The Plant Kingdom: From Mosses to Flowering Plants

From the forest floor to the canopy

On a damp boulder in an Ontario forest you might find a cushion of moss. Nearby, ferns uncurl their fiddleheads in May, white pines tower above, and trilliums bloom on the forest floor. All of these are plants, but they represent very different solutions to one big problem: how to live on land.

Plants are multicellular eukaryotes with cellulose cell walls that make their own food by photosynthesis, using chlorophyll a and b in chloroplasts. They store food as starch and have a two-stage life cycle called alternation of generations.

Problems of life on land, and plant solutions

Plants evolved from green algae that lived in water. On land they needed to:

• avoid drying out → a waxy cuticle, and stomata (pores) that open and close;
• move water up and food around → vascular tissue (xylem and phloem);
• stand up without water to support them → stiff cell walls strengthened with lignin;
• reproduce without swimming sperm → pollen (sperm cells inside a protective coat, carried by wind or animals) and seeds (an embryo with stored food and a protective coat).

The four main groups of plants show these solutions appearing one after another.

Nonvascular plants: mosses and their relatives

Mosses, liverworts and hornworts have no xylem or phloem. Water moves cell to cell by osmosis and diffusion, so they stay small and grow in moist places. They have rhizoids (thin anchoring threads) instead of true roots. Their sperm must swim through a film of water to reach the eggs.

The green, leafy moss you see is the gametophyte. The sporophyte is the thin stalk with a capsule that grows out of it and depends on it for food.

Sphagnum (peat moss) dominates Ontario's northern peatlands, which store huge amounts of carbon.

Seedless vascular plants: ferns, horsetails and club mosses

These plants have vascular tissue, so they have true roots, stems and leaves and can grow taller. In ferns, the leaves are called fronds, and the stem is often an underground rhizome. The fern plant you see is the sporophyte. Brown dots on the underside of the fronds, called sori, release spores. A spore grows into a tiny, heart-shaped gametophyte, about the size of a fingernail. Ferns still need water for sperm to swim.

The young, coiled fronds of the ostrich fern are the fiddleheads sold in spring in Ontario and the Maritimes. (Health Canada advises cooking them thoroughly before eating.)

Seed plants: gymnosperms and angiosperms

Gymnosperms ("naked seeds") carry seeds on the scales of cones. Male cones release pollen carried by wind to female cones. Most are evergreen conifers with needle-shaped leaves and a waxy coating, which helps them survive cold, dry winters. Ontario's provincial tree, the eastern white pine, is a gymnosperm.

Angiosperms ("enclosed seeds") have flowers, which often attract animal pollinators, and seeds that develop inside an ovary that ripens into a fruit. Fruits help spread seeds: maple keys spin in the wind; raspberries are eaten by birds. Angiosperms are by far the largest plant group. Ontario's provincial flower, the white trillium, is one. You will study flowering plants in detail, including monocots and dicots, in the Plants: Anatomy, Growth, and Function unit.

In all seed plants the sporophyte is the large, long-lived plant, and the gametophytes are microscopic: inside pollen grains and inside the ovule.

Alternation of generations

All plants alternate between two multicellular generations:

1. The sporophyte (diploid, 2n: two sets of chromosomes) makes spores by meiosis. Each spore is haploid (n: one set).
2. A spore grows by mitosis into the gametophyte (haploid, n).
3. The gametophyte makes eggs and sperm by mitosis.
4. Fertilization joins egg and sperm into a zygote (2n), which grows into a new sporophyte.

Across plant evolution the gametophyte gets smaller and the sporophyte gets bigger: moss (gametophyte dominant) → fern (sporophyte dominant, gametophyte separate) → seed plants (gametophyte microscopic, protected inside the sporophyte). You will study meiosis and haploid and diploid cells in detail in the Genetic Processes unit.