Chapter 26 Seed Plants

Introduction: Seed Plants

• First plants to colonize land appeared 500 mya, where closely related to the mosses then liverworts and then primitive vascular plants - the pterophytes

• Does not require water for the process of fertilization

• Dominant sporophyte generation

• Reduced size of the gametophyte; microscopic cells enclosed in the tissues of the sporophyte

• Heterosporous - Megaspores (female) and microspores (male)

• SEEDS AND POLLENS

• Gametophytes depend on sporophytes for water and nutrients

• pollen grains - Male gametophytes containing sperm (1n) and encased in a protective coat and can travel very far

• Seed offers the embryo protection, nourishment and a mechanism to maintain dormancy for extended periods - could be tens of thousands of years

• Earliest seed plants is about 350 mya from fossil records

• Pregymnosperms (380 mya; naked seeds) - Gymnosperms (319 mya)

Evolution of Gymnosperns

• The “seed fern” Elkinsia polymorpha is the earliest known gymnosperm (400 mya)

• They produced their seeds along their branches in structures known as cupules (protects the ovule)

• First gymnosperms - Devonian period (390 mya)

• The Ginkgoles - early Permian

• Expansion seen in Mesozoic era to supplanting ferns

• The Jurassic period was the age of cycads (palm-tree-like gymnosperms)

• Angiosperms are the major plant like in most biomes BUT gymnosperms still dominate the ecosystems such as taiga (boreal forests) and the alpine forests at higher mountain elevation - They adapt well to cold and dry conditions

Seeds and Pollen as an Evolutionary Adaptation to Dry Land

• In seed plants, female gametophyte consists a few cells - the egg and supportive cells including the endosperm producing cells for the support of the embryo

• After fertilization, the diploid zygote produces an embryo that will grow into the sporophyte when the seed germinates

• Storage tissues to sustain growth and protective coat give seed their superior evolutionary advantage

• Several layers of hardened tissue prevents desiccation

• Abscisic acid helps to maintain the state of dormancy

• Dispersal through wind, floating, vectors - animals

• Male gametophytes - pollen grains contain a few cells

• Distributed by water, wind, animal pollinator

• Protected from desiccation

• After reaching the female gametophyte - it grows a tube to deliver a male nucleus to the egg cell

• Sperms of all modern gymnosperms and angiosperms do not have flagella but the primitive ones might (Cycads, Ginkgo etc.)

Evolution of Angiosperms, Flowers and Fruits as an Evolutionary Adaptation

• Angiosperms (seeds in a vessel) produce flowers containing male and/or female reproductive structures

• Appeared about 125 MYA in the lower Cretaceous period

• Probably not derived from gymnosperms ancestor. They form a sister clade (a species and it descendants) that developed in parallel with the gymnosperms

• Flowers and fruits both represent an improved reproductive strategy that serve to protect the embryo, increasing genetic variability and range.

• No consensus on the origin of angiosperms

• Most primitive is the Amborella trichopoda

• Modern angiosperms are classified as monocots and eudicots based on the structure of their leaves and embryos

• Gametes are in separate organs which are housed in a flower

• 300,000 species of flowering plants - very diverse

• Flowers have all shapes, colors, small and arrangements

• Most flowers have mutualistic pollinators

• Following fertilization of the egg, the ovule grows into a seed

• The surrounding tissues of the ovary thicken to form a FRUIT that will protect the seed

• Not all fruits develop from the ovary. Some develop from tissues adjacent to the ovary

• Fruits are agents of dispersal - wind, animals eat them

• Cockleburs (velcro)

Gymnosperms

• Gymnosperms means “naked seeds” - diverse

• Anthophyte hypothesis suggests they are sister group of gymnosperms while the “netifer” hypothesis suggests monophyletic group

• Separate female and male gametes, pollination is by wind (pollen cones and ovulate cones)

• Possession of tracheids for the transportation of water and solutes in the vascular system

• Seeds not enclosed in an ovary but are partially sheltered by modified leaves known as sporophylls

• Strobilus - a tight arrangement of sporophylls around a central stalk

• Some seeds are surrounded by sporophyte tissues upon maturation - the layer of sporophyte tissue that surrounds the megasporangium and the embryo is called the integument

• Dominant in Mesozoic era, survive without fresh water, nitrogen poor soil

• Prominent in the coniferous biome or taiga - conifers have selective advantage in cold and dry weather

• Alternation of generations with a dominant sporophyte

• Heterosporous, male and female organs can form in cones or strobili

• Male and female sporangia are produced either on the same plant - monoecius