gymnosperm I Mar 16,00

Set of major groups of extant seed plants:

Ginkgophyta (1 extant species--Ginkgo biloba)

Coniferophyta (the conifers)

Cycadophyta (the cycads)

Gnetophyta (group of 3 genera: Ephedra, Welwitschia, and Gnetum)

Anthophyta (Flowering plants)

Seed plants of Ginkgoales, Coniferales, Cycadales, and Gnetales form a grade called gymnosperms.

1. Name gymnosperm implies that the seeds are "naked" i.e., the ovules are not enclosed; they are exposed such that pollen can land directly on the ovules.

2. emphasize that gymnosperms are not evolutionary group.

3. Gymnosperms are those seed plants that lack the structure that encloses the ovule of angiosperms.

Two extinct groups we want to consider as part of the discussion of diversity of seed plants are:

1. pteridosperms, the so-called seed ferns because leaves look like leaves of filicalean ferns.

2. progymnosperms are the groups among which the seed plants originated.

Seed plants originated near the end of the Devonian and beginning of the Carboniferous Period--about 345mya

1. These seed plants originated from among progymnosperms which were free-sporing plants.

2., free-sporing plants disperse spores --don't retain them as do seed plants.

Progymnosperms represent branch of trimerophyte grade.

We have discussed that pteridophytes (ferns) evolved from among trimerophytes and can likewise think of progymnosperms and seed plants as evolving separately from among the trimerophytes.

Progymnosperms differ from trimerophytes primarily in having a vascular cambium that forms secondary xylem and phloem.

Progymnosperms a grade--not an evolutionary group.

-- Progymnosperms are at base of seed plants which are an evolutionary group.

Summarize attributes of progymnosperms:

1. arborescent with stems that were pseudomonopodial and had some branches that were dichotomous.

2. Stele was often protostele, but had a vascular cambium that gave rise to a secondary xylem and phloem (differ from trimerophytes in this way).

3. Were free-sporing plants.

4. Sporangia were like those of trimerophytes

5. But progymnosperms were heterosporous, unlike trimerophytes.

6. With seed plants, share the trait of having megaspores same size as microspores or smaller.

During Carboniferous Period--after 345mya--see diversification of pteridosperms (so-called seed ferns) until about 280mya in Permian period when they became extinct.

Pteridosperms had large, compound leaves resembling fern fronds.

1. Various shoot forms among Pteridosperms: prostrate, vine-like others more tree-like. The medullosan seed fern was a tree 3-8 meters tall.

2. Ovules borne on some leaves.

3. Ovuliferous leaves basically with same form as foliage leaves.

4. Ovules of early pteridosperms borne in structure called a cupule.

5. Cupules presumed to represent modification of part of leaf lamina to surround the ovule.

6. Microsporangia of pteridosperms also borne on foliage leaf-like sporophyll.

--Microsporangia united to form compound synangia.

7. Stem had a eustele.

8. Vascular cambium of stem gave rise to secondary xylem and phloem.

9. Secondary xylem of seed ferns called manoxylic--meaning loose textured.

a. i.e., its stem has a relatively small cylinder of secondary xylem compared to the amount of pith and cortex.

b. secondary xylem tissue consists of few conducting tracheary elements and mostly nonconducting parenchymatous cells (i.e., those cells that are basically undifferentiated).

Cycads

Cycads comprise about 11 genera and perhaps 160 species.

cycads basically resemble palms.

--shoot form called pachycaul: stem is thick with loose textured wood; plants don't grow very tall and rarely form vegetative branches; they have a rosette of leaves at the top.

cycads can be tall 18 meters (55-60 ft.). or have very short stems or underground stems. We keep the short ones in the greenhouse.

As mentioned above in regard to pachycaul appearance, most cycads do not form vegetative branches.

1. When branches do arise they are most often associated with decapitation or injury to stem.

2. Among most cycads, stem branches (said to be dichotomous) only with reproduction and formation of strobilus.

Primary vasculature of stem form eustele.

Stem is distinctive in that it forms large amounts of pith and cortex relative to vascular tissues.

