At fertilization, one of the sperm cells will finally unite its haploid nucleus with the haploid nucleus of an egg cell. Female cones ovulate cones contain two ovules per scale. One megaspore mother cell megasporocyte undergoes meiosis in each ovule. Three of the four cells break down leaving only a single surviving cell which will develop into a female multicellular gametophyte. It encloses archegonia an archegonium is a reproductive organ that contains a single large egg. Upon fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant.
Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination. The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte that will provide nutrients, and the embryo itself.
In the life cycle of a conifer, the sporophyte 2n phase is the longest phase. The gametophytes 1n , microspores and megaspores, are reduced in size.
This phase may take more than one year between pollination and fertilization while the pollen tube grows towards the megasporocyte 2n , which undergoes meiosis into megaspores. The megaspores will mature into eggs 1n. Life cycle of a conifer : This image shows the life cycle of a conifer.
Pollen from male cones moves up into upper branches where it fertilizes female cones. Gymnosperms are a diverse group of plants the protect their seeds with cones and do not produce flowers or fruits. Modern gymnosperms are classified into four phyla. The first three the Coniferophyta, Cycadophyta, and Gingkophyta are similar in their production of secondary cambium cells that generate the vascular system of the trunk or stem and are partially specialized for water transportation and their pattern of seed development.
However, these three phyla are not closely related phylogenetically to each other. The fourth phylum the Gnetophyta are considered the closest group to angiosperms because they produce true xylem tissue.
Conifers are the dominant phylum of gymnosperms, with the most variety of species. They are typically tall trees that usually bear scale-like or needle-like leaves. Water evaporation from leaves is reduced by their thin shape and the thick cuticle. Snow slides easily off needle-shaped leaves, keeping the load light and decreasing breaking of branches. Adaptations to cold and dry weather explain the predominance of conifers at high altitudes and in cold climates.
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. 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. Each flower makes a fruit, and these fruit fuse together. In aggregate fruits , like strawberries and blackberries, the fruit develops from a flower with many carpels. Each of these carpels develops as a separate fruitlet, that fuse together to form the compound fruit.
Seeds all bear the plant version of the belly button. They have a crescent-shaped scar called a hilum , where the ovule was attached to the wall of the ovary. Right above the hilum, if you look very carefully, you can also see a little pinprick scar that is a vestige of the micropyle. Inside the seed, the tiny sporophyte embryo develops. When it is nearly ready to germinate, the seed contains one or two thick embryonic leaves.
These seed leaves, or cotyledons , will support the tender baby plant while it establishes its roots and starts to grow its regular leaves. Most angiosperms, like roses, marigolds, and maple trees, are members of the Class Dicotyledones, the dicots , sp.
These flowers have seeds with two seed leaves di - cotyledon. Some angiosperms, like lilies, onions, and corn , are in the Class Monocotyledones, the monocots 65, sp. The seeds of monocots have only one seed leaf mono - cot.. There are several other differences between these two groups, which we summarized in the last lab plant structure.
There are seed leaves everywhere in Spring, and its impossible to tell what they will become just by looking at them. Examine slides of Lilium mature anthers. Observe the microsporangia , with all the developing pollen grains inside. Microspores are formed by meiosis, and these haploid cells develop into pollen grains, the male gametophyte in flowering plants.
Find the anthers on the real and model flowers. Examine slides of Lilium pollen tubes. You will see pollen grains in every stage of germination, many with a long pollen tube attached. Examine slides of Lilium embryo sac 8 nucleate stage.
On low power, you can see the overall structure of the ovules very clearly. Try to identify the protective integuments and the tiny opening or micropyle where the pollen tube will enter. You may have to hunt through the slide to find the embryo sac. The material has to be sliced just right to pass through the embryo sac. That's why there are so many sections on each slide. The embryo sac is the female gametophyte of flowering plants.
The pollen tubes grow down through the style and up into the ovary through the micropyle. One male nucleus fertilize the egg nucleus, the other fuses with two other embryo sac nuclei to form a 3N cell that develops into the stored food or endosperm. This process is called double fertilization. The ovules, each with a fertilized egg, will develop into seeds , with the integuments forming the seed coat. Examine the fruits on display. Be able to distinguish between simple dry fruit rice, corn, oats, peanuts , simple fleshy fruit tomatoes, cucumbers, peppers , and the two types of compound fruit , multiple fruit pineapples and aggregate fruit strawberries, blackberries, or raspberries.
Try to visualize, from cross sections of these fruits, how the carpels and ovules were arranged in the flowers that made these fruits. Examine the biomounts of dicot and monocot seeds and seedlings , and any other angiosperm seeds on display. The bracts, known as microsporophylls, are the sites where microspores will develop.
The microspores develop inside the microsporangium. Within the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores.
Further mitosis of the microspore produces two nuclei: the generative nucleus and the tube nucleus. Upon maturity, the male gametophyte pollen is released from the male cones and is carried by the wind to land on female cones. The female cone also has a central axis on which bracts known as megasporophylls are present. In the female cone, megaspore mother cells are present in the megasporangium. The megaspore mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure.
The female gametophyte is contained within a structure called the archegonium. Upon landing on the female cone, the tube cell of the pollen forms the pollen tube, through which the generative cell migrates towards the female gametophyte through the micropyle.
It takes approximately one year for the pollen tube to grow and migrate towards the female gametophyte. The male gametophyte containing the generative cell splits into two sperm nuclei, one of which fuses with the egg, while the other degenerates.
After fertilization of the egg, the diploid zygote is formed, which divides by mitosis to form the embryo. The scales of the cones are closed during development of the seed.
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