Plant Evolution Plants evolved about 475 million years ago from charophytes (a group of green algae) –Most modern plants are photoautotrophs on land

Plant Adaptations to Land Most groups are adapted to dry and often cold habitats through structural modifications –Stomata across epidermal surfaces –Waterproof cuticle –Lignin-reinforced tissues –Xylem and phloem (vascular tissues)

Alternation of Generations Land plants alternate between gametophyte (haploid) and sporophyte (diploid) generations

What are bryophytes? Mosses, liverworts, and hornworts –Nonvascular (no xylem or phloem) –Rhizoids store moisture, anchor gametophyte Sperm swim through water droplets or film of water to eggs Sporophytes stay attached to larger gametophytes Spores that give rise to gametophytes are wind-dispersed

What are seedless vascular plants? Lycophytes, horsetails, whisk ferns, true ferns Dominated by the sporophyte Spore-bearing structures –Strobili of horsetails –Sori of ferns Sperm swim through water to reach eggs

Seed-bearing vascular plants Gymnosperms and flowering plants (angiosperms)

Gymnosperms: Naked Seeds Conifers, cycads, ginkgos, and gnetophytes –Many are well adapted to dry climates Life cycle: No ovaries –Ovules form on exposed surfaces of strobili or (in conifers) female cones

Angiosperms: Flowering Plants Only angiosperms have flowers –Many coevolved with birds, bees, bats, and other animal pollinators Most widely distributed and diverse plant group –Two largest classes: Eudicots and monocots –Magnoliids preceded eudicots and monocots

Life Cycle: Flowering Plants Monocot life cycle: An example of sexual reproduction in flowering plants –Formation of pollen and eggs Double fertilization produces an embryo sporophyte and nutritive tissue that supports it –Protective seeds form in ovaries –Outer ovary tissues later develop into fruits

Genetically Altered Food Crops Botanists alter genes to improve crops for human populations around the world