Gymnosperms: The Naked Seed Plants - Prof. Robert E. Reeder | Papers Biology

bio1120_gymnosperms 3/22/04

BIOL 1120 REEDER

THE GYMNOSPERMS

I. Gymnosperms ("naked" seed plants: not enclosed by an ovary)

A. Representative Groups

1. PHYLUM CYCADOPHYTA (cycads)

a. Very old group important during the triassic period (248 million years ago): "Age of Cycads"

1) Most species extinct today: nine genera with one hundred species remain

b. Are slow growing evergreens retaining many primitive features

1) Motile sperm, although pollination and pollen tube formation has made feature obsolete

c. Are dioecious with separate and immense seed cones and pollen cones on separate plants

1) palm-like and fern-like appearance due to compound leaves

d. Grow in tropics and subtropics with many remaining cycads endangered

1) Sago palm (10 feet tall) of Florida and California is the only native cycad in U.S.

e. Are popular as ornamental plants for landscaping

2. PHYLUM GINKGOPHYTA (Ginkgo)

a. The maiden hair tree is the sole surviving species; Ginkgo is the oldest genus of living trees

1) Fossils dated 200 million years old

b. Used by Tibetan monks for ornamental purposes

1) Cultivation has been instrumental to its survival; no species found in the wild

c. Characteristics of tough, hardy, and resistance to insects, fungi, and urban smog has resulted in its planting

in cities of North America

d. Its leaves are deciduous and turn a golden color

e. Are dioecious and produce flagellated sperm

f. Females bear seeds that smell like rancid butter resulting in only the planting of male trees

3. PHYLUM GNETOPHYTA (Ephedra, Gnetum, Welwitschia)

a. 70 species in three genera remain

b. Share many angiosperm-like features

1) xylem contains more efficient vessel-like elements

2) cone clusters resemble some flower clusters

c. Gnetum contains tropical vines and trees with leaves resembling flowering plants

d. Ephedra contains shrubs found in dry regions like deserts

1) Asiatic Ephedra is the source of ephedrine (asthma medicine)

e. Welwitschia contains a single species found in deserts of southwestern Africa

4. PHYLUM CONIFEROPHYTA (conifers)

a. Members include pines, spruces, hemlocks, firs, redwoods, junipers, cypresses, larches, and yews

b. Largest gymnosperm phylum of 50 genera and more than 500 species

c. Represented by woody trees or shrubs, most of which are evergreens, with a few being deciduous and

shedding their leaves (needles)

d. Most are monoecious with reproductive parts borne on cones

e. Habitat ranges from Arctic to the tropics and are the dominant vegetation in forested areas of Canada,

northern Europe, and Siberia; southern hemisphere areas include South America, Australia, and Malaysia

f. Economical importance: source of more than 75% of wood used in construction, paper manufacturing,

plastics, rayon, lacquer, photographic film, explosives, resins used in production of turpentine, tar, and oils,

and in landscaping

g. Ecologically: food and shelter to animals and other organisms; roots hold soil to cut down on erosion

h. Gametic Life Cycle: Pine (dominant sporophyte and heterosporous with megaspores and microspores

produced on separate cones)

1) Two types of cones produced in the spring

a) In the spring, the larger ovulate or seed cone appear containing megasporangium; composed of

tough, woody scales radiating from a central axis with each scale (sporophyll) bearing a pair of

ovules close to the central axis, where they are out of sight and protected; the red and blue

"berries" of yews and junipers are actually fleshy ovulate cones.

(1) Seed cones are usually found on the upper branches of trees: usually takes two or more

seasons to mature

(2) Two ovules develop toward base of each scale

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Download Gymnosperms: The Naked Seed Plants - Prof. Robert E. Reeder and more Papers Biology in PDF only on Docsity!

BIOL 1120 REEDER

THE GYMNOSPERMS

I. Gymnosperms ("naked" seed plants: not enclosed by an ovary) A. Representative Groups

PHYLUM CYCADOPHYTA (cycads) a. Very old group important during the triassic period (248 million years ago): "Age of Cycads" 1) Most species extinct today: nine genera with one hundred species remain b. Are slow growing evergreens retaining many primitive features 1) Motile sperm, although pollination and pollen tube formation has made feature obsolete c. Are dioecious with separate and immense seed cones and pollen cones on separate plants 1) palm-like and fern-like appearance due to compound leaves d. Grow in tropics and subtropics with many remaining cycads endangered 1) Sago palm (10 feet tall) of Florida and California is the only native cycad in U.S. e. Are popular as ornamental plants for landscaping
PHYLUM GINKGOPHYTA (Ginkgo) a. The maiden hair tree is the sole surviving species; Ginkgo is the oldest genus of living trees 1) Fossils dated 200 million years old b. Used by Tibetan monks for ornamental purposes 1) Cultivation has been instrumental to its survival; no species found in the wild c. Characteristics of tough, hardy, and resistance to insects, fungi, and urban smog has resulted in its planting in cities of North America d. Its leaves are deciduous and turn a golden color e. Are dioecious and produce flagellated sperm f. Females bear seeds that smell like rancid butter resulting in only the planting of male trees
PHYLUM GNETOPHYTA (Ephedra, Gnetum, Welwitschia) a. 70 species in three genera remain b. Share many angiosperm-like features 1) xylem contains more efficient vessel-like elements 2) cone clusters resemble some flower clusters c. Gnetum contains tropical vines and trees with leaves resembling flowering plants d. Ephedra contains shrubs found in dry regions like deserts 1) Asiatic Ephedra is the source of ephedrine (asthma medicine) e. Welwitschia contains a single species found in deserts of southwestern Africa
PHYLUM CONIFEROPHYTA (conifers) a. Members include pines, spruces, hemlocks, firs, redwoods, junipers, cypresses, larches, and yews b. Largest gymnosperm phylum of 50 genera and more than 500 species c. Represented by woody trees or shrubs, most of which are evergreens, with a few being deciduous and shedding their leaves (needles) d. Most are monoecious with reproductive parts borne on cones e. Habitat ranges from Arctic to the tropics and are the dominant vegetation in forested areas of Canada, northern Europe, and Siberia; southern hemisphere areas include South America, Australia, and Malaysia f. Economical importance: source of more than 75% of wood used in construction, paper manufacturing, plastics, rayon, lacquer, photographic film, explosives, resins used in production of turpentine, tar, and oils, and in landscaping g. Ecologically: food and shelter to animals and other organisms; roots hold soil to cut down on erosion h. Gametic Life Cycle: Pine (dominant sporophyte and heterosporous with megaspores and microspores produced on separate cones) 1) Two types of cones produced in the spring a) In the spring, the larger ovulate or seed cone appear containing megasporangium; composed of tough, woody scales radiating from a central axis with each scale (sporophyll) bearing a pair of ovules close to the central axis, where they are out of sight and protected; the red and blue "berries" of yews and junipers are actually fleshy ovulate cones. (1) Seed cones are usually found on the upper branches of trees: usually takes two or more seasons to mature (2) Two ovules develop toward base of each scale

