Flowering plants have flowers and seeds (phanerogamic plants). They differ from gymnosperms because their seeds are located within fruits.
Angiosperm plants are divided into monocotyledonous (monocots) and dicotyledonous (dicots) plants.
The main differences between monocots and dicots are: the number of cotyledons (seed leaves) in seeds, with one in monocots and two in dicots; the pattern of leaf veins, which is parallel in monocots and reticulated in dicots; the multiplicity of petal number, which comes in multiples of three in monocots and multiples of four or five in dicots; and the position of vascular bundles in the stem, which are scattered in monocots and concentrically ringed in dicots.
Grasses, banana trees, sugar cane and orchids are examples of monocots. Sunflowers, oak and water lilies are examples of dicots.
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The androecium is the set of male reproductive structures of flowers. It consists of the stamens formed of the filament and anther. One flower has one androecium that may have one or several stamens. The gynoecium is the set of female reproductive structures of flowers. It is generally composed of a single pistil that includes the stigma, the style and the ovary. The androecium usually surrounds the central gynoecium.
In addition to the androecium and the gynoecium, typical flowers are also made of a peduncle, sepals and petals.
The process by which pollen grains (the male gametophytes of phanerogamic plants) reach female gametophytes is called pollination.
The main forms of pollination are: anemophily, in which pollen is carried by wind; hydrophily, in which pollination is helped by water; entomophily, in which pollen is carried by insects; ornitophily, which is pollination by birds; and chiropterophily, which is the dissemination of pollen by bats.
Characteristics of the flowers of each plant species relate to the type of pollination used by the plant. Colored flowers are specialized in bird and insect attraction. Nocturnal flowers are generally white and perfumed, and many specialized in pollination by bats. Nectar is also a special adaptation to attract pollinator animals. Flowers that produce an exaggerated amount of pollen often use the wind as a pollinator. The position of more external or internal anthers next to the nectar is a way to facilitate the dissemination of pollen, via the wind or animals.
The anthers of each stamen contain pollen sacs. Within the pollen sacs are microspore mother cells, or microsporocytes. These cells undergo meiosis to form microspores. Each microspore undergoes mitosis to form a pollen grain containing one generative cell and one tube cell. The pollen grain is the male gametophyte.
When pollination occurs and the pollen grain makes contact with the stigma (the apex of the pistil), the tube cell elongates its cytoplasm, forming the pollen tube that grows towards the ovary. The generative cell divides to form two sperm nuclei (male gametes), which then migrate through the pollen tube.
The pollen tube, which is the mature male gametophyte of angiosperms, has three cellular nuclei: two sperm nuclei and one tube cell nucleus.
All those nuclei are part of the male gametophyte of the plant and, therefore, each of them is haploid (n).
The flower ovary contains megasporangia enclosed by a tegument with a small opening, the micropyle. Within the megasporangium is a megasporocyte, or megaspore mother cell, which undergoes meiosis to form four megaspores, three of which regress and only one of which is functional. The functional megaspore undergoes mitosis (three times) generating eight cells which, as a whole, form the embryonic sac.
The embryonic sac is the female gametophyte of angiosperms.
The embryonic sac is composed of: three cells that remain next to the micropyle, which are two lateral synergids and the central oosphere (egg); one binucleated cell, the polar nuclei, placed in the central region; and three antipodal cells, which stay in the opposite side to the micropyle.
Since all these cells are produced by the mitosis of the functional megaspore, they are haploid (n).
After pollination, one of the sperm nuclei from the pollen tube unites with the oosphere of the embryonic sac to form the diploid (2n) zygote. The other sperm nucleus fuses with the polar nuclei of the embryonic sac, producing a triploid (3n) cell that, by mitosis, will turn into the secondary endosperm of the seed. The synergids and the antipodal cells degenerate after the fertilization process.
Fertilization in these plants does not depend on water.
Self-pollination occurs when pollen grains from a flowering plant fall into the pistils of the same plant and therefore gametes from the same individual unite to form a zygote. Cross-pollination occurs when pollinators carry pollen grains from a plant to other individual plants of the same species and, as a result, gametes of different individuals form the zygote.
Since it promotes the formation of zygotes containing genes from different individuals (new gene combinations), cross-pollination contributes more to biological diversity.
Dichogamy is the phenomenon of the maturation of female reproductive structures of the plant during a different period than that of the maturation of the male reproductive structures. Dichogamy prevents self-pollination and makes cross-pollination almost obligatory. thus assisting in an evolutionary strategy to promote genetic recombination.
A typical seed is composed of the embryo, endosperm and shell. Within seeds of angiosperms, there are one or two cotyledons (seed leaves, one in monocots, two in dicots).
The endosperm is the tissue within the seed that has the function of nourishing the embryo.
In gymnosperms, the endosperm is haploid (n); it is called a primary endosperm. In angiosperms, the endosperm is triploid (3n); it is called a secondary endosperm.
Cotyledons, or seed leaves, are structures formed by the embryo of angiosperms to absorb nutrients from the endosperm and to store and transfer these nutrients to the embryo. (Cotyledons are auxiliary embryonic structures).
The seeds of monocots have a single cotyledon. The seeds of dicots have two cotyledons.
The main function of fruit is the protection and spreading of seeds.
Fruits are modified flower ovaries.
The fertilization in angiosperms triggers the release of hormones that act on the ovaries. The ovary wall then develops into a fruit that contains seeds.
In some so-called fruits, the actual fruit is not the fleshy part. For example, the fleshy part of the strawberry is not the fruit. The fruits are the small hard dots on the surface of the strawberry. Another example: the fleshy part of the cashew is not the fruit. The fruit is the nut.
The edible part of the onion is the stem of the plant and not the fruit.
Plants that produce single-seeded fruits, for example, mangos and avocados, often have ovaries with only one egg inside. Fruits with more than one seed are produced from plants whose ovaries contain more than one egg.
Infructescences are aggregated fruits formed from inflorescences, which are aggregated flowers. Grape clusters are examples of infructescences. Pseudofruits are “fruits” not made in the ovaries and, in general, their true fruits lack development and are found within the flesh, such as in apples and pears. Parthenocarpic fruits are those made without fertilization, by means of hormonal stimuli, such as bananas.
Fruits contain seeds and can detach from the plant by falling on the ground. They can also serve as food for animals. Therefore, with the emergence of fruits, the seeds of angiosperms could be transported across long distances, thus contributing to the propagation of the species.
During the evolution of plantas, the tendency has been for gametophytes form gametes that are independent from water. In bryophytes and pteridophytes, fertilization is completely dependent on water. In phanerogamic plants, such a dependency does not exist.
Another tendency is the reduction in the size and duration of the gametophyte. In bryophytes, the gametophyte is the lasting stage. In pteridophytes, gymnosperms and angiosperms, it became the temporary stage and its relative size was successively reduced.
A third evolutionary trend relates to the interdependency between gametophytes and sporophytes. In bryophytes, the sporophyte is entirely dependent on the gametophyte to survive. In the remainder of plants, the sporophyte is the independent stage and the once autotrophic gametophyte in bryophytes and pteridophytes became dependent on the sporophyte in phanerogamic plants.
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