1. When secondary growth begins, little secondary tissue formed.

2. Secondary xylem (wood) said to have a loose texture.

3. Few conducting cells differentiate in secondary tissues.

4. Many cells of secondary tissues remain as undifferentiated parenchyma.

5. This "loose texture" manifestation of secondary xylem called manoxylic.as in tree ferns

Contrast manoxylic with so-called pycnoxylic wood.

1. In pycnoxylic, most differentiation of secondary xylem cells leads to tracheary elements with few undifferentiated parenchyma cells.

2. Hence tracheary elements appear to be dense not loose.

Leaves of cycads are pinnately compound.

Cycad shoots bear rosette of leaves.

1. all leaves congregated near apex of shoot.

2. older leaves abscise and new ones formed at about same rate.

When you think of roots in plants, consider them to grow downward through soil.

All cycads are said to produce apogeotropic ( roots that grow upward without the influence of gravity) roots in addition to roots growing downward through soil.

-- Called coralloid roots because of appearance.

--short, stubby, and branch dichotomously.

In cortex of coralloid roots, distinctive cylindrical region inhabited by cyanobacterium Anabaena.

1. Anabaena live mostly in intercellular spaces.

2. Presumed that Anabaena fix nitrogen that cycad utilizes, although this has not been demonstrated.

All cycads are dioecious. (Separate sex plants)

Microstrobilus bears microsporophylls with microsporangia.

1. few dozen to more than thousand sporangia on sporophylls depending on kind of cycad.

2. Sporangia in some congregated in sorus-like clusters on lower side of sporophyll.

Megastrobilus with megasporophylls and ovules.

--2-8 ovules per megasporophyll depending on kind of cycad.

Sporophylls become "woody" in all cycads except megastrobili of Cycas.

1. "woody" implies that they are hard like wood, but not that they are composed of secondary xylem.

2. Sporophylls are not technically composed of wood. (Thick cell walls not xylem)

3. "woody" strobili such as these often called cones.

-- pine cones are "woody" strobili

Cycad cones can become very large--some weighing as much as 88 pounds.

Consider reproductive process:

Inside microsporangium, meiosis forms microspores in which endosporic gametophyte development occurs. Gametophyte forms inside microsporangium.

First mitotic division of microspore nucleus results in two cells.

1. small prothalial cell

2. larger meristematic cell

the larger (meristematic) cell, also called antheridial initial, splits into

1. generative cell

2. tube cell

there are therefore three cells composing the male gametophyte POLLEN when it is released from the sporangium.

Pollen grains are relatively large compared to pollen of other seed plants.

1. Boat-shaped pollen with single aperture.

2. Pollen transferred from male strobilus to female strobilus by insects (Beetles) in at least some cycads.

At the time of pollination, ovule exudes drop of liquid out micropyle.

1. called pollination droplet.

2. pollen adhere to pollination droplet.

3. pollination droplet retracts and brings pollen into contact with nucellus.

4. pollen tube begins to grow through nucellus.

Pollen tube is haustorial and ramifies (branches out) in upper part of nucellus.

As pollen tube grows, generative cell divides to form two cells.

1. sterile

2. spermatogenous cell

--spernatogenous cell splits to form two sperm

Pollen tube grows into archegonial chamber between nucellus and female gametophyte.

1. Pollen tube ruptures, discharging two sperm and liquid into archegonial chamber.

2. Sperm are motile and swim to archegonia.

Sperm enter archegonia and fuse with egg = syngamy.

--Zygote results.

Few points about development of female gametophyte.

1. after meiosis of single sporocyte that is embedded in nucellar tissue, four megaspores form

2. 3 degrade and single megaspore remains.

3. Mitotic divisions of these cells result in multicellularfemale gametophyte.

4. Archegonia form at micropylar end of female gametophyte.

5. Archegonia simple: set of cells surrounding egg called archegonial jacket; and four neck cells.

Ginkgoes

Ginkgo group of seed plants now consists of only one extant plant.

1. Distribution in historic times limited to remote forests of Western China.

2. In China, oldest trees found in association with temples so it is unknown whether the trees have existed in natural setting at all during historical time.

Earliest members of Ginkgo group are fossil genera Dichophyllum and Trichopitys from Upper Carboniferous and Permian.

1. Ginkgo group may have originated about 280 mya.

2. During Jurassic (about 200 mya), ginkgo had world-wide distribution.

Among extant seed plant groups, Ginkgo most closely related to conifers.

