origin of seeds late devonian, 360 mya seed = embryo + food + protective coat advantage: protection...

origin of seeds

• late Devonian, 360 mya• seed = embryo + food + protective

coat• advantage:• protection from desiccation• wait for good conditions• own food to get started

reproductive adaptations of seed plants

• gametophytes smaller• female gametophyte retained on

parent sporophyte• male gametophyte transports

sperm• water not required for fertilization• seeds are means of dispersal

2 types of seed plants

• 1. Gymnosperms “naked seeds”– seeds exposed (on cones)

• 2. Angiosperms “vessel seeds”– seeds inside fruits

gymnosperm life cycle

• heterospory w/2 types of cones:• 1) small cones w/microsporangia• microspores develop into pollen• pollen = immature male gametophyte• reduced in size—no antheridia• whole gametophyte travels (pollination)• fertilization without water

gymnosperm life cycle

• 2) large cones have megasporangia• megasporangium protected by

integuments• ovule = megasporangium +

integuments• seed = mature ovule• integuments become seed coat• megaspore stays in megasporangium

gymnosperm seedexample: Pine

•

embryo (2n)

female gametophyte (n)

seed coat (2n)

embryo is new sporophytefemale gametophyte is stored food

pine life cycle

• 3 years to make seed

• pollination & seed dispersal by wind

gymnosperms--cycads

• prominent w/dinosaurs• today 11 genera 130 sp • tough leaves• look like ferns• but have cones w/seeds• roots assoc. w/cyanobacteria• plants defended by toxic compounds

gymnosperms--Ginkgo

• only genus in group Ginkgophyta• thought extinct• good urban tree• stinky seeds• veins dichotomously

branched

gymnosperms--Gnetophyta3 very different genera, 70 sp

• 1) Ephedra • desert• “mormon tea”• ephedrine

gymnosperms--Gnetophyta

• 2) Gnetum• tropical vine• flat leaves

gymnosperms--Gnetophyta

• 3) Welwitschia mirabilis• Namib desert, 2 leaves, ~1500 yrs

old

gymnosperms--conifers

• very successful. 50 genera, 550 sp• most evergreen• needle leaves adapted for drought

– small surface area, thick cuticle– stomata sunken in valleys

Taxodium distichum bald cypress

Heron Pond, Cache River State Natural Area , S IL

deciduous

relative of redwoods & sequoias

•oldest plants•Bristlecone Pines 4600 years old

gymnosperms--conifers

• most massive plants• Giant Sequoia 26 m circumference,

(8.3 m diameter!) 3000 years old• tallest living plants• Coast Redwoods 367 ft tall,

600-800 yrs old• Watterson Towers only 281 ft

types of growth

• primary growth (up or down):shoot & root apical meristems

• secondary growth (out): cambium— meristem makes shoot & root thick

secondary growth

• vascular cambium:– xylem to inside, phloem to outside

• wood is secondary xylem• cork cambium makes cork to

outside• bark = cork + phloem

– everything outside of vascular cambium

tree rings: seasonal changes in xylem cell size

dendrochronology

• tree rings date historical events• similar patterns in neighboring trees• overlap rings: to get complete

record• need consistent rings• regular wet &

dry cycles

Plant tissue culture Fig 38.14

• plant cells are totipotent:– any cell can grow into whole new plant

• new plant is clone (same genes)• examples: leaf cutting, Wollemi Pine• callus = undifferentiated tissue (wound) • develops roots and shoots• balance of hormones required• biotech: insert gene into callus