Gymnosperms

1. Gymnosperms Chapter 24

2. Gymnosperms • “naked seed” • Seeds are beneath cone scales or other specialized structures • Not enclosed inside a fruit • Nonflowering seed plants • Present throughout world • Particularly prominent in cool, temperate forests

3. Gymnosperms • Living examples • Cycads (Cycas, Dioon, Zamia) • Maidenhair tree (Ginkgo) • Cedar (Cedrus) • Juniper fir (Juniperus) • Pine (Pinus) • Redwood (Sequoia, Metasequoia, Sequoiadendron) • Spruce (Picea) • Mormon tea or joint fir (Ephedra)

4. Origin of Seeds, Pollen, and Wood • Fossils from Devonian and early Carboniferous periods provide information about origin of seed plants • Progymnosperms • Reveal transition or intermediate form between spore-releasing vascular plants and seed plants

5. Origin of Seeds, Pollen, and Wood • Plants with true seeds appeared in Devonian period • Oldest known seed plant, Elkinsia polymorpha • Small tree with fernlike foliage • Ring of separate vascular bundles • Fossils found in West Virginia and Belgium

6. Origin of Seeds, Pollen, and Wood • Steps involved in evolution of seeds • Heterospory • Creates division of labor, enhances sedentary nature of one type of gametophyte and mobility of the other • Retention of megaspore inside megasporangium • Development of pollen to deliver sperm to egg cell

7. Features of Gymnosperms • Seeds and pollen • Megaphylls reduced to simple leaves, needles, or scales • Primary stem vascular system • Ring of separate bundles with phloem toward the outside and xylem toward the center

8. Features of Gymnosperms • Secondary growth from lateral cambium • Secondary xylem to the inside and secondary phloem to the outside • Main stem with lateral branching • Almost all are woody • Most are trees

9. Relationships Among Gymnosperms • Four gymnosperm groups traditionally classified as divisions • Each is monophyletic (possible exception is conifers) • Exact relationships are problematic • Groups • Cycads – possess series of ancestral features • Ginkgo – retains ancestral features, shares similarities with conifers • Conifers – bulk of gymnosperm species • Gnetophytes – vegetative and reproductive traits similar to flowering plants

10. Gymnosperm Dominance • Mesozoic era • 245 million to 65 million years ago • Age of gymnosperms, dinosaurs, and moving continents • Today fewer species and growth forms • Occupy fewer habitats than during Mesozoic era

11. Gymnosperm Vascular System • Primary stem vascular system composed of a ring of bundles • Distinct pith and cortex regions • Vascular cambium produces secondary tissues

12. Gymnosperm Vascular System • Some differences in secondary growth patterns • Cycads produce light wood with abundance of living parenchyma • Ginkgo and other conifers produce dense wood primarily composed of cells that are dead at maturity

13. Gymnosperm Vascular System • Trunks have secondary xylem, phloem, rays, and bark • Absence of vessels • Gnetophytes may have vessels in their wood • Seasonal variation in tracheid size results in annual rings in wood • Phloem • Sieve cells, fibers, ray parenchyma, axial parenchyma

14. Gymnosperm Vascular System • Many produce resin • Mix of organic byproducts of tree’s metabolism • Accumulates and flows in resin ducts • Inhibits insects • Stressed trees produce less resin making them more susceptible to insect infestation

15. Gymnosperm Vascular System • Adapted to survive drought • Needles (leaves) • Thick cuticle • Sunken stomata • Fibrous epidermis • Closely packed mesophyll without intercellular air spaces • Veins only in center of leaf • Thick rather than thin • Lifespan ranging from 3 to 30 years (do not continue growing during life span)

16. Cycads • Contains only 11 genera and about 125 species • Mainly grow in tropics • Zamia integrifolia only species occurring naturally in United States (Florida and southern Georgia) • Contain potent toxins • Can be eaten if prepared correctly

17. Cycads • Stem pith yields edible starch • Many are palmlike in appearance • Slow growing • Specimen 2m in height could be as old as 1,000 years

