Seed PlantsSeed Plants Two groups of seed plants: Two groups of seed plants:

GymnospermsGymnosperms AngiospermsAngiosperms

Gymnosperms include the conifers and cycads and this Gymnosperms include the conifers and cycads and this group originated in the Carboniferous period.group originated in the Carboniferous period.

Angiosperms are the flowering plants. Oldest accepted Angiosperms are the flowering plants. Oldest accepted fossils are from about 125mya and are scarce. By fossils are from about 125mya and are scarce. By 65mya angiosperms were dominant plants on earth. 65mya angiosperms were dominant plants on earth.

Heterospory and alternation of Heterospory and alternation of generationsgenerations

Recall that in mosses the life cycle is dominated by the Recall that in mosses the life cycle is dominated by the gametophyte generation. The sporophyte is dependent gametophyte generation. The sporophyte is dependent on the gametophyte. Both are visible with the naked eye.on the gametophyte. Both are visible with the naked eye.

In ferns, the sporophyte generation is dominant and the In ferns, the sporophyte generation is dominant and the gametophyte is reduced, but still visible to the naked gametophyte is reduced, but still visible to the naked eye. Both are independent.eye. Both are independent.

In seed plants the situation from mosses is reversed. In seed plants the situation from mosses is reversed. The gametophyte generation is dependent on the The gametophyte generation is dependent on the sporophyte. However, the gametophyte generation is sporophyte. However, the gametophyte generation is microscopically small.microscopically small.

Alternation of generations in seedless and seed plants

Origin of gymnospermsOrigin of gymnosperms

Gymnosperms first group to develop Gymnosperms first group to develop seeds. Probably evolved from a seeds. Probably evolved from a heterosporous group called the heterosporous group called the progymnosperms in the Devonian Period progymnosperms in the Devonian Period (408-360 mya).(408-360 mya).

First gymnosperms lacked seeds, but had First gymnosperms lacked seeds, but had developed them by the end of the developed them by the end of the Devonian.Devonian.

Radiation of GymnospermsRadiation of Gymnosperms

Gymnosperms underwent adaptive radiation Gymnosperms underwent adaptive radiation during the Carboniferous and Permian periods during the Carboniferous and Permian periods (360-245 mya).(360-245 mya).

Planet became progressively drier during the Planet became progressively drier during the Permian and gymnosperms replaced ferns and Permian and gymnosperms replaced ferns and relatives as the dominant plants. Gymnosperms relatives as the dominant plants. Gymnosperms were the dominant plants during the Age of were the dominant plants during the Age of Dinosaurs (Mesozoic 245-65 mya).Dinosaurs (Mesozoic 245-65 mya).

Advantages of heterosporyAdvantages of heterospory

Ancestor of gymnosperms was Ancestor of gymnosperms was heterosporous.heterosporous.

Evolution of heterospory was a significant Evolution of heterospory was a significant development because it led to the development because it led to the evolution of seeds.evolution of seeds.

Gymnosperm - heterosporyGymnosperm - heterosporyIndependent and dominant sporophyte

megaspores

mega-gametophyte

eggs

zygote

embryo

meiosis

n2n

sperm

microspores

micro-gametophyte

Advantages of heterosporyAdvantages of heterospory

Female gametophyte greatly reduced in Female gametophyte greatly reduced in size, so much so that it could be enclosed size, so much so that it could be enclosed and protected within a layer of sporophyte and protected within a layer of sporophyte tissue (the ovule).tissue (the ovule).

In seed plants the female gametophyte is In seed plants the female gametophyte is not dispersed and is protected from drying not dispersed and is protected from drying out and other hazards.out and other hazards.

Advantages of heterosporyAdvantages of heterospory

The male gametophyte (n) is dispersed in The male gametophyte (n) is dispersed in seed plants, but is protected by a layer of seed plants, but is protected by a layer of sporophyte tissue (2n). sporophyte tissue (2n).

