- What are angiosperms, the flowering plants? What is the main feature that distinguishes them from the gymnosperms? Flowering plants have flowers and seeds (phanerogamic plants). They differ from gymnosperms by having their seeds within fruits.
- What are the two main groups into which flowering plants are divided? Angiosperm plants are divided into monocotyledonous (monocots) and dicotyledonous (dicots).
- What are the main morphological differences between monocot plants and dicot plants? The main differentiation criteria between monocots and dicots are: number of cotyledons (seed leaf) in seeds, one in monocots and two in dicots; pattern of leaf veins, parallel in monocots, reticulated in dicots; multiplicity of petal number, multiples of three in monocots, multiples of four or five in dicots; position of vascular bundles in the stem, scattered in monocots, concentrically ringed in dicots. Grasses, banana tree, sugar cane, orchids are examples of monocots. Sunflowers, oaks and waterlilies are examples of dicots.
- What are the androecium and the gynoecium? What are the other structures of flowers? Androecium is the set of male reproductive structures of flowers. It comprehends the stamens formed of filament and anther; one flower has one androecium that may have one or several stamens. Gynoecium is the set of female reproductive structures of flowers. It generally is composed of a single pistil that includes the stigma, the style and the ovary. The androecium usually surrounds the central gynoecium. Besides the androecium and the gynoecium typical flowers are also made of peduncle, sepals and petals.
- What is pollination? What are the main forms of pollination? The process in which pollen grains (the male gametophytes of phanerogamic plants) reach the female gametophyte is called pollination. The main forms of pollination are: anemophily, in which pollen is carried by wind. Hydrophily, pollination helped by water; entomophily, pollen carried by insects; ornitophily, pollination by birds; chiropterophily, pollen dissemination 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 generally are white and perfumed, many specialized in pollination by bats; the nectar is also a specialization to attract pollinator animals; flowers that produce an exaggerated amount of pollen often use the wind as pollinator; the position of anthers more external or internal next to the nectar is a way to facilitate the pollen dissemination respectively by the wind or by animals.
- How are the male gametophytes and the male gametes formed in angiosperms?In the anthers of each stamen there are pollen sacs. Within the pollen sacs there are microspore mother cells, or microsporocytes. These cells undergo meiosis forming microspores. Each microspore by mitosis forms 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 forming two sperm nuclei (male gametes) that migrate through the pollen tube.
- How many cellular nuclei does the pollen tube of angiosperms have? What is the ploidy of each of these nuclei? The pollen tube that 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 thus each of them is haploid (n).
- How is the female gametophyte formed in angiosperms? In the flower ovary there are megasporangia enclosed by a tegument having a small opening, the micropyle. Within the megasporangium there is a megasporocyte, or megaspore mother cell, that undergoes meiosis forming four megaspores of which three regress and only one is functional. The functional megaspore undergoes (three) mitosis generating eight cells that as a whole form the embryonic sac.
- What is the embryonic sac? Which are the cells that form the embryonic sac? What are their ploidies? The embryonic sac is the female gametophyte of angiosperms. The embryonic sac is composed of three cells that remain next to the micropyle, two lateral synergids and the central oosphere (egg); one binucleated cell, the polar nuclei, is placed in the central region; three antipodal cells stay in the opposite side to the micropyle. Since all these cells come by mitosis from the functional megaspore they are haploid (n).
- After pollination how does fecundation occur in angiosperms? In these plants is fecundation dependent on water? After pollination one of the sperm nuclei from the pollen tube unites with the oosphere of the embryonic sac forming the diploid (2n) zygote. The other sperm nucleus fuses with the polar nuclei of the embryonic sac originating 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 fecundation process. Fecundation in these plants is independent from water.
- What is the difference between self pollination and cross pollination? Which of these two modes of pollination contributes more to the plant diversity? Self pollination occurs when pollen grains from a flowering plant fall into the pistils of the same plant and thus gametes from the same individual unite to form a zygote. Cross pollination occurs when pollinators carry pollen grains from a plant to reach other individual plants of the same species thus gametes of different individuals form the zygote. Since it promotes formation of zygotes containing genes from different individuals (new gene combinations) cross pollination contributes more to biological diversity.
- What is dichogamy? Dichogamy is the phenomenon of the maturation of female reproductive structures of the plant in a different period to the maturation of the male reproductive structures. Dichogamy prevents self pollination and makes cross pollination almost obligatory so assisting in an evolutionary strategy to promote genetic recombination.
- What are the typical structures of the seed? What is endosperm? A typical seed is composed of the embryo, endosperm and shell. Within seeds of angiosperms there are one or two cotyledons (seed leaf, one in monocots, two in dicots). The endosperm is the tissue within the seed that has the function of nourishing the embryo.
- How different are the endosperm of gymnosperms and the endosperm of angiosperms? In gymnosperms the endosperm is haploid (n), it is called primary endosperm. In angiosperm the endosperm is triploid (3n), it is called secondary endosperm.
- What are cotyledons? 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). Seeds of monocots have a single cotyledon. Seeds of dicots have two cotyledons.
- What are the main functions of fruits? The main functions of fruits are the protection and spreading of seeds.
- From which floral structure do fruits come? Fruits are modified ovaries of the flowers.
- How are fruits formed? The fecundation in angiosperms triggers the release of hormones that act upon the ovaries. The ovary wall then develops into a fruit that contains the seeds.
- Are fruits always the flesh part of the “fruits”? Is the edible part of the onion a fruit? In some so-called fruits the actual fruit is not the flesh part. For example, the flesh part of the strawberry is not the fruit. The fruits are the small hard dots on the surface of the strawberry. Another example: the flesh 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.
- Why are there plants having single-seeded fruits and plants having fruits with more than one seed? Plants that produce single-seeded fruits, for example, mango and avocado, often have ovaries with only one egg inside. Fruits with more than one seed are originated from plants whose ovaries have more than one egg.
- What are infructescences, pseudofruits and parthenocarpic fruits? Infructescences are aggregated fruits formed from inflorescences, 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, like in apples and pears. Parthenocarpic fruits are those made without fecundation, by means of hormonal stimuli, like bananas.
- What is the evolutionary importance of the fruits for the angiosperms? The fruits contain seeds and they can detach from the plant falling on the ground or can serve as food for animals. Therefore with the emergence of fruits the seeds of angiosperms could be transported across long distances contributing to the propagation of the species.
- What are the trends of the gametophyte in the evolution of plants? A tendency of the gametophyte evolution in plants has been towards the formation of gametes that are independent from water. In bryophytes and in pteridophytes the fecundation is totally dependent on water. In phanerogamic plants such dependency does not exist. Another tendency is the reduction in the size and duration of the gametophyte. In bryophytes the gametophyte is indeed 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 remaining plants the sporophyte is the independent stage and the once autotrophic gametophyte in bryophytes and pteridophytes became dependent upon the sporophyte in the phanerogamic plants.
The Angiosperms Question and Answer
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