18. Cycads • Distinctive features • Large compound leaves • Rarely branch • Wood has a lot of parenchyma in xylem • All are dioecious • Seed producing cone  large and often protected by sharp prickles or woody plates • Pollen strobili  large and upright • Vectors of pollen transfer are beetles or wind

19. Cycads • Seeds • Often covered with fleshy, brightly colored seed coat to attract animal dispersers

20. Ginkgo • Single living representative  maidenhair tree (Ginkgo biloba) • Grows wild only in warm-temperate forests of China • Planted throughout world as urban street tree because of pollution tolerance • Tree is symbol of longevity

21. Ginkgo • Uses for herbal supplements made from leaves • Brain dysfunction • Cardiovascular fitness • Seeds • Important food in Asia • Mildly toxic • Poisoning rare, but more likely to occur in children

22. Ginkgo • Dense wood similar to conifer wood • Fan-shaped leaves • Often divided into two lobes • Turn brilliant golden color in fall before dropping • Dioecious • Due to foul-smelling, fleshy seed coat, planting of pollen-producing trees is preferred as ornamental

23. Conifers • Most widely known and economically important gymnosperms • Approximately 650 species • “cone bearer” • Conifers that lack woody cones • Junipers, podocarps, yews, plum yews

24. One clade includes: Araucariaceae Podocarpaceae Other clade includes: Taxaceae Cupressaceae Sciadopityaceae Cephalotaxaceae Conifer Clades

25. Pinaceae • Includes pines, firs, spruces • Economically important for wood, pulp, turpentine, resin, and as ornamentals • Constitutes bulk of conifer forests • Leaves needlelike  single or in clusters called fascicles • Most are monoecious

26. Pinaceae • Pines (Pinus) – largest genus in family • 93 species • Large, long-lived trees with asymmetrical shape • Bristlecone pines (Pinus longaeva) – oldest living organisms • May be more than 5,000 years of age

27. Pinaceae • Pines • Cones usually shed once seeds have matured and spilled out • Closed-cone pines keep scales closed until heated by fire

28. Pinaceae • Firs (Abies) • Symmetrical, cylindrical, or pyramidal in shape • Annual growth marked by symmetrical whorl of branches • Seed cones shatter at maturity rather than falling as a unit • About 40 species • Restricted to cooler parts of Northern Hemisphere

29. Pinaceae • Spruces (Picea) • About 40 species • All in Northern Hemisphere • Resins • Once collected and chewed by Native Americans and European immigrants • Spruce Gum • Gum manufacturers eventually shifted to paraffin and then chicle latex from an angiosperm tree

30. Pinaceae • Hemlocks (Tsuga) • Pyramidal with slender, horizontal branches and drooping tops • About 10 species in North America and Asia

31. Pinaceae • Douglas firs (Pseudotsuga) • 5 species • Pseudotsuga menziesii • Most heavily cut timber tree in United States • Dominates Pacific Northwest, Cascade, and Rocky Mountain regions

32. Pinaceae • Larches and tamaracks (Larix) • Unusual among conifers in being deciduous • American larch frequently found at edge of bogs • Cedars (Cedrus sp.) • Native to North Africa and Asia • Widely planted as ornamentals in North America • Asymmetrical to pyramidal in shape • Important timber trees since biblical times

33. Cupressaceae • Includes junipers, cypresses, redwoods • Can be monoecious or dioecious • More than 130 species with world-wide distribution • Juniper cones can be eaten and are used to flavor gin • Cone scales • Woody in cypress • Fleshy in juniper

34. Cupressaceae • Dawn redwood and bald cypress  deciduous • Coast redwood  possibly tallest tree in world • Sierra redwood  most massive tree in world • Largest living tree, the General Sherman tree, estimated to weigh 625 metric tons • Dawn redwood • Known only from fossil record until living trees were discovered in China’s Szechwan Province

35. Taxaceae • Includes yews • Shrubs or trees with dark-colored, broadly linear, sharp-pointed leaves • Dioecious • Only conifers that lack cones • English yew (Taxus baccata) • Famous for bows made of its wood • Pacific yew (Taxus brevifolia) • Found to contain anticancer compound, TAXOL

