The Evolution of Seed PlantsSeed plants are the most highly derived of the land plants Fossil record of seed plants and some extinct seedless relatives.Gymnosperms were very dominant for millions of years, but Angiosperms are the dominant seed plant today.Characteristics of Seed Plants1. Sperm do NOT swim to the egg (removes the need for water in order to go through fertilization)2. Have vascular tissue (xylem and phloem) Xylem contains tracheidXylem of Gnetophytes and angiosperms contain tracheid and vessel elements3. Secondary GrowthGrowth that contributes to increase in diameter due to accumulation of xylem (wood) (rings of a tree part in the middle is the live part of the tree)Metal Pipe “swallowed” by secondary growthAdvantages of secondary growth:Provides support to grow taller (primary growth)Facilitates light absorption and seed dispersal4. Gametophytes greatly reduced.Gold part is the gametophyte Blue is the sporophyte Develop while attached to sporophyte (provides protection)Nutritionally dependent on sporophyte5. All are heterosporousSporophyte (2n) produces two types of spores via meiosis:Microspore- (n) develops into tiny microgametophyte (pollen grain) which produces 2 sperm by mitosis (dispersed by wind or animals)Megaspore- (n) develops a microscopic megagametophyte which produces one egg by mitosis which is retained in megagametophyte (Fertilization= fusion of the sperm and egg)
Following fertilization, the diploid zygote divides mitotically to produce an embryonic sporophyte.