Progymnosperms & Seed Plants

• progymnosperms are seedless plants
• progymnosperms were the first plants with extensive secondary growth (lots of wood and had secondary phloem)
• progymnosperms had one main axis (monopodial), elaborate branching, and bifacial vascular cambium (produces secondary xylem, secondary phloem , tracheids with circular, bordered pits). They are seedless
• There are two types of progymnosperms: aneurophyton and archaeopteris
• aneurophyton is shrubby, has 3D branching, no leaves, lobed protostele, homosporous (one type of spore)
• archaeopteris has homosporous and heterosporous species, has secondary growth and periderm, the eustele first evolved in this group, and has megaphylls (fern like)
• a transition from branches to leaves occurred during the existence of Archeopteris
• The first fossils of seeds were 365 MY BP. Seed plants were highly successful. They are all heterosporous, vascular with megaphylls
• In seedless plants, the gametophyte typically independent of sporophyte, small and vulnerable. In seed plants, the female gametophyte is retained in sproangium on sporophyte (protected and nourished), and male gametophyte is transported to female
• Developments necessary for the evolution of seed: heterospory (seperation of genders), endosporic megagametophyte, increased resources for female and decreased for male, retention of megagametophyte (and thus egg and embryo) on the sporophyte
• there are 2 groups in seed plants: gymnosperms (naked seed) and angiosperms (seed in vessel - ovary )
• gymnosperms have seeds often on surface of sporophylls (scales), there are approximately 840 species (mostly trees and shrubs), 4 phyla
• angiosperms have seeds that are enclosed in a fruit (mature ovary of a flower), approximately 300,000 species, 1 phylum (anthophyta)
• seed plants have a small, fleshy megasporangium (nucellus) which remains on the sporophyte
• in the megasporangium, there is a megaspore mother cell (2n) that undergoes meiosis to turn into 4 megaspore haploid cells which 3 degenerate and 1 only becomes a functional megaspore (1n)
• Functional megaspore develops into an endosporic megagametophyte on the sporophyte
• ovule = megagametophyte and surrounding tissues
• microsporangium = pollen sac
• microsporangium develops on the sporophyte, has many 2n microspore mother cells that undergo meiosis which rform 4 microspores (1n). each microspore forms a microgametophyte (1n)= pollen grain
• microgametophytes are dispersed from the parent, NOT microspores
• Pollination is the transfer of pollen grains from pollen sac to ovule
• In gymnosperms: pollen grain enters micropyle and pollen grain germinates forming a pollen tube
• pollen tube may anchor the pollen grain &/or absorb nutrients from the nucellus (=hasutorial pollen tube), &/or convey sperm near or to archegonia. no free water is needed for fertilization
• fertilization = 2 male gametes (sperm) enter an archegonium and 1 fuses with egg nucleus to form zygote then form embryo. 1 archegonium develops into 1 embryo = polyembryony (many embryos)
• In gymnosperms: early embryogeny is characterized by divisions of nuclei without cell wall formation. 4-1000 nuclei formed before cell walls form = free nuclear division
• as the embryo develops: integument turns into seed coat. ovule turns into seed
• seed = mature ovule containing embryo
• advantages of seed: embryo has a food store, protective seed coat prolongs survival (compared to a spore), can wait for optimal germination conditions, easily transported