Seed Plants  Two groups of seed plants: Gymnosperms Gymnosperms Angiosperms Angiosperms  Gymnosperms include the conifers and cycads and this group originated.

Presentation on theme: "Seed Plants  Two groups of seed plants: Gymnosperms Gymnosperms Angiosperms Angiosperms  Gymnosperms include the conifers and cycads and this group originated."— Presentation transcript:

1 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.

2

3 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.

4 Alternation of generations in seedless and seed plants

5 Origin of gymnosperms  Gymnosperms first group to develop seeds. Probably evolved from a heterosporous group called the progymnosperms in the Devonian Period (408-360 mya).  First gymnosperms lacked seeds, but had developed them by the end of the Devonian.

6 Radiation of Gymnosperms  Gymnosperms underwent adaptive radiation during the Carboniferous and Permian periods (360-245 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 245-65 mya).

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

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

9 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.

10 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

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

12 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.

13 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.

14

15

16 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.

17 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.

18

19 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.

20 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.

21 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.

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

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

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

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

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