- MICROSPOROPHYLL - a sporophyll that produces male spores (microspores)
- MEGASPOROPHYLL - a sporophyll that produces female spores (megaspores)
(colloquially, they're called "cones")
So a "pine cone" is actually a whorled cluster of sporophylls: each woody bract is a sporophyll that bears sporangia.
Although conifers are monoecious (male and female reproductive structures are borne on a single individual), they bear separate male and female cones.
The big, woody pine cones with which you are familiar are clusters of MEGASPOROPHYLLS. They are females! (They bear the seeds).
The male strobili appear in the springtime, release their pollen and drop off the tree in a very short time. These are clusters of MICROSPOROPHYLLS. They are males.
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Up until now, we've said that when a spore hits the ground in a suitable location, it has the potential to grow (mitotically) into a gametophyte (which will make eggs or sperm or both).
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In the seedless tracheophytes (ferns and their allies), the gametophyte grows up independently of the sporophyte in most cases, but it lives only for a short time (one season) before "giving birth" to the new sporophyte, which crushes the gametophyte out of existence as it grows to maturity.
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In the SEED PLANTS, the spores never "hit the ground." Instead, they remain on the parental sporophyte until they are finished growing into the male or female gametophytes!
The pine tree gives the classic example of this!
1. In the springtime, whorls of microsporophylls (male pine cones) are produced on the outer ends of the lower branches. Each microsporophyll bears two microsporangia.
2. Inside the immature microsporangium, diploid MICROSPORE MOTHER CELLS undergo meiosis to produce haploid microspores.
3. The microspores are not released into the environment! (Unlike the case in the seedless plants!). Rather, each microspore remains inside the microsporangium and matures into a male gametophyte.
4. The male gametophyte is the haploid POLLEN GRAIN. It's a complete male plant, whose only job is to produce TWO SPERM.
5. When the pollen grains are mature, the little male cone opens, and the microsporangia split, releasing pollen to the wind.
6. Pines, like other wind-pollinated plants, produce huge quantities of pollen, simply because wind isn't very efficient at delivering the pollen right the the waiting female.
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Meanwhile, back at the female...
1. In springtime, each sporophyte tree produces tiny (about 1 cm long) FEMALE cones (clusters of megasporophylls) at the tips of the uppermost branches.
2. While pollen is flying around, the megasporophylls are spread apart to allow pollen to sift between and become trapped.
3. By early summer, the megasporophylls shut tight, trapping the pollen inside.
4. Once trapped, the pollen GERMINATES and begins growing a tube. Eventually, it will send its sperm through this tube to the waiting female gametophyte.
Female development:
5. Each megasporophyll has two megasporangia on the upper side of the leaf. Inside each megasporangium is a diploid Megaspore Mother Cell which will undergo meiosis to produce four megaspores. (Three of these die; the remaining one develops into a....
megaspore!) This megaspore is not released into the environment.
6. The megaspore develops inside the megasporangium, inside a specialized structure called an ovule (which contains several layers of maternal sporophyte tissue, including what will become the seed coat and a nutrient tissue called NUCELLUS).
7. The megaspore develops into the female gametophyte, which is little more than a cell containing archegonia (the female sex organs; remember?)
8. The archegonium produces an ovum (egg).
9. The maturation process from megaspore mothercell to female gametophyte to ready ovum takes about one year!
10. By the time the ovum is ready for fertilization, the poor little pollen has completed growth of his tube and has released sperm.
11. The fusion of ovum nucleus and sperm nucleus INSIDE THE OVULE is known as fertilization. NOTE THAT POLLINATION AND FERTILIZATION ARE SEPARATED BY ABOUT A YEAR, IN THE CASE OF A PINE TREE!
12. Once the ovum and sperm have united, the ovule is called a SEED.
12. Fertilization produces a ZYGOTE inside the ovule. This will develop into a pine tree embryo for about another six months or so. Inside the seed, nutritive NUCELLUS tissue deposited by the original sporophyte will serve to give the little pine embryo its first nutrients and "head start" in life: one of the biggest advantages conferred by a seed over a spore!
13. As this is occurring, the female pine cone is getting more brown and dry. By the time the SEED is "ripe" and ready to be released, the pine cone (now about two years old) is brown and woody and looks like the typical pine cone you've all seen!
14. When the seeds are ready, the bracts separate
again, and the seeds are either dispersed by wind (these have
little "wings" on them, which allow the wind to carry
them like little helicopters) or animals! (squirrels tend to forget
where they hid them!)
This is the general life cycle of all the GYMNOSPERM (literally, "naked seed" plants:
Phylum Cycadophyta - "Sago palms"
Phylum Gingkophyta - ginkgoes
Phylum Coniferophyta - pines, junipers, cypress, etc.
Phylum Gnetophyta - gnetophytes
Let's have a look at some pics...
The ANGIOSPERMS (Flowering Plants) take it one step further! All the parts of the flower are modified leaves!
The microsporophylls of the flower are the STAMENS, consisting of the ANTHER and the FILAMENT.
The megasporophylls of the flower are the CARPELS, each consisting of an OVULARY (where the ovules develop), the STYLE, and the sticky STIGMA (which serves as a "landing pad" for the pollen.
All the carpels collectively are called the PISTIL. A flower may have a pistil consisting of only one carpel (one seed chamber, with one or more seeds) or many carpels (more than one seed chamber, each with one or more seeds).
Once pollen has landed on the stigma, it grows a long tube that reaches down into the ovulary, where it delivers its two sperm to a single ovule. Inside the ovule, the female gametophyte is waiting with her ovum.
Let's have a look at the whole cycle, just for the agony of it....
Note that unlike the conifers, the female gametophyte of the angiosperm LACKS ANTHERIDIA. She consists of no more than a mass of cytoplasm containing 8 genetically identical nuclei in a very specific configuration.
A single female gametophyte like the one pictured above resides in every ovule. One pollen grain will successfully grow a tube to each ovule and deliver two sperm to the female gametophyte.
One sperm will fuse with the ovum to become the zygote, which will grow into the embryo and eventually be the new sporophyte.
The other sperm will fuse with both polar nuclei, and this 3n nucleus will then proliferate into a mass of cytoplasm, the ENDOSPERM, that will serve as the embryo's nutrient reservoir.
NOTE THAT IN THE CONIFERS, THE NUTRITIVE NUCELLUS COMES FROM THE MOTHER SPOROPHYTE'S TISSUES. In anthophytes, the endosperm serves the same function as the nucellus, but is of entirely different origin. The two substances, nucellus and endosperm, are ANALOGOUS--NOT HOMOLOGOUS!
