Conifer seeds are very complex structures, containing cells from three generations of the tree. Can you figure out which tissues come from which generation of the conifer?
Just as Gymnosperms forced non-seed plants into the ecological background, the evolution of Angiosperms, sometime during the Cretaceous, forced gymnosperms into restricted habitats. Wherever the earth was cold or dry, gymnosperms could prevail.
But in all other habitats, flowering plants rapidly became the dominant plant life. Flowering plants are able to survive in a greater variety of habitats than gymnosperms. Flowering plants mature more quickly than gymnosperms, and produce greater numbers of seeds. The woody tissues of angiosperms are also more complex and specialized.
Their seeds are enclosed in a fruit for easy dispersal by wind, water, or animals. The leaves of angiosperms are mostly thin, extended blades, with an amazing diversity of shapes, sizes, and types.
The surface of the pollen grain has a complex three-dimensional structure. This structure is unique for each species, like a floral thumbprint. It also means that pollen grains, which are abundant in the fossil record, allow us to reconstruct ancient plant communities, and these communities in turn tells us about ancient climates.
All angiosperms produce flowers , reproductive structures that are formed from four whorls of modified leaves. Most flowers have showy petals to attract pollinators, bribing insects and other animals with nectar, to get them to carry the male gametophyte through the air to another flower.
Animal pollination is common in angiosperms, in contrast to the mostly wind-pollinated gymnosperms. The ovules in angiosperms are encased in an ovary, not exposed on the sporophylls of a strobilus, as they are in gymnosperms. Angiosperm means "covered seed". The ovules develop into seeds , and the wall of the ovary forms a fruit to contain those seeds. Fruits attract animals to disperse the seeds. Flowers consist of four whorls of modified leaves on a shortened stem: sepals , petals , stamens an anther atop a slender filament , and one or more carpels.
Imagine a broad leaf with sporangia fastened along the edges of the leaf. Some ferns actually look like this. Now fold that leave over along the midrib, and you've enclosed the sporangia in a protected chamber. You've just made a carpel. The carpels are fused together to form a pistil , which consists of a stigma upper surface , a style long, slender neck , and an ovary round inner chamber at the bottom containing one or more ovules.
The flower is analogous to the strobilus of pines and more primitive plants, except that only the inner two whorls stamens and carpels actually bear sporangia. The base of the flower is called the receptacle , and the tiny stalk that holds it is the pedicel. The life cycle of flowering plants is described in more detail below. Microspores develop in microsporangia in the anthers , at the tip of the stamen. Each anther has four microsporangia.
Microspores develops by meiosis from the microspore mother cell. These microspores develop into pollen grains. Pollen grains are the male gametophytes in flowering plants. Inside the pollen grain, the microspore divides to form two cells, a tube cell and a cell that will act as the sperm. Cross walls break down between each pair of microsporangia, forming two large pollen sacs.
These gradually dry out and split open to release the pollen. Meanwhile, inside the ovary, at the base of the carpel, the ovules, are developing, attached to the wall of the ovary by a short stalk. The megasporangia is covered by an integument , protective tissues that are actually part of the parent sporophyte.
The megaspore mother cell divides by meiosis to produce four haploid megaspores. Three of these megaspores degenerate, and the surviving fourth megaspore divides by mitosis.
Each of the daughter nuclei divides again, making four nuclei, and these divide a third time, making a grand total of eight haploid nuclei. This large cell with eight nuclei is the embryo sac. This embryo sac is the female gametophyte in flowering plants. One nucleus from each group of four migrates to the center. These are called the polar nuclei. The remaining three nuclei of each group migrates to opposite ends of the cell.
Cell walls form around each group of three nuclei. The mature female gametophyte thus consists of only seven cells, three at the top, three at the bottom, and a large cell in the middle with two nuclei. One cell of the bottom three cells will act as the egg. When the pollen grain reaches the stigma of the carpel, it germinates to form a pollen tube. This pollen tube will grow through the neck or style, all the way down to the bottom of the carpel, to a small opening called the micropyle.
