Tracheophytes: How Plants Became Tall

A moss tops out at ankle height. A redwood reaches 115 metres. Both are land plants, but only one of them can move water up its own body. The difference is a set of pipes, and the plants that have them are called tracheophytes.

This post explains what those pipes are, how they work, and which plants have them.

What the Name Means

Tracheophyte comes from two Greek words. Trakheia means windpipe, and phuta means plants. It is a good name. The earliest conducting cells are hollow tubes joined end to end, with walls that are barred like the rings of a windpipe.

Rhynia had a bare upright stem and no true leaves, but it did carry conducting tissue — Image: Matteo De Stefano, CC BY-SA 3.0, via Wikimedia Commons

The group is also called the vascular plants, from the Latin vasculum, a duct. About 300,000 species are accepted, which is every land plant except the mosses. The earliest known tracheophyte is a plant called Cooksonia, from the mid-Silurian period, roughly 425 million years ago.

You will sometimes see vascular plants called "higher plants". That is an old label and it is not scientific. It implies a ladder of life with mosses at the bottom, which evolution does not work that way. Every living group has been evolving for exactly as long as every other one.

The Two Tissues That Define the Group

Botanists define a vascular plant by three things. It has conducting tissue, its dominant life stage is the sporophyte, and it has true roots, stems and leaves. The conducting tissue is the important one, and there are two kinds.

Xylem: dead tubes that carry water

Xylem moves water and dissolved minerals upward from the roots. The cells that do the work are dead by the time they are fully grown. An empty rigid tube is a far better pipe than a living one, because nothing inside it gets in the way of the water.

In flowering plants the pipes are wide vessels. In every other vascular plant they are narrower cells called tracheids. A tracheid wall contains lignin, the same stiff substance that stiffens wood, and it is what stops the tube collapsing under the tension of the water inside it.

The vascular bundles sit in a ring near the outside of the stem, each one carrying xylem inboard and phloem outboard — Image: Curtis Clark, public domain, via Wikimedia Commons

Phloem: living tubes that carry sugar

Phloem carries the products of photosynthesis, mostly sucrose, from the leaves to the growing shoots and roots. Its conducting cells are alive. That is the opposite of xylem, and the contrast is worth remembering: one tissue is a plumbing system and the other is a living conveyor.

Phloem cells are called sieve-tube members, joined end to end across perforated walls called sieve plates. A mature sieve-tube member has no nucleus and no ribosomes of its own. Neighboring cells called companion cells keep it alive. This one arrangement is why a sieve-tube member cannot simply repair itself when damaged.

The soft inner bark is phloem. Stripping it off a living tree is what girdling does, and it kills the tree from the inside — Image: Juha Kämäräinen, CC BY-SA 3.0, via Wikimedia Commons

Why they always come as a pair

Xylem and phloem sit right next to each other, and botanists call the combination a vascular bundle. Neither tissue works on its own for long. The xylem delivers water and minerals that the phloem needs to keep running, and the phloem delivers the sugar that powers the living cells doing the transport.

How Water Rises Without a Pump

It is tempting to picture a plant pumping water upward from its roots. It does not, and it barely spends any energy on the job at all. The mechanism is transpiration pull.

Water leaves the leaves through pores called stomata. As it evaporates from the wet cell walls inside a leaf, it curves the meniscus in the tiny tubes, and surface tension resists. Evaporation at the top of the water column pulls on the water below it. Each molecule escaping hauls up the next one.

Most of a plant's water leaves through these adjustable pores. Close them and the pull stops — Image: Charles Tschirhart, CC BY-SA 3.0, via Wikimedia Commons

It works because water molecules hydrogen-bond to each other, so they line up in a connected thread the whole height of the tree. Osmosis into the root helps push water in from the soil, but the pull is what does the lifting.

This is why a plant in a sealed jar with a healthy plant does so well while the sealed one wilts. It also explains why no water moves at all during high humidity, darkness or drought. With no evaporation there is no pull.

