Seed Plants

Evolution of the seed  Seeds represent an extreme form of heterospory  Seed – mature ovule with embryo –Megasporangium surrounded by integuments  Events leading to evolution of seed –Retention of megaspores in megasporangium (nucellus) –Reduction in number of megaspore mother cells –Only one megaspore survives –Reduced megagametophyte –Young sporophyte developes in megagametophyte which is in megasporangium –Integument envelopes megasporangium –Apex of megasporangium modified for pollen

Fossils  Oldest seeds from Devonian (365 mya)  Elkinsia  Archaeosperma

Elkinsia

Archaeosperma

Paleozoic plants showing potential stages in the evolution of the seed

Progymnosperms  Seedless vascular plants – but likely progenitors of seed plants  Unlike other seedless vascular plants, progymnosperms had secondary vascular tissue (both xylem and phloem) and its structure is very like that of modern conifers  Some had a eustele and were heterosporous

Reconstruction of progymnosperm Triloboxylon

Reconstruction of progymnosperm Archaeopteris

Reconstruction of branch of Archaeopteris

Extinct Gymnosperms  Three groups of extinct gymnosperms  Seed ferns  Cordaitales  Bennettitales – may be ancestors of angiospems –Had a flower – like structure

Living Gymnosperms  Gymnosperm means “naked seed” – ovules & seeds exposed on surface of sporophylls  Most have polyembryony  Pollen grain – no water required! –In Conifers and Gnetophytes, sperm are non- motile  Pollen tube –In Ginkgo and cycads, transitional. Pollen tube is present, but sperm swim to egg

Coniferophyta  Xylem composed of tracheids  Non-motile sperm conveyed to egg by pollen tube  Leaves usually needle-like or scale-like –Fascicles in pines  Ovulate and microsporangiate cones on same plant; ovulate cone compound

Pine life cycle  In general, representative of gymnosperms, but has many peculiarities specific to pines  Highlights –Male gametophyte consists of 4 nuclei when dispersed, 2 sperm produced near pollination –Female produces several archaegonia, all of which may be fertilized (polyembryony #1) –Each archaegonium produces 4 embryos (polyembryony #2)

Douglas fir forest

Sequoia

Pacific Cypress

Pacific Yew

Common Juniper

Pine farm, with cloned pines to be planted

Frazier fir with cones

Some “living fossils”  Metasequoia – the dawn redwood  Wollemia pine – discovered in 1994 near Sydney, Australia

Wollemia in nature, and a fossil with a living branch

Bristlecone Pine

Cycadophyta  Xylem of tracheids  Produce flagellated motile sperm –Pollen tube not the main conveyer of sperm to egg  Ovulate and microsporangiate cones on separate plants  Leaves “palm-like”  Often contain toxins  Pollen may be carried from male to female by insects

Ginkgophyta  Xylem composed of tracheids  Motile sperm –Pollen tube not true conveyer of sperm to egg  Ovulate and microsporangia on separate plants  Ovules fleshy, stinky  Leaves fan shaped

Gnetophyta  Most like flowering plants  Xylem both tracheids and vessel elements  Non-motile sperm –Pollen tube conveys sperm to eggs  Ovulate and microsporangiate cones compound, mostly on separate plants  Double-fertilization in Ephedra  Pollination often by insects, cones produce nectar  Leaves various