(3) Each ovule contains a megasporangium embedded in nutritive tissue (nucellus) (4) Each ovule or megasporangium will eventually form a female gametophyte bearing two or more archegonia and is destined to be a seed; two ovules would ultimately result in two seeds (5) Seeds will eventually develop on the shelf-like scales of a female cone (not enclosed) (6) Each ovule, or immature seed, consists of megasporangium which contains a reproductive cell (diploid megaspore mother cell) that undergoes meiosis to produce four large haploid megaspores; the three nearest the micropyle degenerate leaving one which will form an immature female gametophyte b) Also in the Spring, smaller staminate or pollen cones appear containing microsporangium; short- lived and composed of soft tissue which breaks apart soon after pollen formation. (1) Pollen cones are generally produced on the lower branches; clusters of 30-70 cones (2) Sporophylls of the pollen cone (leaf like structures bearing microsporangium) have two microsporangia at their base containing numerous diploid microspore mother cells

The cycle a) The immature male gametophytes originate within pollen sacs (microsporangia), which develop on the scales (sporophylls) of the smaller male cones (1) Within the microsporangium: meiosis occurs in each of the numerous microspore mother cells to produce tetrads of haploid microspores; each one develops into a pollen grain having two winglike projections externally that will facilitate wind-aided pollen distribution (2) When the pollen grains are mature (intrasporal divisions result in a 4-celled immature male gametophyte), the microsporangia, now called pollen sacs, will open by a longitudinal slit liberating millions of pollen grains in the same spring; liberated pollen grain is now considered an immature male gametophyte. (3) Each male cone annually releases an estimated 1 to 2 million pollen grains (4) Spruce and firs also have winged pollen grains, while hemlock and Douglas fir are wingless b) Each female gametophyte slowly matures as well (1) The megasporangium wall becomes a thick, protective integument that completely surrounds the female gametophyte except for a small opening called the micropyle. (a) This thick integument encloses nucellus surrounding a megasporangium, collectively called an ovule. (b) One of integumentary layers eventually becomes the seed coat. (2) A sticky fluid secreted by the mass of sporophyte tissue exudes from the micropyle and spreads out on the surface of the cone scale (pollination fluid). c) The female cone's scales will spread slightly at the same time in the spring when pollen is shed, which allows admittance of pollen grains to the scale surface, where they are trapped by the sticky fluid (pollination). d) Pollination typically occurs in the spring or early summer depending on the latitude and species. (1) It takes about 15 months after pollination for the pollen tube to reach an archegonium. (2) Some conifers require up to a year for the gametophytes to mature following pollination and lead to fertilization the next spring or summer with seeds released in the fall. (3) Other conifers reproduce much faster and release their seeds in the same year: north temperate conifers such as spruce, fir and hemlock. e) As the fluid dries, it contracts and draws the pollen grains through the micropyle; also the seed cone scales slowly grow together aiding the pollen grain f) The pollen tubes, which have penetrated the megasporangium, cease to grow with the approach of winter and will be renewed rapidly with the onset of spring. g) During the maturing of the male gametophyte (pollen grain), it will contain four cells: a pollen tube nucleus, a stalk nucleus, and a body nucleus formed by the generative cell. (1) The body nucleus is essentially a single-celled antheridium. (2) Development of the male gametophyte will be completed within the pollen chamber or within tissues of the megasporangium. h) The pollen grain, following pollination, will germinate producing a pollen tube. (1) While the pollen tube grows, the generative cell mitotically divides; one of the two resulting cells divides again forming two ameboid sperm cells. (2) The male gametophyte has now reached maturity; parallels maturing of the female gametophyte. i) By late spring or early summer of the second year, the female gametophyte is mature containing two to six archegonia, each with a single egg due to mitotic divisions.

Gymnosperms: The Naked Seed Plants - Prof. Robert E. Reeder, Papers of Biology

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Download Gymnosperms: The Naked Seed Plants - Prof. Robert E. Reeder and more Papers Biology in PDF only on Docsity!

BIOL 1120 REEDER

THE GYMNOSPERMS