The whole structure is calledThe whole structure is called

a pollen grain.a pollen grain.

Pollen grain

Gymnosperm Life CycleGymnosperm Life Cycle

In gymnosperms dominant generation is In gymnosperms dominant generation is the tree (sporophyte),the tree (sporophyte),

Pine trees (and other gymnosperms) Pine trees (and other gymnosperms) produce both ovulate and pollen cones.produce both ovulate and pollen cones.

Gymnosperm Life CycleGymnosperm Life Cycle

Pollen cone contains sporangia that Pollen cone contains sporangia that undergo meiosis to produce haploid undergo meiosis to produce haploid microspores that develop into male microspores that develop into male gametophytes (n). gametophytes (n).

Pollen is wind dispersed and some lands Pollen is wind dispersed and some lands on ovulate cones.on ovulate cones.

Gymnosperm Life CycleGymnosperm Life Cycle

Ovulate cone contains many scales, each Ovulate cone contains many scales, each with two ovules. with two ovules.

Ovule contains megaspore mother cell Ovule contains megaspore mother cell that undergoes meiosis and eventually that undergoes meiosis and eventually produces the female gametophyte (n) that produces the female gametophyte (n) that produces eggs.produces eggs.

Gymnosperm Life CycleGymnosperm Life Cycle

Eggs are fertilized by sperm from the male Eggs are fertilized by sperm from the male gametophyte. Usually only one fertilized gametophyte. Usually only one fertilized egg develops into an embryo.egg develops into an embryo.

Embryo is surrounded and nourished by a Embryo is surrounded and nourished by a food supply (the female gametophyte food supply (the female gametophyte tissue). Both are protected by a seed coat tissue). Both are protected by a seed coat derived from the parent tree. derived from the parent tree.

Gymnosperm Life CycleGymnosperm Life Cycle

Seed thus consists ofSeed 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, Seed falls from cone, and if lucky, develops into a tree.develops into a tree.

Advantages of seedsAdvantages of seeds

Seed coat provides Seed coat provides protectionprotection and and gametophyte tissue provides gametophyte tissue provides nourishmentnourishment for developing embryo. for developing embryo.

DormancyDormancy: the developing embryo is : the developing embryo is protected and can wait a long time to protected and can wait a long time to germinate when conditions are good.germinate when conditions are good.

Advantages of having seedsAdvantages of having seeds

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

Seeds vs sporesSeeds vs spores

Seeds are better than spores because Seeds are better than spores because spores have a short lifetime. spores have a short lifetime.

Spores are thinner walled and more Spores are thinner walled and more vulnerable to pathogens and damage.vulnerable to pathogens and damage.

Modern GymnospermsModern Gymnosperms

Gymnosperms have “naked” seeds that Gymnosperms have “naked” seeds that are not enclosed in an ovary (as are not enclosed in an ovary (as angiosperm seeds are).angiosperm seeds are).

There are four extant phyla.There are four extant phyla.

CycadsCycads

130 species in 2 130 species in 2 familiesfamilies

Old World Old World tropicstropics

Large palm-like Large palm-like leaves and large leaves and large cones.cones.

GinkgoGinkgo

1 species1 species Unknown in wild, Unknown in wild,

previously previously widespreadwidespread

Seed coat is Seed coat is fleshy.fleshy.

Widely planted Widely planted street treestreet tree

ConifersConifers

600 species in 7 600 species in 7 familiesfamilies

Most important Most important gymnospermsgymnosperms

Pine, spruce, fir, Pine, spruce, fir, cedar, etc.cedar, etc.

Complex seed Complex seed conescones

Needled leaves Needled leaves reduce water lossreduce water loss

GnetophytesGnetophytes 90 species, 90 species,

3 genera3 genera Double Double

fertilizationfertilization Transition to Transition to

angiosperms?angiosperms?

Ephedra

Welwitschia

Gnetum