The male gametophyte has two cells. One is the tube cell, the other will act as a sperm. As the pollen tube grows closer to the embryo sac, the sperm nucleus divides in two, so the mature male gametophyte has three haploid nuclei. While the pollen tube is entering the ovule, the two polar nuclei in the female gametophyte fuse together, making one diploid nucleus.
The two sperm nuclei enter the embryo sac. One sperm nucleus fuses with the egg nucleus to form a diploid zygote. The other sperm nucleus fuses with the fused polar nuclei to make a triploid cell. This 3N cell will divide repeatedly to form the endosperm, the stored nutritive material inside the seed. The integuments develop into the tough outer seed coat, which will protect the developing embryo from mechanical harm or dessication. Thus the ovule, the integuments and the megasporangium they enclose, develops into the seed.
The walls of the ovary then develop into the fruit. There is an incredible diversity of flower structure, not only in the number of sepals, petals, stamens, and carpels, but also in the way these modified leaves are attached with respect to the ovary. Linnaeus used these very characteristics to sort out the different related groups of flowering plants in his invention of binomial nomenclature, genus and species.
All of these differences can affect the final physical appearance of the fruit. The ovary wall has three layers, each of which can develop into a different part of the fruit. Simple fruits are fruits that develop from a single ovary. They can be either dry , like grains, nuts and legumes, or fleshy , like apples, tomatoes and cucumbers.
Compound fruits develop from a group of ovaries. They can be either multiple fruits or aggregate fruits. In multiple fruits , like the pineapple, the group of ovaries come from separate flowers. 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. Plants do the opposite—they breathe in CO2 and breathe out oxygen during photosynthesis. Because angiosperms photosynthesize so much, they are some of the best oxygen makers around.
Angiosperms have been so successful because of their compact DNA and cells. Angiosperms - you are one magnificent bunch of plants.
The elimination of the necessity of water to transport the sperm grom the microgametophyte to the megagametophyte for fertilization to occur. This development made it possible for seed plants to complete their life cycles in relatively dry environments, compared to those of non-seed-producing vascular plants.
Seed production is an adaptation of great significance for the survival and dispersal of plants. In fact, this was part of the competititve advantage that allowed the gymnosperms to supercede the other vascular plants as the dominant type of vegetation on land.
Only the later evolution of flower and fruit allowed another group of seed plants the angiosperms to displace the gymnosperms from their preeminent position. Gymnosperms are seed-bearing plants that lack the combination of specialized features that characterize the flowering plants.
The name gymnosperm, means naked seed. Gymnosperms, then, are all fruitless seed plants. And they are made up of a heterogeneous group of plants characterized by the production of naked seeds.
Estimates form fossil records indicate that gymnosperms must have evolved approximately million years ago from non-seed producing ancestors of the extinct division of Progymnospermophyta , which were fern-like in appearance. They lack the folded, marginally-sealed carpels that characterize the flowering plants. The pollen-receptive structures are the ovules rather than the stigmatic portion of the carpels. Most gymnosperms lack vessels in their xylem unlike flowering plants which have both vessels and tracheids , except for the gnetophytes , which have vessels.
Considering the relatively small number of living gymnosperms about species in 65 genera , they are remarkably diverse in their reproductive structures and leaf types. Gymnosperms, like angiosperms the flowering plants , differ from seedless plants like mosses and ferns in not requiring water for sperm to swim in to reach the egg.
This means that the movement of pollen male gamete to ovule female gamete in seed plants relies on airborne transport, not water transport. Consequently, most gymnosperms produce huge amounts of pollen. In gymnosperms, pollen is found located in stamen-like structures called strobili various types of cones. The pollen grains of Pinus and several other genera have bladder-like wings.
