On either side of the pollen grain, two ear-like structures emerge. These air sacs may help orient the pollen grain toward the ovule. Xerophytic leaves are adapted to withstand drought conditions. In conifers, we see a wide range of xerophytic leaves with different morphologies that can be shaped by their local environment.
Consider the leaves of the coast redwood and the giant sequoia, shown below. Though these two trees belong to different genera-- Sequoia and Sequoiadendron , respectively--they are sister taxa.
However, the coast redwood has adapted to life on the coast, where the giant sequoia has evolved in inland, higher elevation forests with much more extreme climatic conditions.
How can this be seen in the structure of their leaves? Conifers Conifers are the most species-rich lineage of gymnosperms. Conifers share the following characteristics: Monoecious. Plants produce both male and female strobili on the same plant. Wind pollinated with "winged" pollen air sacs Xerophytic leaves with a low surface area to volume ratio. Primarily evergreen, but some species are deciduous ex. Dawn redwood and larch.
Unlike the woody seed cones, pollen cones are small, fragile, and ephemeral. Microgametophytes are formed within the microsporangia of the microstrobilus, or pollen cone. These structures are all diploid. Within the microsporangium, there are microsporocytes also called microspore mother cells , diploid cells that undergo meiosis to produce haploid microspores. Microspores grow by mitosis into microgametophytes, AKA pollen. From the original figure caption: "During the first year of growth the cutinized epidermis is replaced by protective growth of cork rich periderm.
The outer periderm consists of layers of cork cells, the phellem, which produces waterproofing suberin. Conifers include familiar evergreen trees such as pines, spruces, firs, cedars, sequoias, and yews. A few species are deciduous, losing their leaves in fall.
The European larch and the tamarack are examples of deciduous conifers. Many coniferous trees are harvested for paper pulp and timber.
Diversity of conifers : Conifers are the dominant form of vegetation in cold or arid environments and at high altitudes. Shown here are the a evergreen spruce Picea sp. Notice the yellow leaves of the tamarack. Cycads thrive in mild climates. They are often mistaken for palms because of the shape of their large, compound leaves. Cycads bear large cones and may be pollinated by beetles rather than wind, which is unusual for a gymnosperm.
They dominated the landscape during the age of dinosaurs in the Mesozoic, but only a hundred or so species persisted to modern times. Cycads face possible extinction; several species are protected through international conventions. Because of their attractive shape, they are often used as ornamental plants in gardens in the tropics and subtropics. Cycad leaves : This Encephalartos ferox cycad has large cones and broad, fern-like leaves.
The single surviving species of the gingkophytes group is the Gingko biloba. Its fan-shaped leaves, unique among seed plants because they feature a dichotomous venation pattern, turn yellow in autumn and fall from the tree. For centuries, G. It is planted in public spaces because it is unusually resistant to pollution. Male and female organs are produced on separate plants. Typically, gardeners plant only male trees because the seeds produced by the female plant have an off-putting smell of rancid butter.
Gingko biloba is the only surviving species of the phylum Gingkophyta. Gnetophytes are the closest relative to modern angiosperms and include three dissimilar genera of plants: Ephedra , Gnetum , and Welwitschia.
Like angiosperms, they have broad leaves. In tropical and subtropical zones, gnetophytes are vines or small shrubs. Because ephedrine is similar to amphetamines, both in chemical structure and neurological effects, its use is restricted to prescription drugs. Like angiosperms, but unlike other gymnosperms, all gnetophytes possess vessel elements in their xylem.
Privacy Policy. Skip to main content. Seed Plants. Search for:. Characteristics of Gymnosperms Gymnosperms are seed plants that have evolved cones to carry their reproductive structures.
Learning Objectives Discuss the type of seeds produced by gymnosperms. Key Takeaways Key Points Gymnosperms produce both male and female cones, each making the gametes needed for fertilization; this makes them heterosporous. Megaspores made in cones develop into the female gametophytes inside the ovules of gymnosperms, while pollen grains develop from cones that produce microspores. Conifer sperm do not have flagella but rather move by way of a pollen tube once in contact with the ovule. A giant cycad today might reach feet max.
They are unisexual or dioecious, having separate male and female plants. Dioecious means two houses, vs. Only one genus of cycad Zamia is native to North America.
The Seminoles ate the starchy roots of Zamia pumila , found in southern Florida. In India, Japan, and Sri Lanka, sago flour is often made from cycad stems it is also made from real palms, which are angiosperms. Cycads are widely grown as ornamental landscape plants. Cycads also enrich the fertility of barren soil, because they are symbiotic with nitrogen-fixing cyanobacteria.
Cycads are extremely slow growing, and can live 1, years or more. They are wind pollinated, a strategy which requires immense amounts of airborne pollen. A few may have been pollinated by beetles attracted to the edible pollen grains.
This may be the humble beginnings of the complex animal pollination developed by flowering plants. The pollen sacs and ovules are born on scalelike sporophylls in compact cones. Unlike pine cones, the cones of cycads are often very large in relation to the plant. Ginkgo trees are commonly seen in cities today. They are attractive shade trees, reaching feet or more, with beautiful yellow foliage in the Fall. They are very resistant to air pollution and insects. That the sole remaining species did not join its brethren in extinction we owe to the ancient Chinese and Japanese, who cultivated it in their temple gardens for centuries.
Their may no longer be a single living wild tree. It is a popular tree for bonsai, because the leaves will readily miniaturize, and the branches are easy to shape. The species name biloba comes from the two distinct lobes of its fan-shaped leaves, very different from the straplike or needle shaped leaves of other gymnosperms. The common name maidenhair tree comes from the similarity of ginkgo leaves to fronds of the maidenhair fern. Ginkgos and cycads show a transitional stage between the primitive ferns and the more advanced conifers and flowering plants.
