The Seed Plants: Gymnosperms & Angiosperms

1. The Seed Plants: Gymnosperms & Angiosperms

2. Last day… evolution of plants adapted to land, from ‘bryophytes’ to ‘seedless vascular plants’ Today, the 2 groups that contain most living plants, the Gymnosperms & Angiosperms

3. Success of seed plants related to advantages of producing seeds: fertilized embryo & its food supply, surrounded by a protective coat

4. Along with seeds, 4 associated evolutionary changes: 1) reduction of gametophyte stage

5. 2) consistently heterosporous: microspores  male gametophytes, megaspores female gametophytes - separate gametophytes will live different lives

6. 3) Ovules & production of eggs: sporophyte produces protective layer of tissue (integument) enclosing megasporangum & its megaspore Ovule = integument + megasporangium + megaspore

7. Megaspore germinates within ovule, develops into tiny female gametophyte - produces egg which will eventually be fertilized

8. Seed contains tissue from three generations of plants: 1) seed coat from mature sporophyte 2) food supply from female gametophyte 3) embryo is new sporophyte generation Seed is protected, capable of dispersal, can remain dormant for very long periods, & has food supply to begin growth

9. 4) Pollen grains: male gametophyte surrounded by pollen wall (partly secreted by sporophyte) - a separate, multicellular generation, even if reduced to an extreme (2 cells, or 3 after germination)

10. Pollen grain capable of long dispersal, tough & resistant, & sperm nuclei do not require external H2O for fertilization

11. Seed Plants divided into 2 main groups: Gymnosperms (‘naked seeds’) & Angiosperms (‘container seeds’)

12. Some ancestors of seed plants found in fossil record ~ 380 MYA, & ‘seed ferns’ by 360 MYA Carboniferous forests dominated by lycophytes, horsetails & ferns but early gymnosperms by 305 MYA Drier conditions of Permian period favored gymnosperms over seedless vascular plants, & dominated through whole Mesozoic era

13. Look at life cycle of pine to illustrate for gymnosperms Tree is sporophyte, has ovulate cones w. megasporangia & pollen cones w. microsporangia Integument does not entirely cover megasporangium (‘naked seed’) so pollen lands on megasporangium Megaspore produced by meiosis

14. Megaspore develops into female gametophyte, which produces eggs

15. Fertilization eventually occurs, one zygote becomes embryo Ovule becomes seed: embryo, food supply & seed coat

16. Four living phyla of gymnosperms: Phylum Cycadophyta Cycads or ‘Sago Palms’ – only ~300? spp. living, but Mesozoic was ‘Age of Cycads’ - warmer areas (including southeast USA) Used as ornamental plants Seeds or starch from stem consumed, but neurotoxins may remain!

17. Phylum Gingkophyta – the Gingko - one species, ‘living fossil’, taxon common in Mesozoic, rediscovered in central China - delightful smelling fruits, leaves as traditional medicine

18. Phylum Gnetophyta – gnetophytes - <100 species in 3 genera Ephedra, 40 spp. in arid regions, ‘Mormon Tea’ in US - ephedrine from some, medicine or formerly ‘supplement’ Gnetum, 35 spp. trees, shrubs, vines, in tropical Asia Africa - some may be insect pollinated?

19. Welwitschia mirabilis – unique spp. of Namib Desert - just 2 straplike leaves (get torn up) to 6.2 m - may live > 1000 yrs.? - absorbs dew for moisture

20. Phylum Coniferophyta (Pinophyta)– the conifers Largest group of gymnosperms (~600 spp.) - some of the largest & oldest organisms - needle-leaved, usually ‘evergreen’

21. Dominant plants of boreal forests, & in some temperate areas - very important for lumber, pulp, as well as ecologically

22. But by far the most important plants… the angiosperms Phylum Anthophyta – ‘the flowering plants’ - > 260,000 spp., almost every habitat

23. Key innovations are flowers & fruits Flower – structure specialized to facilitate transfer of pollen between plants; specialized shoot w. 4 rings of modified leaves Sepals – basal, often green, enclose flower before opening Petals – interior to sepals, often brightly colored

24. Stamens – filament supports anther where microspores produced, develop into pollen grains Carpels – sticky stigma for receiving pollen, style leads down to ovary where 1+ ovule is, produces megaspores which develop into female gametophytes

25. Fruit – mature ovary of a flower, thickens around seeds - may include some additional tissues as well - protects seeds & often enhances dispersal - may be fleshy or dry

26. Flowering plants are diploid sporophytes, produce microspores and/or megaspores - asexual reproduction, no gametes or fertilization… - microspores develop into male gametophytes (inside pollen grain), megaspores develop into female gametophyte = embryo sac

27. Pollen transferred to stigma, normally cross-pollination Female gametophyte now has 8 nuclei (7 cells) - 2 sperm cells carry out double fertilization: one fertilizes egg, other fuses w. 2 nuclei in central cell - central cell becomes triploid endosperm (food supply for seed)

28. Embryo, w. endosperm & integuments, develop into seed - surrounding ovary tissue forms fruit

29. Angiosperms probably split from gymnosperms about 305 MYA, but subsequent history not well known - living lineages shared a common ancestor ~150 MYA

30. Earliest branch in phylogeny is Amborella, shrub found only on New Caledonia - lacks vessels in xylem

31. Other early branches include water lilies & the star anise & relatives - star anise family has female gametophyte w. only 4 nuclei, some spp. used as spices and medicines

32. Magnoliids are more speciose (~8,000 spp.), including magnolias, laurels, & black pepper plant

33. The 2 big groups of angiosperms are the Monocots (~70,000 spp.) & the Eudicots ( ~ 170,000? spp.) Monocots include orchids, palms, lilies, grasses…

34. Dicots include… lots! (oaks, peas, roses, potatoes, etc.)

35. A number of distinctive characteristics generally make Monocots & Eudicots fairly easy to distinguish

36. The value of seed plants to humans is… priceless - six species (maize, rice, wheat, potatoes, cassava & potatoes) provide 80% of calories consumed - coffee, tea, spices & sugar also impt. - source of building material, fuel, pulp, etc.

37. Many drugs from seed plants, currently or originally discovered in plants - most plants not investigated yet for potential uses

38. No less important to organism around the world, at least in terrestrial habitats