F. Plants: Anatomy, Growth, and Function · Plants: Anatomy, Growth, and Function

Monocots and Dicots: Comparing Structures and Evolution

F3.2 F2.1 F2.3About 65 minutes

Learning goals

I can compare monocots and dicots by their seeds, leaves, stems, roots and flowers.
I can classify an unfamiliar flowering plant as a monocot or dicot using several pieces of evidence.
I can explain how monocots are thought to have evolved from dicot-like ancestors.
I can give Canadian examples of important monocots and dicots.

Words to know

angiosperm
A flowering plant: a plant whose seeds develop inside an ovary (which becomes a fruit).

cotyledon
A seed leaf: part of the embryo that stores food or absorbs it from the endosperm, and sometimes becomes the first leaf to photosynthesize.

monocot
A flowering plant with one cotyledon, usually with parallel leaf veins, scattered vascular bundles, fibrous roots and flower parts in threes.

dicot
A flowering plant with two cotyledons, usually with net leaf veins, a ring of vascular bundles, a taproot and flower parts in fours or fives.

eudicot
"True dicot": the large group that includes most dicots; they share pollen grains with three pores or grooves.

endosperm
Food-storing tissue in a seed that nourishes the embryo; large in most monocot seeds such as corn and wheat.

common ancestor
An ancestral species from which two or more later groups evolved.

Two big families of flowering plants

The lawn grass under your feet and the maple tree above it are both flowering plants, but they are built very differently. Grass leaves are long and narrow with veins running side by side; maple leaves are broad with a web of veins. Grass roots form a tangled mat; maples have deep main roots. Grass never makes wood; a maple trunk is mostly wood.

Botanists sort flowering plants (angiosperms) into two main groups based on these differences: monocots and dicots. The names come from the number of cotyledons (seed leaves) in the embryo: mono means one, di means two.

Remember from earlier lessons

In Grade 10 you saw that monocot stems have scattered vascular bundles and dicot stems have a ring. In this unit you have also met fibrous and taproot systems (Lesson 2), parallel and net venation (Lesson 4), and the cambium that makes wood (Lesson 3). This lesson puts it all together. For how angiosperms fit into the whole plant kingdom (mosses, ferns, conifers and flowering plants), see the Diversity of Living Things unit.

Five structural differences

Seeds. A monocot embryo has one cotyledon. In grains such as corn and wheat, most of the seed is endosperm, and the cotyledon absorbs food from it. A dicot embryo has two cotyledons, which in beans and peas are large and store the food themselves.
Leaves. Monocot leaves usually have parallel veins and often a sheath around the stem. Dicot leaves usually have net veins (pinnate or palmate) and a petiole.
Stems. Monocot stems have scattered vascular bundles and no vascular cambium, so most monocots don't make true wood. Dicot stems have bundles in a ring, with cambium between xylem and phloem, so many dicots can become woody.
Roots. Monocots usually have a fibrous root system, and in cross-section their roots have xylem and phloem in a ring around a central pith. Dicots usually have a taproot, with an X-shaped core of xylem.
Flowers. Monocot flower parts usually come in threes or multiples of three (3 or 6 petals, 6 stamens). Dicot flower parts usually come in fours or fives or their multiples.

A sixth, microscopic clue: most dicot pollen grains have three pores or grooves, while monocot pollen has one.

The five main differences between monocots and dicots. No single feature is foolproof, so botanists look at several together.

Monocots	Dicots
Grasses and grains: wheat, corn, rice, oats, barley	Trees: maples, oaks, birches, apples (not conifers)
Lilies, onions, garlic, tulips	Beans, peas, soybeans, lentils
Orchids, irises, trilliums	Sunflowers, canola, roses, strawberries
Cattails, sedges, bananas, palms	Tomatoes, potatoes, carrots, dandelions

Examples. Note that conifers such as pines and spruces are not monocots or dicots: they are gymnosperms, with no flowers.

Exceptions: why you use several features

Nature doesn't always follow the rules. Ontario's provincial flower, the white trillium, is a monocot: its flower parts come in threes (3 sepals, 3 petals, 6 stamens), and it grows from an underground stem. But its three broad leaves have net veins, like a dicot. Some dicots, such as plantain weeds, have leaves with nearly parallel main veins. And palm trees are monocots that grow tall, tough trunks without a true vascular cambium.

So a botanist never classifies a plant using one feature. Use the whole set of evidence, and give the most weight to the features that are most consistent (cotyledon number, flower-part number and stem anatomy).

How did monocots and dicots evolve?

The first flowering plants appear in the fossil record more than 100 million years ago. Fossils and, especially, comparisons of DNA show that:

The earliest flowering plants had two cotyledons, like dicots. Some of their descendants, such as water lilies and magnolias, still belong to early-branching groups.
Monocots evolved from one of these early dicot-like lineages. They are a single natural group: all monocots share a common ancestor that had one cotyledon.
Most of the remaining "dicots" form another natural group, the eudicots ("true dicots"), identified by their three-pored pollen.

This means the old group "dicots" is not one natural branch of the tree of life: it is "all flowering plants except monocots", which includes several separate early lineages as well as the eudicots. Many textbooks still use "monocot" and "dicot" because the structural differences are real and useful, but biologists today usually say monocots and eudicots. You will learn how new groups arise from ancestral ones in the Evolution unit.

A simplified family tree of flowering plants (not to scale). Monocots branched off from early dicot-like ancestors; the eudicots are a separate branch. The exact order of the three right-hand branches is still being studied, so they are shown splitting at the same point.

