Flowering plants

Flowering plants, or angiosperms, are plants from the class Angiospermae that produce seeds enclosed in an ovary and are characterized by the possession of fruits and flowers. They are better adapted to life on land than mosses for several reasons. The most important adaptations angiosperms went through to become more successful terrestrial organisms than mosses were the development of their vascular tissues and the specialization of their organs used for sexual reproduction, the flower.

The vascular tissue, xylem functions in the conduction of water and minerals and also provides mechanical support. In addition to the primary cell wall that all plants have, xylem cells have a secondary cell wall that give them additional strength. Sometimes, the walls of xylem cells have pits, or places where the secondary cell wall is absent. However, most xylem cells are dead at maturity since they are only cell walls lacking cellular components, and contain only the material being transported.

There are two kinds of xylem cells, tracheids and vessel elements. In tracheids, which are long and tapered, water passes from one tracheid to another through pits on the overlapping tapered end of the cells. Vessel elements are shorter and wider than tracheids, and have less or no taper at their ends. A column of vessel elements is called a vessel. Water passes from one vessel element to the next through areas without both primary and secondary cell walls. These areas are called perforations and are literally holes between cells. Because of the perforations, water movement through vessel elements is more efficient than through tracheids. As a result, vessels are considered a more evolutionary advanced feature. They are found most prominently among the flowering plants.

Refinements in the vascular tissue of angiosperms, especially the xylem, played a role in spreading flowering plants across diverse terrestrial habitats. The vascular tissue of flowering plants includes tracheids, which function in both mechanical support of the plant, and water transport throughout the plant. In addition to these tracheids, angiosperm xylem also has fiber cells, specifically developed for support. Vessel elements, another type of xylem cell, are also found in most flowering plants. Vessel elements are arranged end-to-end into continuous tubes called xylem vessels, which are more efficient in transporting water than tracheids. In mosses, most lack conducting tissues that can distribute water and organic compounds within thick tissues. Although some have specialized tissues for such a purpose, their cell walls lack a lignin coating that would be considered characteristic of a vascular plant system. Such an absence of lignified vascular tissues also limits the height of mosses. This reduces their apical dominance, which means they are easily outdone by flowering plants that do have lignin. The refinements to the vascular tissue of angiosperms played a role in giving angiosperms an edge over mosses, whose vascular tissues remain largely rudimentary in comparison.

The flower is the organ for sexual reproduction of angiosperms and consists of male and female organs. The flower's male organ is known as the stamen. It consists of a thin, stalklike filament with a sac at the top. This structure is called the anther, and produces haploid spores. The haploid spores develop into pollen grains. The haploid nuclei within the spores will become the sperm nuclei, which fertilize the ovum. The flower's female organ is termed the pistil. It consists of three parts: the stigma, style, and the ovary. The stigma is the sticky top part of the flower which catches the pollen. The tubelike structure connecting the stigma o the ovary at the base of the pistil is known as the style. The style permits the sperm to reach the ovules. The ovary, the enlarged base of the pistil, contains one or more ovules. Each ovule contains the monoploid egg nucleus.

Petals are specialized leaves that surround and protect the pistil. They attract insects with their characteristic color and odors. This attraction is essential for cross-pollination. The pollen grains are transferred from the anther to the stigma during this process. Agents of cross-pollination include wind, insects, and water. The flower is often brightly colored and emits a pleasant odor which attracts insects and birds, which help to spread the male gametophytes. Carrying pollen directly from plant to plant is more efficient than relying on pollen that is carried by wind since it prevents self-pollination, which does not create diversity. When the pollen grain reaches the stigma, it releases enzymes that enable it to absorb and utilize food and water from the stigma and to generate a pollen tube. The pollen tube is the remains of the evolutionary gametophyte.

The fruit, in which most seeds develop, is formed from the ovary walls, the base of the flower, and other consolidated flower pistil components. Thus, the fruit represents the ripened ovary. The fruit may be fleshy, for example a tomato, or dry, a nut. It serves as a means of seed dispersal, since it enables the seed to be carried more frequently or effectively by air, water, or animals, through their consumption and elimination of the fruit. Eventually, the seed is released from the ovary, and will germinate under proper conditions of temperature, moisture, and oxygen.

The development of flowers and fruit in angiosperms is another factor that gives flowering plants an advantage over mosses when living on land. The flower is a specialized structure designed for reproduction. In essence, flowers are specialized shoots with four sets of modified leaves: sepals, petals, stamens, and carpels. These modified leaves cover the flower before it opens. The petals are brightly colored to attract insects and other pollinators, but plants that rely on wind pollination generally lack brightly colored petals. Stamens are male reproductive organs in a flower, whereas the carpel houses the female reproductive organs. A stamen consists of a supportive filament, and an anther, where pollen is produced. A carpel houses the stigma, which opens into a tube called the style, which leads to the ovary of the plant. The evolution of flowers was an important step towards angiosperms being successful land because pollination by both wind and pollinators took advantage of terrestrial animals and wind patterns. Fruits are mature ovaries, which develop from fertilized ovules, which thicken and enlarge after fertilization. Fruits also protect dormant seeds within them, and aid in their dispersal. Modifications in fruits help to disperse seeds in a variety of ways. For example, dandelions and maples have fruit that enhance seed dispersal by wind. Other plants have fruit such as burrs that cling onto passing animals, or are stored in nutritious, often sweet-tasting fruit that will be eaten by animals and deposited in their excrement, allowing the seeds to be deposited far away from their parent plant. In mosses, their offspring are dispersed through spores. Most sporophytes consist of a foot, an elongated stalk called a seta, and the sporangium or capsule, which produces the spores. The moss capsule is the organism's site of meiosis and spore production, with the capsule capable of storing up to 50 million spores. When immature, the capsule is covered with a protective cap called the calyptra, made of gametophyte tissue. When the capsule is ready to release its spore, the capsule loses its calyptra. The upper part of the capsule, called the peristome, is specialized for gradual spore discharge, and as a result often sprinkles rather than dumps out the spores within the capsule. The periodic release of spores allows them to take advantage of wind gusts that can carry the spores over long distances. While clever to take advantage of the wind, a moss's means of dispersal are somewhat limited compared to the means that a flowering plant has at its disposal. As mentioned earlier, fruit produced by angiosperms can enable their seeds to be dispersed by wind, by being carried by animals, or by being deposited in the feces of organisms that eat the fruit, not to mention the many other means that angiosperms have of dispersal. Also, a weakness of spore distribution is that if there were no winds to carry the spores to other locations, the moss will have failed to spread its spores and enjoy reproductive success. By having so many means of dispersal available, angiosperms have a much higher chance of proliferation, and spreading their seeds. As is obvious, angiosperms are undoubtedly better suited to living on land than mosses.

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