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After completing this section, students should be able to:
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| Seed plants can be broken down into two different categories, gymnosperms and angiosperms. |
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| Gymnosperms have seeds without a protective coat, known as "naked" seeds. Seeds are defined as protective structures in which the embryonic plants can be dispersed and lie dormant until conditions are favorable for its survival. The seed includes the embryo plant (dormant sporophyte), a store of nutritive tissue and an outer protective coat. |
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The plant's life cycle is called the alternation of generations because it spends half of it's life as a diploid (2n) organism and half of it's life as a haploid (1n) organism, literally "alternating between generations." 
Diploid gymnosperms are called sporophytes
because they are the spore producing generation. Haploid gymnosperms
are called gametophytes
because they are part of the gamete producing generation. In gymnosperms,
unlike other plants, the gametophytes are completely independent of the
sporophytic generation. Also, the size of the gametes is smaller than the
sporophytes. Gymnosperms are heterosporous ( produce two different types
of spores) and produce two different sporangia. Microspores
are the male gametophytes formed in the microsporangia and megaspores
are
the female gameteophytes formed in the megasporangia by a mother
megaspore through meiosis. A mother megaspore actually produces four cells
but three of the cells are discarded and only one of the cells becomes
a megaspore. Surrounding the megasporangium are one or two layers
of integument, protective tissue. Together, the megasporangium, the integument,
and the megaspore, are called the ovule.
As the female gametophyte grows in the ovule, it produces two or more archegonia,
each containing a single egg. This entire maturing process may take up
to or more than a year.
Gymnosperms produce two different kinds of cones which then produce the
two types of spores. The haploid microspores from the male cone, develop
into the male gametophyte, a wind borne pollen
grain. The female cones are larger than the
male cones because each cone has ovule-bearing scales that are much thicker
and tougher. When the ovule matures it secretes a sticky liquid that catches
the pollen gains as they are blown around. While the sticky liquid
is drying, the pollen grain matures and is drawn to the ovule. Two non-motile
sperm are released by the pollen grain and transferred through the
pollen tube that has been created to fertilize
the egg.
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Conifers have needle like leaves that can be up to 10 cm long and only 1- 3 mm in diameter. This characteristic long, thin structure allows the conifers to be well adapted to areas where the amount of moisture is variable, for example, in areas that have seasonal rainfall, long cold winters or sandy soil. In the center of the needle are the veins. The tracheid veins, or conducting cells, carry the water and the sieve cells transport the sugars. Surrounding the veins are the cells that carry out photosynthesis. On the flat side of the needle, there are ducts that release resin, the sticky liquid that closes any "wounds" in the epidermis. The outer layer of cells, called the epidermal cells, provide a hard protective coating to the leaf but contain stomata--openings that exchange gases. |
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Angiosperm Reproduction In the reproduction of angiosperms, the pollen grains are usually carried to the stigma of another flower by animals, insects, or wind. Once there, they germinate, developing pollen tubes which grow toward the ovule and provide a way for the sperm cells of the mature male gametophyte to fertilize the female gametophyte. Because many angiosperms are pollinated by insects, those flowers that attract fewer but regular visitors have an advantage over those that attract all kinds of animals because their pollen is not wasted. The many varied shapes, colors and odors of angiosperms allow sensory recognition by pollinators and encourage the advantageous regularity. Fruit- Its Advantages Fruit is just the mature ovary of an angiosperm that contains the seeds. For the purpose of survival, the seeds of the parent plants need to be carried far enough away so that the offspring will have ample open ground and sunlight to grow. When mammals eat the edible, fleshy fruit, they also consume the indigestible seeds which then pass through the digestive tract and are deposited some distance away from the parent plant. Many seeds are protected from mutilation when they're consumed by having a discouraging bitter or toxic taste. Plants in the mustard family, horse radish and mustard, for example, have a very pungent odor and taste that help guard against predation. Milk weeds produce a chemical toxic to the heart of vertebrates and nicotine and caffeine produced by some plants are defenses against predators too. |
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