The Seedless Tracheophytes

Seedless tracheophytes reproduce by spores. They split into three familiar lines: ferns, horsetails, and clubmosses.

Ferns

Ferns are the seedless tracheophytes most people recognize. Their leaves are called fronds, and a young fern uncurls its fronds from a spiral, a shape called a crozier. That shape is the one in the fiddlehead you eat if you grow your own.

A fern is a seedless vascular plant with a dominant sporophyte generation — Image: Jml3, CC BY-SA 3.0, via Wikimedia Commons

Horsetails

Horsetails, genus Equisetum, are often treated as ferns. They have jointed hollow stems and a whorl of thin branches at each node, which gives them the nickname of bottlebrush. They also contain silica, which is why they are called scouring rush.

Each segment breaks off at the joint. Same vascular tissue, much simpler stem — Image: Liné1, CC BY-SA 3.0, via Wikimedia Commons

Clubmosses, and the single-vein leaf

Lycophytes are the oldest living lineage of vascular plants, and they are not ferns at all. They evolved on a separate branch. Their leaves are microphylls, which have a single unbranched vein. Almost every other vascular plant has megaphylls, leaves with a branching network of veins. One vein against many is the cleanest way to tell the two groups apart.

Lycophytes are the oldest living vascular lineage, and their leaves have one vein each — Image: Tigerente, CC BY-SA 3.0, via Wikimedia Commons

Living lycophytes are small creeping plants. Their ancestors were not. The tree-like Lepidodendron grew over 40 metres and formed the swamp forests of the Carboniferous. When those forests buried and compressed, they became coal.

The coal forests were built largely from lycophytes, close relatives of today's clubmosses — Image: Tim Bertelink, CC BY-SA 4.0, via Wikimedia Commons

Why "Seedless" Is Not a Real Group

Textbooks divide tracheophytes into seedless plants and seed-bearing plants. It is an easy split, and it is also wrong in a way that matters.

Seedless tracheophytes do not share a recent common ancestor. Ferns and horsetails are more closely related to seed plants than they are to clubmosses. On the tree of life, the clubmosses split off first, then everything else. Grouping ferns with clubmosses puts together two branches that a plant never had as a pair.

Biologists still say pteridophyte, and the International Association of Pteridologists still exists, so the term has not disappeared. But Pteridophyta is no longer accepted as a formal group. The school division is a memory aid, not a claim about ancestry.

The Seed-Bearing Tracheophytes

Seed plants, or spermatophytes, split into gymnosperms and angiosperms. The difference is simply whether the seed is enclosed.

Gymnosperms: naked seeds

Gymnosperm means naked seed, from the Greek. The seed sits on the surface of a scale or a modified leaf, usually gathered into a cone. No fruit surrounds it. Conifers are by far the largest group, with roughly 600 to 630 species. Cycads number about 338, the gnetophytes around 95 to 100, and there is exactly one ginkgo.

Cycads are palm-like but are not flowering plants, and they are not conifers — Image: Ian Alexander, CC BY-SA 3.0, via Wikimedia Commons

Ginkgo biloba is the only living species of its whole group. The fan-shaped leaf is unmistakable — Image: Eecherplaz, CC BY-SA 3.0, via Wikimedia Commons

Conifers carry both male and female cones on the same tree, and all of them are wind-pollinated. About 65 percent of gymnosperms as a whole carry separate male and female people.

Angiosperms: seeds inside fruit

Angiosperms are the flowering plants, and they are the largest group by a wide margin. Their ovules sit inside an ovary, which develops into the fruit. You are eating the seed-bearing structure whenever you eat an apple or a pea pod.

Why the Sporophyte Took Over

Tracheophytes run their lives with the sporophyte stage dominant. That is the diploid generation, with two sets of chromosomes, which produces spores. Mosses are the other way round, with the haploid gametophyte dominant.

