GymnospermsChapter 18

Two major groups of vascular plants:A. Seedless plants - reproduce via sporesB. Seed plants - reproduce via seeds1. Seed = a structure in which the embryo (the young sporophyte) is shed from the parent plant, enclosed within a resistant coat, together with a supply of food that aids its establishment2. The majority of extant plants are seed plants

SeedsConsist of an embryoStored foodSeed coatModern seed plant the ovule consist of a nucellus envelope by one or two integuments with a micropyle (apical opening)When fertile the nucellus contains a megagametophyte composed of nutritive tissue and archegoniaAfter fertilization the integuments develop into a seed coat; a seed is formed

Characteristics of seed plants:A. Megaphylls B. Heterospory C. A reduced megagametophyte retained within the megaspore D. A megaspore retained within a fleshy megasporangium called a nucellusE. Pollen - a structure which carries the male gamete to the female gamete Seed plants do not require water for fertilization

Evolution of an ovuleRetention of the megaspores within the megasporangium (fleshy nucellus)- the megasporangium no longer releases the sporesReduction of megaspore mother cells to one functional megaspore in the megasporangiumFormation of an endosporic (within the wall) megagametophyte that is no longer free-living- retained within the megasporangium

Evolution of an ovuleDevelopment of the embryo (young sporophyte) within the megagametophyte retained within the megasporangium

Evolution of an ovuleFormation of an integument that completely envelops the megasporangium except for the micropyleModification of the apex of the megasporangium to receive microspores or pollen grains

Evolution of seedsA. The seed habit arose by 365 million years ago viafusion of vegetative tissues around the megasporangium1. This additional protective layer is called an integument2. The integument has a small opening, the micropyle, through which fertilization takes place3. Ovule = an integumented megasporangium4. Following fertilization the integument will become the seed coat

ClassificationA. There are five phyla of extant seed plants: 1. Four of the phyla have naked ovules borne on modified sporopylls. These are called "gymnosperms" = naked seed2. In the remaining phylum (Anthophyta) the ovules are enclosed within a protective structure called an ovary (flower like reprodcutive structures)

PHYLA OF GYMNOSPERMS

Cycadophyta (cycads)Ginkophyta (maidenhair tree or Ginko)Coniferophyta or Pinophyta (conifers)Gnetophyta (gnetophytes)

ConiferophytaI. Coniferophyta - commonly called conifersA. About 50 genera and 550 speciesB. Common members include the pines, hemlocks, spruces, firs, yews, cypresses, junipers and redwoodsC. Conifers are most common at the higher higher latitudes, towards the polesD. Conifers arose by 300 million years ago

E. Conifers include some of the largest organisms on earth, e.g. Coast redwoods may reach 380 feet tall and giant sequoias may reach diameters of 36 feet. A bristlecone pine, dated at 4,900 years old, is one of the oldest organisms on earth.F. Conifers are one of the most economically important groups of plants. They supply building materials and paper pulp

All conifers are woody and they have a bifacial vascular cambium that produces xylem to the inside and phloem to the outside

1. As the tree grows the center xylem becomes lignified to provide additional support2. Lignin is a chemical deposited in the secondary wallsa. Heartwood = center, nonfunctional, lignified secondary xylemb. Sapwood = outer, functional, non-lignified secondary xylem

3. With initiation of secondary growth the epidermis is replaced by a periderm produced by the cork cambiuma. Bark = all tissue external to the vascular cambium.

Longleaf pines

Foliar unit

Open seed cone

Pine seedling

G. Most conifers are evergreen, but there are a few deciduous species, e.g. bald cypress and larch

Swamp bald cypress

Common bald cypress

Larch meadow

Fir

Juniper

Sequoiadendron

Broken branch

Dawn redwood

Conifer reproduction1. Megasporangia and microsporangia are borne in separate megastrobili and microstrobili (cones)

Ovulating pine cones

2. Microstrobili (male cones) are relatively small and they dry up and wither away shortly after shedding their pollen. The strobili consist of a central axis with pairs of microsporangia on the underside of microsporophylls. Within the immature microsporangia microsporocytes (microspore mother cells) undergo meiosis to produce four haploid microspores. Each microspore develops into a winged pollen grain which consists of:

a. Two prothallial cellsb. One generative cellc. One tube celld. This four celled pollen grain is the immature male gametophyte. The pollen is shed at this stage and it carried by the wind to an immature megastrobilus (female cone)

3. During the spring pollination season the megastrobili (female cones) have their cone scales open. The pollen in the wind is caught and held by a sticky secretion. As the secretion dries up the pollen is drawn into the micropyle. The cone scales then grow together

Pollen cone

4. The pollen tube begins to digest through the nucellus towards the developing megagametophyte

Another Pine life cycle figure

5. Twelve months after pollination the generative cell divides to form a sterile cell and a spermatogenous cella. The spermatogenous cell then divides to produce two sperm cells. At this stage the male gametophyte is mature

6. Megastrobili (female cones) are larger than the microstrobili. A pair of ovules sit on top of a seed-scale complex which is subtended by a sterile bract. Each ovule contains a multicellular nucellus surrounded by a thick integument with an opening (micropyle) facing inward

7. Each nucellus contains a megasporocyte (megaspore mother cell) which undergoes meiosis to produce four haploid cells. Three degenerate and the remaining one develops into the megagametophyte over a six month period. This takes place up to six months after pollination. Development of the megagametophyte is therefore a full year behind formation of the pollen responsible for its fertilization

8. At 13 months after pollination, inside the megagametophyte, a large number of free nuclear divisions take place, forming 2,000 free nuclei. Then cell walls form to make solid tissue. This gametophytic tissue will become the stored food inside the mature seed

9. 15 months after pollination 2 - 3 archegonia, each containing an egg, form. At this time the pollen tube reaches the archegonia and it discharges both sperm nuclei into the archegonia. One sperm fertilizes the egg, the other degenerates

10. As the developing zygote divides to form the embryo four tiers of cells are produced. These form suspensors which push the developing embryo deep into the gametophyte tissue (developing stored food). Simultaneously the integument forms the seed coat

11. The seeds mature and they are shed during the second autumn, when the cone scales open. Therefore if takes about 2 1/2 years from pollination to the shedding of seeds

GnetophtyaWelwitschia mirablis

GnetophytaLiana

Gneto LeavesGnetophyta can have opposite or whorled type leaves also here is an Example of Strap like leaves Welwitschia Mirablis

Leaves continueAngiosperm-like Leaves of GnetumGnetaceae

Scale-like Leaves of Ephedra

Ginkophyta

Cycadophyta