Both gymnosperms cone-bearing plants and angiosperms flowering plants produce pollen as part of sexual reproduction. In gymnosperms pollen is produced in microsporangiate cones male cones or pollen cones , while in angiosperms pollen is produced in the anthers part of the stamen within the flower.
Each pollen grain typically consists of one to a few cells. The wall of the pollen grain consists of two layers, the exine outer wall and intine inner wall. The exine may be smooth or ornamented with spines, warts, granules, pores or furrows. The Birds and the Bees Animals like birds and insects can be pollinators. When insects and birds get nectar out of a flower, they pick up some pollen as they move from flower to flower, and also can leave some pollen behind. Wind can also help move pollen from one flower to another.
Dicots Angiosperms in this group grow two seed-leaves. Their leaves usually have a single main vein that starts at the base of the leaf blade, or three or more main veins that spread out from the base of the leaf. Most plants are dicots, including most trees, shrubs, vines, fruit and vegetable plants and flowers. There are about , species of dicots. Monocots These angiosperms start with one seed-leaf. Images CC BY 3.
There are over , species of angiosperms. In botany, a fertilized, fully-grown, and ripened ovary is a fruit. As the seed develops, the walls of the ovary in which it forms thicken and form the fruit, enlarging as the seeds grow. Many foods commonly-called vegetables are actually fruit. Eggplants, zucchini, string beans, and bell peppers are all technically fruit because they contain seeds and are derived from the thick ovary tissue.
Acorns are nuts and winged, maple whirligigs whose botanical name is samara are also fruit. Botanists classify fruit into more than two dozen different categories, only a few of which are actually fleshy and sweet. Mature fruit can be fleshy or dry. Fleshy fruit include the familiar berries, peaches, apples, grapes, and tomatoes. Rice, wheat, and nuts are examples of dry fruit. Another distinction is that not all fruits are derived from the ovary.
For instance, strawberries are derived from the receptacle, while apples are derived from the pericarp, or hypanthium. Some fruits are derived from separate ovaries in a single flower, such as the raspberry. Other fruits, such as the pineapple, form from clusters of flowers. Additionally, some fruits, like watermelon and oranges, have rinds. Regardless of how they are formed, fruits are an agent of seed dispersal. The variety of shapes and characteristics reflect the mode of dispersal, whether it be wind, water, or animals.
Wind carries the light dry fruit of trees and dandelions. Water transports floating coconuts. Some fruits attract herbivores with color or perfume, or as food.
Other fruits have burs and hooks to cling to fur and hitch rides on animals. They can, therefore, glide for great distances. Angiosperms are seed-producing plants that generate male and female gametophytes, which allow them to carry out double fertilization. Explain the life cycle of an angiosperm, including cross-pollination and the ways in which it takes place.
As with gymnosperms, angiosperms are heterosporous. Therefore, they generate microspores, which will produce pollen grains as the male gametophytes, and megaspores, which will form an ovule that contains female gametophytes.
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. Life cycle of angiosperms : The life cycle of an angiosperm is shown. Anthers and carpels are structures that shelter the actual gametophytes: the pollen grain and embryo sac. Double fertilization is a process unique to angiosperms. The ovule, sheltered within the ovary of the carpel, contains the megasporangium protected by two layers of integuments and the ovary wall.
Within each megasporangium, a megasporocyte undergoes meiosis, generating four megaspores: three small and one large. Only the large megaspore survives; it produces the female gametophyte referred to as the embryo sac. The megaspore divides three times to form an eight-cell stage. Four of these cells migrate to each pole of the embryo sac; two come to the equator and will eventually fuse to form a 2n polar nucleus.
The three cells away from the egg form antipodals while the two cells closest to the egg become the synergids. When a pollen grain reaches the stigma, a pollen tube extends from the grain, grows down the style, and enters through the micropyle, an opening in the integuments of the ovule.
The two sperm cells are deposited in the embryo sac. A double fertilization event then occurs. One sperm and the egg combine, forming a diploid zygote, the future embryo.
The other sperm fuses with the 2n polar nuclei, forming a triploid cell that will develop into the endosperm, which is tissue that serves as a food reserve. The zygote develops into an embryo with a radicle, or small root, and one monocot or two dicot leaf-like organs called cotyledons. This difference in the number of embryonic leaves is the basis for the two major groups of angiosperms: the monocots and the eudicots.
Seed food reserves are stored outside the embryo in the form of complex carbohydrates, lipids, or proteins. The cotyledons serve as conduits to transmit the broken-down food reserves from their storage site inside the seed to the developing embryo. The seed consists of a toughened layer of integuments forming the coat, the endosperm with food reserves, and the well-protected embryo at the center. The fruit of the Aesculus or Horse Chestnut tree : These seeds are enclosed a protective outer covering called the seed coat, usually with some stored food.
After fertilization and some growth in the angiosperm, the ripened ovule is produced. The formation of the seed completes the process of reproduction in seed plants started with the development of flowers and pollination , with the embryo developed from the zygote and the seed coat from the integuments of the ovule.
Some species of angiosperms are hermaphroditic stamens and pistils are contained on a single flower , some species are monoecious stamens and pistils occur on separate flowers, but the same plant , and some are dioecious staminate and pistillate flowers occur on separate plants. Both anatomical and environmental barriers promote cross-pollination mediated by a physical agent wind or water or an animal, such as an insect or bird.
Cross-pollination increases genetic diversity in a species.
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