Each male of a pine tree cone annually releases an estimated million pollen grains. Pollination in gymnosperms involves a pollination droplet that protrudes from the micropyle when pollen grains are being shed.
The droplet provides a large, sticky surface that catches the normally wind-borne pollen grains of gymnosperms so that the ovule is more likely to be fertilized.
After pollination the droplet evaporates and contracts, carrying the pollen grains into the pollen chamber and into contact with the ovule. The most dramatic differences between gymnosperms and other plants involve pollen and seeds and the organs that bear them.
These features differ significantly from those of comparable organs of flowering plants. Strobilus cone — the reproductive structure in gymnosperms. In conifers, this consists of an ovoid, cylindrical, or spherical cluster of sporophylls cone scales arranged around a central axis.
The place where seeds of gymnosperms are produced; essentially, an evolutionarily modified branch. There are 4 major divisions of plants within the gymnosperms :. Ginkgophyta Ginkgo: maidenhair tree ,. Cycadophyta Cycads ,. Gnetophyta Gnetophytes , and. Pinophyta or Coniferophyta the conifers.
Ginkgo biloba , or maidenhair fern because of the resemblance of its fan-shaped leaves to those of maidenhair ferns, is the only living representative of the division. It is the oldest known genus and species of living trees. Fossil ginkgo leaves and wood have been discovered that date back million years, and are nearly identical to those of the modern-day ginkgo.
It is exclusively dioecious and deciduous. It has distinctive fan-shaped leaves with dichotomous venation; it is deciduous. Both attributes resemble angiosperms. However, the fleshy seed coat of Ginkgo can easily be mistaken for a fruit. The seeds of Ginkgo include a massive integument outer coating of an ovule; later becoming testa of a seed that consists of a fleshy outer layer, a hard, stony middle layer, and an inner layer that is dry and papery.
Mature seeds have the size and appearance of small plums, but the fleshy integument has a nauseating odor like a vomitorium and irritates the skin of some people. Nevertheless, pickled Ginkgo seeds are a delicacy in some parts of Asia.
The earliest reliable record of gymnosperms dates their appearance to the Carboniferous period — million years ago. In the Mesozoic era — Angiosperms took over by the middle of the Cretaceous period The two innovative structures of pollen and seed allowed seed plants to break their dependence on water for reproduction and development of the embryo, and to conquer dry land.
The pollen grains carry the male gametes of the plant. The small haploid 1 n cells are encased in a protective coat that prevents desiccation drying out and mechanical damage. The seed offers the embryo protection, nourishment and a mechanism to maintain dormancy for tens or even thousands of years, allowing it to survive in a harsh environment and ensuring germination when growth conditions are optimal.
Seeds allow plants to disperse the next generation through both space and time. With such evolutionary advantages, seed plants have become the most successful and familiar group of plants. Paraphyletic groups do not include descendants of a single common ancestor. Gymnosperm characteristics include naked seeds, separate female and male gametes, pollination by wind, and tracheids, which transport water and solutes in the vascular system.
Pine trees are conifers and carry both male and female sporophylls on the same plant. Like all gymnosperms, pines are heterosporous and produce male microspores and female megaspores.
In the male cones, or staminate cones, the microsporocytes give rise to microspores by meiosis. The microspores then develop into pollen grains.
Each pollen grain contains two cells: one generative cell that will divide into two sperm, and a second cell that will become the pollen tube cell. In the spring, pine trees release large amounts of yellow pollen, which is carried by the wind. Some gametophytes will land on a female cone. The pollen tube grows from the pollen grain slowly, and the generative cell in the pollen grain divides into two sperm cells by mitosis.
One of the sperm cells will finally unite its haploid nucleus with the haploid nucleus of an egg cell in the process of fertilization. Female cones , or ovulate cones, contain two ovules per scale. One megasporocyte undergoes meiosis in each ovule.
Only a single surviving haploid cell will develop into a female multicellular gametophyte that encloses an egg. On fertilization, the zygote will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant.
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