1. the growth pattern or architecture of entire tree is like many conifers.

2. Although as the arborescent sporophyte ages, its branching becomes more irregular.

3. Largest Ginkgos are those around temples in China and Japan where they reach height of about 30 meters.

shoots of Ginkgo may be dimorphic, they may form shoots with elongate internodes called long shoots or shoots with nonelongated internodes called short shoots or spur shoots.

1. These short shoots may convert into a long shoot growth pattern when newly formed internodes elongate rather than remaining nonelongated.

2. Likewise, long shoots may convert to short shoots when newly formed internodes remain nonelongated rather than undergoing elongation growth.

3. Both long shoots and short shoots form leaves.

Leaves of Ginkgo quite distinctive.

1. fan-shaped lamina. With a lot of variation in lobing

2. venation composed of dichotomously branching vascular bundles with no midrib (i.e., no distinctive large vein in the center of the lamina).

Ginkgo is dioecious.

1. It forms male strobili with microsporangia and microspores and female strobili with megasporangia and megaspores on different individuals.

2. Reproductive strobili are formed only in the axils of leaves on short shoots.

The female strobilus consists of an axis with two ovules at its tip.

(note that strobili you will examine in lab are somewhat aberrant because they form four ovules at tip of each strobilus).

The male strobilus consists of an axis with numerous stalk-like appendages.

1. Each appendage bears two microsporangia.

2. Meiosis of sporocytes inside microsporangium results in formation of set of microspores.

3. Endosporic gametophyte development occurs.

4. Wall around gametophyte thickens and becomes resistant to environment.

5. Walled gametophyte is pollen grain.

6. When pollen grain dispersed male gametophyte consists of 4 cells: two prothallial cells, generative cell and tube cell.

Pollen grain boat shaped and single aperture.

Pollen is carried by wind to female plants.

Ovule forms pollination droplet to which pollen grains adhere.

1. Pollination droplet retracted, pulling pollen into ovule and into contact with nucellus.

2. Pollination occurs in spring.

3. When pollen arrives on surface of nucellus, cells of the nucellus begin to degrade.

4. Pollen tube, the elongated and branched tube cell of the pollen grain, emerges and begins growth in the nucellus.

5. Pollen tube single cell, but forms large number of filamentous branches that ramify between cells of nucellus.

6. This is haustorial pollen tube--in sense, thought to feed like parasite from degraded contents of nucellus.

Pollination also signals onset of sporogenesis in ovule.

1. That is, it is time at which single sporocyte in ovule undergoes meiosis, forming 4 megaspores.

2. 3 megaspores degrade, one megaspore remains viable.

3. Single megaspore undergoes endosporic development of female gametophyte.

4. Female gametophyte consists of numerous cell at maturity.

5. Archegonia are formed at micropylar end of female gametophyte.

6. Usually two archegonia per ovule, may be from 1-5.

7.Archegonia consist of two neck cells and large egg cell at maturity.

Fertilization of egg occurs the fall following pollination.

1. Part of male gametophyte--a sac-like region of pollen tube--becomes suspended in archegonial chamber--the space above female gametophyte.

2. Inside of sac-like region of pollen tube:

a. generative cell divides to form sterile cell and spermatogenous cell.

b. Spermatogenous cells then divides to form two sperm.

3. Pollen tube ruptures releasing fluid and two sperm.

4. Both sperm swim to archegonium, only one usually enters and accomplishes fertilization.

5. The other sperm may enter another archegonium, degrade outside of archegonium or inside if it entered same as first sperm.

Zygote develops into embryo inside of seed.

1. Seed forms hard outer wall that consists of middle cell layers of integument that gain thickened walls.

2. Outer part of integument degrades, forming mucilaginous layer around outside of seed and develops rancid smell as it does.

3. Rancid smell and mucilage presumed to attract animals that might consume and hence disperse seeds.

Inner part of Ginkgo seeds--the female gametophyte and embryo--are eaten by people in China and Japan.

Extracts of Ginkgo leaves recommended in Chinese folk medicine to increase health of heart and lungs.

--Current research on Ginkgo extracts involves its potential value in treatments for asthma, toxic shock, Alzheimer's Disease and circulatory disorders.