Bellwork: How can seeded plants reproduce without standing water Bellwork: How can seeded plants reproduce without standing water? Describe their general life cycle

Seed plants Section 22.3

What is a seed? A plant embryo and food supply encased in a protective coating Living plant is diploid Early development stage of sporophyte stage of plant life cycle

The first seed plants First fossils date bake to 360 million years ago Several evolutionary stages chart the development of the seed DNA evidence suggests all seed plants share a common ancestor Key evolutionary steps: No standing water required for fertilization Through reproductive process involving cones or flowers Pollination Embryos in seed

What is the relationship between gametophyte and sporophyte in seed plants? Male and Female gametophyte grow and mature directly within the sporophyte Gametophytes either develop in cones or flowers Two groups exist on the basis of which structures are present Gymnosperms – seeds on scales of cones Angiosperms – seeds in flowers Layer of tissue to protect the seed

What is the role of pollen? Entire male gametophyte is contained within a pollen grain Sperm do not swim through water Carried by wind or insects to fertilize eggs Process called pollination

How does a seed form? After fertilization, zygote in seed grows into a tiny plant Embryo will stop growing whilst it is small and contained within a seed Can remain this way for weeks, months or years Seed coat provides protection Can survive long periods of bitter cold, extreme heat or drought Embryo will begin to grow again when conditions are once again right Uses nutrients from stored food supply until it can carry out photosynthesis on its own

How does fertilization take place in gymnosperms? Name means – ‘naked seed’ Seeds exposed on scales of cones Cones are produced by mature sporophyte plant Two types of cones produced – pollen cones and seed cones One pollen grain is entire male gametophyte – one haploid nuclei will divide to produce two sperm cells Familiar seed cones/female cones produce female gametophytes Larger than pollen cones Two ovules found near base In ovules meiosis produces female gametophytes that contain hundred and thousands of egg cells – when mature each gametophyte contains a few large eggs cells ready for fertilization

The life cycle continued Entire cycle takes two years to complete Male pollen carried by wind to female cones Pollen grains caught in sticky secretion on the scales of the female cone Pulled into the ovule No need for standing water Pollen grains are able to produce pollen tubes which contains two haploid sperm nuclei One fertilizes the egg, the other disintegrates Diploid zygote produced Grows into embryo Encased to form a seed Scattered by wind to form a new plant

The life cycle overviewed

Key points questions What adaptations allow seed plants to reproduce without water? What is a Gymnosperm How does fertilization of a Gymnosperm take place?