36. Cephalotaxaceae • Includes plum yews • Consists of less than 10 species of Chinese shrubs and trees • Dioecious

37. Podocarpaceae • About 140 species • Most are restricted to Southern Hemisphere • Most are dioecious • Cones are attractive to birds (vector for seed distribution) • Some excellent lumber trees are in this lineage • Many species widely planted as ornamentals

38. Araucariaceae • More than 30 species exclusively native to Southern Hemisphere • Genera Araucaria, Agathis, Wollemia • Relatively large trees • Cones disintegrate when ripe • Some, such as Norfolk Island pine, widely planted as ornamentals throughout world

39. Life Cycle of Pinus • Pine sporophytes are trees (except Pinus mugo, which is a shrub) • Heterosporous • Pollen  produced in strobili • Female gametophytes  produced in ovulate or seed cones • Strobili and ovulate cones differ in size, architecture, longevity, and location on tree

40. Life Cycle of Pinus • Pollen strobili • Average 1 cm in length and 5 mm in diameter • Produced in groups, usually on lower branches of trees • Each strobilus composed of many microsporophylls spirally attached to an axis

41. Life Cycle of Pinus • Two microsporangia develop on underside of each sporophyll • Microsporangium lined with layer of nutritive cells called the tapetum • Inside microsporangium, microsporocytes undergo meiosis, form haploid microspores • Resulting pollen grain (immature male gametophyte) contains two nuclei that undergo further divisions

42. Life Cycle of Pinus • Pollen grains • Yellow and lightweight (wind dispersal) • Generally released in spring • Have two inflated wings that help orient pollen grain on the pollination droplet of ovule

43. Life Cycle of Pinus • Ovulate cone • Familiar cone associated with pines and other conifers • Composed of many woody scales spirally attached to axis beneath it • Young ovulate cone often reddish and softer and smaller than male strobilus • Develop singly in early spring at tips of young branches in upper part of tree

44. Life Cycle of Pinus • Ovulate cone • Two ovules develop on upper surface of each scale • Megasporangium and its integument layer form ovule of pine • Megasporocyte divides by meiosis producing four megaspores • Only one of the megaspores develops into megagametophyte

45. Life Cycle of Pinus • Ovulate cone • Megaspore slowly grows into female gametophyte (several months to just over a year) • Two or more archegonia develop at micropylar end of gametophyte (digest nucellus for energy) • Mature ovule consists of integument, thin layer of remaining nucellus, female gametophyte that is undifferentiated except for several archegonia at one end • Each archegonium has enclosed egg • Space between micropyle and nucellus  micropylar chamber (where pollen grains begin to grow pollen tubes)

46. Life Cycle of Pinus • Pollination • Transfer of pollen from male strobilus to ovulate cone • Usually undergo cross-pollination • More likely due to placement of ovulate cones above pollen strobili • Occurs when ovulate cone is about nine months old

47. Life Cycle of Pinus • Pollination • Pollination drop • Exuded from micropyle • Chemically similar to flower nectar • Passively traps pollen grains that touch it • Chemical signal • Diffuses from trapped pollen to ovule • Triggers absorption of liquid, draws pollen grains through micropyle into micropylar chamber

48. Life Cycle of Pinus • Pollination • Pollen grain germinates • Grows through nucellus toward egg • Pollen grain undergoes nuclear division in tube • Final division produces two sperm nuclei • Pollen tube, containing two sperm nuclei and several vegetative nuclei, is mature microgametophyte

49. Life Cycle of Pinus • Fertilization • Sperm nuclei discharged directly around egg cell • One sperm nucleus enters egg and fuses with egg nucleus • Fertilized egg becomes diploid zygote • Divides and forms proembryo • Apical cells of proembryo develop into embryo

50. Life Cycle of Pinus • Seeds • Cone scales open and seeds fall out when seeds are mature • Wing attached to seed aids in dispersal • Seeds usually spread immediately on ripening, in late summer or early fall