Seed Plants  Two groups of seed plants: Gymnosperms Gymnosperms Angiosperms Angiosperms  Gymnosperms include the conifers and cycads and this group originated in the Carboniferous period.  Angiosperms are the flowering plants. Oldest accepted fossils are from about 125mya and are scarce. By 65mya angiosperms were dominant plants on earth.

Heterospory and alternation of generations  Recall that in mosses the life cycle is dominated by the gametophyte generation. The sporophyte is dependent on the gametophyte. Both are visible with the naked eye.  In ferns, the sporophyte generation is dominant and the gametophyte is reduced, but still visible to the naked eye. Both are independent.  In seed plants the situation from mosses is reversed. The gametophyte generation is dependent on the sporophyte. However, the gametophyte generation is microscopically small.

Alternation of generations in seedless and seed plants

Origin of gymnosperms  Gymnosperms first group to develop seeds. Probably evolved from a heterosporous group called the progymnosperms in the Devonian Period ( mya).  First gymnosperms lacked seeds, but had developed them by the end of the Devonian.

Radiation of Gymnosperms  Gymnosperms underwent adaptive radiation during the Carboniferous and Permian periods ( mya).  Planet became progressively drier during the Permian and gymnosperms replaced ferns and relatives as the dominant plants. Gymnosperms were the dominant plants during the Age of Dinosaurs (Mesozoic mya).

Advantages of heterospory  Ancestor of gymnosperms was heterosporous.  Evolution of heterospory was a significant development because it led to the evolution of seeds.

Gymnosperm - heterospory Independent and dominant sporophyte megaspores mega- gametophyte eggs zygote embryo meiosis n 2n sperm microspores micro- gametophyte

Advantages of heterospory  Female gametophyte greatly reduced in size, so much so that it could be enclosed and protected within a layer of sporophyte tissue (the ovule).  In seed plants the female gametophyte is not dispersed and is protected from drying out and other hazards.

Advantages of heterospory  The male gametophyte (n) is dispersed in seed plants, but is protected by a layer of sporophyte tissue (2n).  The whole structure is called a pollen grain. Pollen grain

Gymnosperm Life Cycle  In gymnosperms dominant generation is the tree (sporophyte),  Pine trees (and other gymnosperms) produce both ovulate and pollen cones.

Gymnosperm Life Cycle  Pollen cone contains sporangia that undergo meiosis to produce haploid microspores that develop into male gametophytes (n).  Pollen is wind dispersed and some lands on ovulate cones.

Gymnosperm Life Cycle  Ovulate cone contains many scales, each with two ovules.  Ovule contains megaspore mother cell that undergoes meiosis and eventually produces the female gametophyte (n) that produces eggs.

Gymnosperm Life Cycle  Eggs are fertilized by sperm from the male gametophyte. Usually only one fertilized egg develops into an embryo.  Embryo is surrounded and nourished by a food supply (the female gametophyte tissue). Both are protected by a seed coat derived from the parent tree.

Gymnosperm Life Cycle  Seed thus consists of Embryo (2n) Embryo (2n) Layer of nutrient (female gametophyte (n)) Layer of nutrient (female gametophyte (n)) Seed coat (2n from parent) Seed coat (2n from parent)  Seed falls from cone, and if lucky, develops into a tree.

Advantages of seeds  Seed coat provides protection and gametophyte tissue provides nourishment for developing embryo.  Dormancy: the developing embryo is protected and can wait a long time to germinate when conditions are good.

Advantages of having seeds  Dispersal: The seed coat can be modified for dispersal (e.g. by developing wings or becoming fleshy). Angiosperm seeds are enclosed in a bribe (fruit) and animals move them.

Seeds vs spores  Seeds are better than spores because spores have a short lifetime.  Spores are thinner walled and more vulnerable to pathogens and damage.

Modern Gymnosperms  Gymnosperms have “naked” seeds that are not enclosed in an ovary (as angiosperm seeds are).  There are four extant phyla.

Cycads  130 species in 2 families  Old World tropics  Large palm-like leaves and large cones.

Ginkgo  1 species  Unknown in wild, previously widespread  Seed coat is fleshy.  Widely planted street tree

Conifers  600 species in 7 families  Most important gymnosperms  Pine, spruce, fir, cedar, etc.  Complex seed cones  Needled leaves reduce water loss

Gnetophytes  90 species, 3 genera  Double fertilization  Transition to angiosperms? Ephedra Welwitschia Gnetum