Hundreds of millions of years ago, the Earth was dominated by ferns and conifers. Then, about million years ago, the first flowering plants appeared on the scene. They quickly spread to all parts of the world, changing the landscape from muted green to a riot of vibrant colour. The reasons behind the incredible success and diversity of flowering plants have been debated for centuries. Charles Darwin himself called it an "abominable mystery", fearing this apparent sudden leap might challenge his theory of evolution.
Simonin and co-researcher Adam Roddy, of Yale University, wondered if the size of the plant's genetic material - or genome - might be important. Gymnosperm seeds are often configured as cones. Do angiosperms have seeds? Angiosperms are vascular plants. They have stems, roots, and leaves. Unlike gymnosperms such as conifers and cycads, angiosperm's seeds are found in a flower. Angiosperm eggs are fertilized and develop into a seed in an ovary that is usually in a flower.
What is the role of a flower? The primary purpose of a flower is reproduction. Since the flowers are the reproductive organs of plant, they mediate the joining of the sperm, contained within pollen, to the ovules — contained in the ovary. Pollination is the movement of pollen from the anthers to the stigma. Is an apple a monocot or dicot?
The apple is a dicot, meaning it has two cotyledons or seed leaves. Some flowering plants are monocots and have only one seed leaf or cotyledon. Corn is a monocot. The embryo can be seen in the area where the seed comes to a point. When did angiosperms become dominant? 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? This has helped angiosperms grow and spread way faster than other plants and has pretty much led them to world domination! If you want more transport methods in a leaf, you need more cells to make them, but that would mean a bigger leaf, which would need even more veins and stomata - see the issue here?
Angiosperms have small cells that can make a dense network of veins and stomata, like a bunch of side-by-side subway routes! So the next question is, how do you get smaller cells? This image shows the amount of space a nucleus and the DNA within can take up in a cell.
Click for more detail. You might picture DNA as a tiny little chain, but when you are working within tiny, tiny cells, that DNA can take up a lot of space. If there was a way to get rid of a bunch of DNA, you can have smaller cells. Smaller cells can leave room for more veins between cells, and for more specialized cell structures, like stomata.
The scientists came to this idea by looking at the genomes of a bunch of plant species not just angiosperms. They measured which plant species had the smallest genomes and therefore, the least amount of DNA in each cell.
A lot of information on plant DNA has already been recorded. This graph shows that the density of stomata goes up as genome size goes down. Well, there are a few ways. They can look at special cell images from powerful microscopes and measure physical size. They can measure the concentration of the amount of phosphate in a counted number of cells, as phosphate is an important part of DNA molecules. Or they can figure out the concentration of a single gene that has been marked with a dye they can do this by looking at how light is absorbed by a sample with all of those specific genes dyed.
These are just a few ways that scientists can measure genome size. So the scientists from this project gathered together all of this info from other scientists and looked at plant fossils to help them think about angiosperm history. They found that angiosperms were the only group of plants that went through a genome downsize during this period.
This is what made them so successful. An evolutionary tree that shows how genome size compares between different groups of land plants. We should be happy that angiosperms are such a common and popular group. We breathe in oxygen and breathe out CO2.
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