From Algae to Flowers: Plant Diversity and Evolution

Several lines of evidence show that plants evolved from green algae, specifically a group of freshwater algae called charophytes. Plants and green algae share the same photosynthetic pigments (chlorophyll a and b), both store food as starch, both build cell walls from cellulose, and DNA comparisons place charophytes as the closest living relatives of land plants. Living in shallow fresh water that sometimes dried up may have pre-adapted these algae for life on land. Once on land, plants gained advantages: more sunlight, more carbon dioxide (which diffuses far faster through air than through water), and almost no competitors or herbivores at first. The table shows the main problems and the structures that solved them.

Animals have one multicellular stage: the diploid body, which makes haploid gametes by meiosis. Plants are different. Their life cycle alternates between two multicellular generations. The sporophyte is diploid (2n) and makes spores by meiosis. A spore is haploid (n), and unlike a gamete it does not fuse with anything: it divides by mitosis to grow into the gametophyte, a haploid multicellular plant. The gametophyte makes gametes (eggs and sperm) by mitosis, since its cells are already haploid. Fertilization joins two gametes into a diploid zygote, which grows by mitosis into a new sporophyte, and the cycle repeats. The big evolutionary trend is that the gametophyte shrinks over time. In mosses the green plant you see is the gametophyte; in ferns the leafy plant is the sporophyte; in seed plants the gametophyte is reduced to a few cells hidden inside the pollen grain and the ovule.

Bryophytes have no xylem or phloem, so water moves from cell to cell by diffusion and osmosis. That limits them to a few centimetres tall and to damp places, and their sperm must swim through a film of water to reach the egg. The moss sporophyte is a brown stalk with a capsule that grows out of the green gametophyte and depends on it for food. Sphagnum moss forms vast peatlands in the Hudson Bay Lowlands of northern Ontario that store huge amounts of carbon. Ferns and their relatives, the club mosses and horsetails, belong to the earliest groups of plants with vascular tissue, so they could grow tall: around 300 million years ago, forests of giant ferns, horsetails and club mosses grew in swamps whose buried remains became coal. Ferns still release spores, from clusters called sori on the undersides of the fronds, and each spore grows into a tiny heart-shaped gametophyte that needs water for fertilization.

Seed plants broke the dependence on water. Pollen carries the male gametophyte through the air, so fertilization can happen on a dry hillside. The fertilized ovule becomes a seed: an embryo, a food supply and a protective coat that can wait out winter or drought. Gymnosperms ("naked seeds") carry seeds exposed on the scales of cones and are mostly wind-pollinated. Their needle leaves, with thick cuticles and sunken stomata, resist water loss in cold, dry winters, which is why conifers such as black spruce and jack pine dominate Canada's boreal forest. Angiosperms added the flower, which attracts animal pollinators, and the fruit, which forms from the ovary and helps disperse seeds. Animal pollination is more precise than wind, and fruits recruit birds and mammals to plant seeds far away. Flowering plants appear in the fossil record about 130 million years ago and are now by far the most diverse plant group.