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