their seeds inside fruits, rose to dominate most land ecosystems. The diploid sporophyte era had begun, and plants could now live and reproduce in the driest places on Earth.