Life on Earth Kingdom Plantae Part IV

The Seed Plants: Gymnosperms

Origin of the Seed

Female Gametophyte Scenario

• Megasporangium produces only one functional megaspore • Megaspores are not released from the plant • Megaspore develops into the female gametophyte within the old megasporangium

Origin of the Seed

(cont.)

• Archegonia are formed with egg cells and fertilized

in situ

• Embryo (new sporophyte) develops surrounded by female gametophyte tissue and old megasporangium wall ( nucellus )

Origin of the Seed

(cont.)

• Megasporophyll surrounds the structure and acts as integument ( seed coat ) • The mature ovule ( seed ) is released from the plant (old sporophyte)

Origin of Pollen

Male Gametophyte Scenario

• Micorsporophylls produce microsporangia • Microspores (N) are not released from the microsporangium but divide to form a 4 celled microgametophyte (pollen grain) • Four cells are 2 prothallial cells , 1 generative cell and 1 tube cell

Pine Pollen

Origin of Pollen

(cont.)

• Pollen grains (male gametophytes) are released from the “strobilus” or “cone” • The pollen grains germinate forming a pollen tube • The generative cell divides to form sperm nuclei which travel down the pollen tube to fertilize the egg

Why evolve seeds and pollen?

• Embryo is well protected in the seed coat • Seeds can be “dormant” and wait for suitable growth conditions • Transfer of pollen (male to female) does not require water like sperm

Why evolve seeds and pollen? (cont.)

• Embryo has a ready supply of nutrients for early development (female gametophyte tissue) • A seed is an excellent dispersal unit (wind, animals)

Seed Plant Divisions

•

Gymnosperms

– plants with “naked” seeds not found in a fruit – includes 4 Divisions •

Angiosperms

– seed plants with “hidden” seeds, i.e. formed inside a fruit – only one Division

Gymnosperms

• • • •

Coniferophyta Cycadophyta Ginkgophyta Gnetophyta

Division Coniferophyta

• Most species are monoecious, woody perennials producing cones • Some are deciduous, most are evergreen with needle like leaves • Extended life cycle (3 years)

Primary vs. Secondary Growth

•

Primary tissues

are derived from

meristems

– Zones of cell division in root tips, shoot tips, axillary buds etc.

– Primary growth generally creates

elongation

of stems, branches, and roots or creates reproductive organs

Primary vs. Secondary Growth (cont.)

•

Secondary tissues

are derived from

cambia

– Cambial activity generally causes increase in

girth

of the plant (root, stem) – Woody plants develop 2 cambia: •

vascular cambium

(producing secondary xylem and phloem) •

cork cambium

(producing “bark” layers)

PITH

Pine Anatomy

(stem) RESIN DUCT VASCULAR CAMBIUM SECONDARY XYLEM SECONDARY PHLOEM

RADIAL

Wood Sections

TRANSVERSE TANGENTIAL Wood Rays

Pine Needles

Pine Life Cycle

• Trees produce both male ( staminate ) and female ( ovulate ) cones • Male cones are usually lower on the tree to assist with out-crossing • Pollination

takes

place in the spring when female cones are small (about 1/2 inch)

Pine Life Cycle

(cont.)

• Development of the mature ovule with archegonia takes more than a year • Pollen tube digests its way slowly through the nucellus to the archegonium for fertilization • Nearly another year is required before the embryo is mature and the female cone opens to release the seeds

Pine Life Cycle

Pinus

(male and female cones

)

Male Cone

(longitudinal Section)

Conifer Diversity

• Highly diverse in both Northern and Southern Hemispheres – Includes pine, spruce, hemlock, fir, Douglas fir, yew, juniper, cypress, sequoia, larch, podocarps, araucarias and others

Taxodium

(bald cypress)

Giant Sequoia Taxus (yew) with female cones

Sequoia and Sequoiadendron

Araucaria family –Wollemi “pine,” discovered living in Australia, 1994, known from Jurassic fossils

Aucariaceae ( Southern Hemisphere ) Norfolk Island Pine

Division Cycadophyta

• Perennial, dioecious plants with coarse palm-like leaves • Female cones are often very large • Only 10 genera known many with highly restricted ranges, primarily tropical • Produce flagellated sperm

Cycas revoluta

and

C. media

With “female cone” and seeds

Dioon edule

Encephalartos

Division Ginkgophyta

•

Ginkgo biloba

only species • Probably extinct in the wild (originally from China) • Separate male and female trees (dioecious)

Division Ginkgophyta

(cont.)

• Female ovules produce butyric acid (plant males!) • Male trees produce small, fleshy pollen cones; motile sperm are produced • Tolerates air pollution well

Ginkgo biloba

Ginkgo Leaves and Ripe Ovules

Division Gnetophyta

•

Gnetum

,

Ephedra

(Mormon Tea) and

Welwitschia

very different organisms • Have vessels in the xylem • Do not produce multicellular archegonia • Have double fertilization

Ephedra male cones

Gnetophyta

(cont.)

• • •

Gnetum

is a genus of tropical vines with flowering plant-like leaves

Ephedra

is a desert shrub with several species in the SW United States (called Mormon Tea)

Welwitschia mirabilis

is a bizarre, tap rooted, perennial plant of the desert areas of SW Africa

Ephedra nevadensis

Welwitschia mirabilis

Habitat view in SW Africa

Welwitschia mirabilis in Namib

Desert, Namibia

Welwitschia mirabilis -female

(with ovulate cones)

Welwitschia mirabilis - male (with staminate cones)

Gnetum

With Ovules

Importance of Gymnosperms

• Major source of lumber for construction • Primary source of pulp for paper manufacture • Many ornamentals • Source of resins for turpentine, rosin (“naval stores”) • Juniper (“berries”) used to flavor gin • Source of amber and even lemon flavoring • Anticancer drug taxol from Pacific yew bark/ English yew leaves

Importance of Gymnosperms

Originally took 4 trees to produce a single human dose!

Now, using leaves of common English yew, a synthetic process makes the drug.

TAXOL: 31 years from “bark” to “business”