Worked example

Let's classify a mystery plant

A plant from a wetland has long narrow leaves with veins running side by side, a mat of thin roots, and flowers with 6 tepals (petal-like parts) and 6 stamens. A cross-section of its stem shows vascular bundles spread throughout. Is it a monocot or a dicot? How confident are you?

Leaves: parallel veins → monocot evidence.
Roots: fibrous mat → monocot evidence.
Flowers: 6 tepals and 6 stamens are multiples of 3 → monocot evidence.
Stem: scattered bundles → monocot evidence.
Four independent features all point the same way, so confidence is high. (A plant like this could be an iris or a lily-like wetland plant.)

A monocot, with high confidence because four separate features agree.

Worked example

Let's handle an exception

A student sees a white trillium in a forest in May. It has 3 broad leaves with net veins and a flower with 3 green sepals and 3 white petals. She says: "Net veins, so it's a dicot." Evaluate her reasoning.

Net venation is one piece of evidence for dicots, but venation has known exceptions.
The flower has parts in threes (3 sepals, 3 petals; it also has 6 stamens): strong monocot evidence, because flower-part number is a more reliable feature.
Trilliums are, in fact, classified as monocots; DNA evidence agrees.
Better reasoning: check several features and weigh the more consistent ones (flower parts, seed, stem anatomy) more heavily.

Her conclusion is wrong because she relied on a single, less reliable feature. Using flower-part number, the trillium is a monocot.

Just for fun

Ontario's floral emblem, the white trillium, is a monocot that refuses to follow the vein rule, but it is excellent at counting to three: 3 leaves, 3 sepals, 3 petals, 6 stamens and a 3-part stigma.

Hands-on activity

Seed and flower detective

You need: about 6 dried beans (kidney, lima or pinto), 6 popcorn kernels or dried corn kernels, 2 cups of water, a magnifying glass, and (optional) a few flowers from a garden or grocery bouquet and a few leaves.

Soak the beans overnight and the corn for 1 to 2 days (change the water daily).
Bean: peel off the seed coat with your fingernails and gently split the bean into its two halves. Find the two cotyledons and the tiny embryo (a little curved root and two miniature leaves) at one edge. Sketch and label.
Corn: look at the flat side of a kernel for a lighter, oval patch: the embryo. Ask a grown-up to slice the kernel lengthwise with a knife. Find the embryo (with its single cotyledon) and the large, starchy endosperm. Sketch and label.
Flowers and leaves: count petals (and stamens if you can) on each flower; note leaf venation. Classify each plant as monocot or dicot and record your evidence in a table.

What to look for: beans split neatly into two cotyledons; corn has one embryo beside a big endosperm. Lilies, alstroemeria and tulips usually show parts in 3s or 6s; apple blossoms, buttercups, geraniums and many others show 5s. Cultivated roses and carnations often have extra petals bred into them, so count stamens or sepals instead if petals seem odd. Wash hands after handling soaked seeds; don't eat them.

Your turn

What does "cotyledon" mean, and how many do monocots and dicots have?

Compare the arrangement of vascular bundles in monocot and dicot stems.

Why can oak trees become woody but most monocots can't?

A flower has 4 petals and 8 stamens. Monocot or dicot?

Name three monocots and three dicots that are important crops in Canada.

Why do botanists use several features to classify a plant as monocot or dicot?

Are pines monocots or dicots?

More practice: warm-up

What vein pattern do most monocot leaves have?

What kind of root system do most dicots have?

In which group are flower parts usually in threes?

What large food-storing tissue fills most of a corn kernel?

What is a eudicot?

More practice: on your own

A cross-section of a root shows xylem and phloem in a ring around a central pith. Which group does the plant belong to? Explain.

A plant has net-veined leaves, a taproot and flowers with 5 petals. It is classified as a dicot. Which of these features would you trust most, and why?

Explain what it means to say monocots evolved "from" dicot-like ancestors.

Why do some biologists say "dicots" isn't a natural group?

Grasses are excellent for erosion control on Prairie roadsides. Which monocot feature explains this?

Challenge

A botanist finds a fossil flowering plant with leaves that have net veins and pollen grains with three pores. Which group would she likely place it in, and why might she be careful?

Corn and beans are both seeds, but you eat them for different stored foods. Using seed structure, explain where the stored food is in each and why.

Suggest why most of the world's major grain crops (wheat, rice, corn, barley) are monocots, using their structure and growth.

Check your understanding

Quiz

Which combination describes a typical monocot?

A flower with 5 petals and 10 stamens most likely belongs to a:

Most monocots don't produce wood because they lack:

The white trillium has net-veined leaves and flower parts in threes. It is classified as a:

Which statement about monocot evolution is best supported?

Where is most of the stored food in a bean seed?

For parents and teachers

Big idea: F3.2 asks students to compare monocots and dicots in structure (seeds, stems, flowers, roots) and evolution. The structural comparison is straightforward; the evolution part asks students to understand that monocots evolved from dicot-like ancestors, so "dicots" is a convenient but not natural group.

How to help: The seed dissection needs an overnight soak (two days for corn). Popcorn kernels work but are hard; fresh or frozen corn kernels are easier. A grown-up should slice the corn kernel. A bouquet from the grocery store usually has both monocots (lilies, alstroemeria) and dicots (daisies, carnations).

Common difficulties:

Relying on one feature (usually leaf veins) and getting fooled by exceptions.
Thinking conifers are dicots because they are trees. Conifers are gymnosperms, outside this comparison.
Saying dicots evolved from monocots. It is the other way round.