Gymnosperms

gymnosperms and angiosperms

• Heterospory • Bifacial cambium: Cambium is a layer of meristem tissue (plant stem cells) that forms a sleeve around the trunk under the bark Bifacial means that new cells are produced to both sides of the cambial layer • Seeds (but not the only time seeds evolved but led to present day seeds)

• Woody plants with secondary vascular tissue (stem widening growth) • Heterosporous • Retention of megaspores on surface of highly modified sporophylls (similar to amniotic egg) • Fertilisation by pollen, freed from dependence on external water • Five main groups: cycads, gingko, pines, gnetophytes, cupressophytes

• Mostly tropical or subtropical • Separate male (microsporangiate) and female (megasporangiate) plants • Loosely clustered megasporophylls with multiple ovules • Other cycads have more tightly clustered female clones with 2 ovules per megasporophyll • Male sporophylls look more like cones • Female sporophylls can look more like leaves

• Currently 1 species: a relict of past diversity • Gingkophyta appeared in Permian 270 mya • Dioecious (unisexual sporophytes) • Deciduous • Strobili form a fleshy bulb to help with dispersal by animals

• Leaves needle-like with thick cuticle and hypodermic, sunken stomata, resin ducts • Can survive dry and cold environments • Separate male and female cones that can occur on the same plant

• Three very different genera • Can have vine-like growth forms or fleshy strobili or only two massive leaves

A diverse and ecologically and economically important lineage

• Start with a pine tree (sporophyte generation) • Diploid strobili form (small male cones and large female cones) • microspores (pollen grains) are produced and dispersed before the megagametophyte develops • Pollen then fertilises the eggs in the archegonium in the megagametophyte • Zygotes become embryos (then one dies, leaving one) and form sporophytes (seedling and then eventually tree)

gametophyte is the cone, sporophyte is the tree