The usual explanation is spore dispersal. A plant that grows tall holds its spores higher up, where the wind carries them further. That is only possible with a rigid, woody body that can stand up, and a rigid body needs vascular tissue to move water to its top. The two innovations pull each other along.

The Groups in One Table

Group	How it reproduces	Vascular tissue	Leaves
Clubmosses	spores	xylem and phloem	microphylls, one vein
Horsetails	spores	xylem and phloem	small and scale-like
Ferns	spores	xylem and phloem	megaphylls, branching veins
Gymnosperms	seeds, not in fruit	xylem and phloem	needles, scales, or fan-shaped
Angiosperms	seeds inside fruit	xylem and phloem	megaphylls, branching veins

Key Terms in Plain English

These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.

Term	What it means here
Tracheophyte	a vascular plant: one with xylem and phloem and true roots, stems and leaves
Xylem	the dead tubes that carry water and dissolved minerals up from the roots
Phloem	the living tubes that carry sugar made in the leaves to the rest of the plant
Sporophyte	the stage of the life cycle that makes spores. In a tracheophyte it is the plant you actually see
Microphyll	a leaf with a single unbranched vein, as the clubmosses have
Megaphyll	a leaf with a branching vein network, as ferns and seed plants have
Gymnosperm	a seed plant whose seeds sit unenclosed, usually on the scales of a cone
Angiosperm	a seed plant whose seeds sit inside a fruit

Terms used in this article, defined as they are used above.

Key Takeaways

• Tracheophytes are vascular plants. The name means windpipe, from the shape of the earliest conducting cells.
• They are defined by xylem and phloem, by a dominant sporophyte stage, and by true roots, stems and leaves.
• Xylem cells are dead at maturity. Phloem cells are alive. One is plumbing, the other is a living conveyor.
• Water rises by transpiration pull, not by pumping, and it costs the plant almost no energy.
• Clubmoss leaves have one vein. Nearly every other vascular plant has branching veins. That is the lycophyte split.
• Seedless tracheophytes are not one natural group. Ferns are closer to seed plants than to clubmosses.
• Gymnosperm seeds sit unenclosed, usually on cone scales. Angiosperm seeds sit inside a fruit.
• Lycophyte trees over 40 m tall built the Coal forests. Their living relatives are small creeping plants.

Frequently Asked Questions

What makes a plant a tracheophyte?

Three things: it has xylem and phloem, its dominant life stage is the sporophyte, and it has true roots, stems and leaves. Some groups have since lost one of those, but all living tracheophytes started with them.

Are ferns tracheophytes?

Yes. Ferns have both xylem and phloem, true roots and stems, and a dominant sporophyte. They reproduce by spores, which makes them seedless tracheophytes.

Why can a tree be so tall? Only a small plant gets water by diffusion?

Diffusion is too slow to lift water more than a few millimeters. Tall tracheophytes use xylem instead, and the lifting is done by evaporation at the leaves pulling on a continuous column of water molecules.

What is the difference between a microphyll and a megaphyll?

A microphyll has one unbranched vein and is found only in lycophytes. A megaphyll has a branching network of veins and is found in every other vascular plant group, from ferns to flowering plants.

Are gymnosperms still around?

Very much so. There are over 1,000 living gymnosperm species, about 600 of them conifers. Conifers dominate northern and mountain forests, and ginkgo is still grown worldwide as a street tree.

Why do older books call them "higher plants"?

Because they used to be ranked as more advanced than mosses and algae. That idea came from the scala naturae, a ladder-of-life scheme that biology abandoned. Botanists now consider the term unscientific, and it is not used.

Sources: Wikipedia articles on tracheophyte, vascular plant, xylem, phloem, vascular tissue, pteridophyte, lycophyte, fern, gymnosperm, seed plant and stoma. Images: Wikimedia Commons, with authors and licenses noted in each caption.