Seed Plants I : Seed Plants Without Flowers ("Gymnosperms")…
Seed Plants I : Seed Plants Without Flowers
The cambium arose just once, in one group of plants that then give rise to a monophyletic group of woody plants.
very soft, spongy, parenchymatous wood
hard, strong wood with little parenchyma
They have naked ovules located on flat sporophylls.
They are flowering plants with carpels
They have more relictual progymnosperms.
Reproduction is heterosporous.
Evolution of Seeds
is surrounded by a layer of tissue called integument.
a hole in the integument which permits sperm cells to swim to the egg after the megaspore had developed into a megagametophyte and had produced eggs.
microspores settle here.
Pteridospermophyta: Seed ferns
Pteridospermophyta , all extinct
evolved from Aneurophytales
Cycadophyta , cycads extant
Microsporophyll in cycads may bear many microsporangia
Sperm cells of cycads have hundreds of flagella and must swim to the egg cell.
Cycadeoidophyta , cycadeoids, all extinct
Vegetatively, cycadeoids such as Cycadeoidea were similar to cycads. They had a broad cortex and a tough outer protective layer formed by persistent leaf bases.
Family Voltziaceae , Lebachia
Two types of shoots
Family , Taxaceae
Ginkgophyta: Maidenhair Tree
Ginkgo leaves are broad.
dichotomous venation like leaves of cycads and seed ferns.
Microsoporophylls occur in small, cone like clusters
Ovules occur in pairs at the end of a stalk-like megasporophyll.
strongly resembles dicots, having broad leaves and woody stems.
Ovules are not borne in cones.