They have flagellated sperm, but the male gametophyte grows a pollen tube, a long filament through which the sperm can safely swim to the egg. The pollen grains of other seed plants grow similar tubes. The megasporangia, which contains the eggs, form tiny female strobili on the tips of special branches on the female tree. The microsporangia, which produce the pollen grains, are in male strobili that hang down like little pine cones on the male tree.
The seed that forms on the female trees is covered with a thick fleshy coat which makes the seed look like a little fruit which it is technically not.
So be very careful if you plant one of these wonderful trees and select a male tree!! Although in fairness to the female tree, its seed is prized in China as a source of medicinal drugs. This odd little group of gymnosperms are mainly xerophytes, plants that are adapted to dry conditions.
They share a close common ancestor with flowering plants. Each genera has some species that produce nectar, and attract insects.
It was recently discovered that double fertilization, a trait we thought was unique to flowering plants, also occurs in Ephedra , one of the three surviving genera of gnetophytes.
Ephedra , incidentally is the natural source of the alkaloid ephedrin, used to treat hay fever, sinus headaches, and asthma. Its medicinal properties have been known for at least 5, years! Most gnetophytes are stem plants, like Ephedra, branched photosynthetic stems with no leaves.
Gnetum has leaves like those of modern flowers. But the third genus, Welwitschia , is one of the strangest plants on earth. Welwitschia really looks like something out a science fiction novel. It grows in the deserts of southwestern Africa.
Most of the plant is deep underground, with a root stretching down to the water table. The top appears above the soil as a squat cup- shaped stem with two strap-shaped leaves.
These are the only leaves the plant will ever grow, and they may live a hundred years or more and reach several meters, usually torn into strips. Male or female strobili grow from the margins of the upper stem. Division Coniferophyta - sp. The conifers are the largest and most successful group of living gymnosperms.
Many of our familiar forest trees are conifers, including pines, spruces, firs, hemlocks, yews, redwoods and cypress trees. They are an ancient group, dating back mya. They evolved during the Permian, toward the end of the Paleozoic, at a time when the climate was very cool and dry. Their special water conducting cells, called tracheids, allowed them to thrive in these climates and these same adaptations let them continue to dominate in colder and dryer environments today, such as northern latitudes, mountain slopes, and sandy soils.
Because they are superior competitors in such habitats even today, they are the only Division of gymnosperms to successfully compete with the flowering plants. Most conifers are evergreens, with the larch and the bald cypress being notable exceptions.
Their needle-shaped leaves are also an adaptation to conserve water. Needles usually occur in small bundles, each bundle emerging from a base that is actually a greatly truncated branch. Conifers have tremendous economic importance, as a source of timber and for byproducts such as pitch, tar, turpentine, and amber and other resins. Millions are sold each year as Christmas trees. All conifers produce cone shaped strobili, both male cones often called pollen cones and female cones often called seed cones or ovulate cones.
Both male and female cones are usually produced on the same tree, but not at the same time, so the trees do not fertilize themselves. Female cones are large and conspicuous, with thick woody scales. Seed cones can persist on the tree for several years after fertilization.
A few species, like junipers and the locally common podocarpus front of Richardson , have seeds that are covered with a fleshy coating, and resemble small berries.
The sporangia produced by the sporophytes are located at the bases of the sporophylls, and collected in the strobilus we call a pine cone. The microspore mother cell in the microsporangia produces the haploid pollen grains. Each scale or sporophyll in the male cone has two microsporangia on its lower surface. Each pollen grain consists of only four cells.
When the immature pollen grain finally reaches the seed cone, the megaspore mother cell in the megasporangium produces four haploid megaspores. Three of these megaspores degenerate, and only the fourth germinates into the female gametophyte. The female gametophyte consists of two or more archegonia, with a single egg in each one.
All eggs are usually fertilized. Each visible scale in the seed cone is really a much reduced lateral branch in itself. So each scale is homologous with the entire male cone. The megasporangium, which is called a nucellus in seed plants, is covered with a layer of protective cells called an integument, which is open at one end.
This tiny opening, the micropyle, marks the point where the male pollen tube will grow into the megasporangium. The megasporangium, together with its integument, makes up the ovule. Seeds develop from ovules. Each scale in the seed cone has two ovules on the upper surface of the scale, and so will ultimately bear two seeds side by side.
The pollen grains formed in the microsporangia of pines have tiny wing on either side. Because they are wind-pollinated? The ovulate cones open to receive pollen, then may close again to protect the developing embryos. When pollen grains land on the ovulate cones, they grow a long pollen tube. By the time this tube reaches the archegonia, about 15 months after pollination, the male gametophyte is fully mature. The pollen tube enters through the micropyle.
The sperm nucleus divides in two, and the pollen tube discharges two sperm. One sperm nucleus degenerates, the other fertilizes the egg. It takes the female gametophyte about 15 months to mature, and about the same time for the pollen tube of the male gametophyte to reach it.
The seed develops within the megasporangium. The seed is the structure containing the embryonic plant and the stored nutrition to support it. A section of the surface of the scale usually detaches along with the seed, giving the seed a little wing to help disperse it farther from the tree.
Conifer seeds are very complex little structures, containing cells from three generations of the tree. The nutritive tissues inside the seed are actually the haploid body cells of the female gametophyte. The seed also contains the developing diploid sporophyte, the little embryonic conifer. The outer wrapping of the seed, the tough and protective seed coat, is formed from the diploid cells of the parent sporophyte.
Pine seeds, along with acorns, are the most important source of plant food for North American wildlife. Examine the cycads and cycad frond on display. How do the leaves of cycads differ from those of angiosperms? Cycad leaves are full of potent neurotoxins, carcinogens, and other toxic chemicals?
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