The vast array of angiosperm floral structures is for sexual reproduction. The angiosperm life cycle consists of a sporophyte phase and a gametophyte phase. The cells of a sporophyte body have a full complement of chromosomes (i.e., the cells are diploid, or 2n); the sporophyte is the typical plant body that we see when we look at an angiosperm. The gametophyte arises when cells of the sporophyte, in preparation for reproduction, undergo meiotic division and produce reproductive cells that have only half the number of chromosomes (i.e., haploid, or n). A two-celled microgametophyte called a pollen grain germinates into a pollen tube and through division produces the haploid sperm. (The prefix micro- denotes gametophytes emanating from a male reproductive organ.) An eight-celled megagametophyte called the embryo sac produces the egg. (The prefix mega- denotes gametophytes emanating from female reproductive organs.)
Angiosperms are vascular plants, and all vascular plants have a life cycle in which the sporophyte phase (vegetative body) is the dominant phase and the gametophyte phase remains diminutive. In the nonvascular plants, such as the bryophytes, the gametophyte phase is dominant over the sporophyte phase. In bryophytes, the gametophyte produces its food by photosynthesis (is autotrophic) while the nongreen sporophyte is dependent on the food produced by the gametophyte. In nonseed vascular plants, such as ferns and horsetails, both the gametophyte and sporophyte are green and photosynthetic, and the gametophyte is small and without vascular tissue. In the seed plants (gymnosperms and angiosperms), the sporophyte is green and photosynthetic and the gametophyte depends on the sporophyte for nourishment. Within the seed plants, the gametophyte has become further reduced, with fewer cells comprising the gametophyte. The microgametophyte (pollen grain), therefore, is reduced from between 4 and 8 cells in the gymnosperms to a 3-celled microgametophyte in the angiosperms. A parallel reduction in the number of cells comprising a megagametophyte (ovule) has also taken place: from between 256 and several thousand cells in the gymnosperms to an 8-celled megagametophyte in most of the angiosperms. The significance of the reduction in megagametophyte cells appears to be related to pollination and fertilization. In many gymnosperms, pollination leads to the formation of a large gametophyte with copious amounts of stored starch for the nourishment of the potential embryo regardless of whether fertilization of the ovule can actually take place (i.e., whether the pollen is from the same species as the ovule). If the pollen is from a different species, fertilization or embryo development fails, so that the stored food is wasted. In angiosperms, however, the megagametophyte and egg are mature before the food is stored, and this is not ever accomplished until after the egg has been adequately fertilized and an embryo is present. This reduces the chances that the stored food will be wasted.
The process of sexual reproduction (Figure 16) depends on pollination to bring these gametophytes in close association so that fertilization can take place. Pollination is the process by which pollen that has been produced in the anthers is received by the stigma of the ovary. Fertilization occurs with the fusion of a sperm with an egg to produce a zygote, which eventually develops into an embryo. After fertilization, the ovule develops into a seed, and the ovary develops into a fruit.
A transverse section of the anther reveals four areas of tissue capable of producing spores. These tissues are composed of microsporocytes, which are diploid cells capable of undergoing meiosis to form a tetrad (four joined cells) of haploid microspores. The microspores become pollen grains and may eventually separate.
During pollen development, the layer of cells beneath the dermis of the anther wall (the endothecium) develops thickenings in the cell walls. The cell layer immediately inside the endothecium (the tapetum) develops into a layer of nutritive cells that either secrete their contents into the area around the microsporocytes or lose their inner cell walls, dissociate from each other, and become amoeboid among the microsporocytes. The pollen grains develop a thick wall of at least two layers, the intine and the exine. The intine, or inner layer, consists primarily of cellulose and pectins. The exine, or outer layer, is composed of a highly decay-resistant chemical called sporopollenin. The exine usually has one or more thin areas, or pores, through which the pollen tubes germinate, and the thick area of the exine is usually highly sculptured. The number of pores and pattern of exine sculpturing are characteristic within an angiosperm family, genus, and often within a species.
The terminology to describe the various sculpturing patterns and position and number of pores is highly complex and only a basic description as related to functional aspects of sculpturing is given here. For example, smooth or essentially smooth pollen is loosely correlated with wind pollination, as in oaks (Quercus) and grasses (corn, Zea mays). Many plants pollinated by birds, insects, and small mammals have highly sculptured patterns of spines, hooks, or sticky threadlike projections by which pollen adheres to the body of the foraging pollinator as it travels to other flowers.
Each microspore (pollen grain) divides mitotically to form a two-celled microgametophyte; one cell is a tube cell (the cell that develops into a pollen tube), and the other is a generative cell, which will give rise to two sperm as a result of a further mitotic division. Thus, a mature microgametophyte consists of only three haploid cells—the tube cell and two sperm. Most angiosperms shed pollen at the two-celled stage, but in some advanced cases it is shed at the mature three-celled stage. When the pollen grains are mature, the anther wall either splits open (dehisces) longitudinally or opens by an apical pore.
Because the sporopollenin is resistant to decay, free pollen is well represented in the fossil record. The distinctive patterns of the exine are useful for identifying which species were present as well as suggesting the conditions of early climates. The proteins in the pollen walls are also a major factor in hay fever and other allergic reactions, and the spinose sculpturing patterns may cause physical irritation.
An ovule is a saclike structure that produces the megaspores and is enclosed by layers of cells. This megasporangium is called the nucellus in angiosperms. After initiation of the carpel wall, one or two integuments arise near the base of the ovule primordium, grow in a rimlike fashion, and enclose the nucellus, leaving only a small opening called the micropyle at the top. In angiosperms the presence of two integuments is plesiomorphic (unspecialized), and one integument is apomorphic (derived). A single large megasporocyte arises within the nucellus near the micropyle and undergoes meiotic division, resulting in a single linear tetrad of megaspores. Three of the four megaspores degenerate, and the surviving one enlarges. The resulting megagametophyte produces the female gametes (eggs). This development (called megagametogenesis) involves free-nuclear mitotic divisions. The cell wall remains intact while the nucleus divides until the megagametophyte, or embryo sac, is formed. The embryo sac typically has eight nuclei. Free-nuclear mitotic division is also found in gametophyte formation in gymnosperms.
Four nuclei migrate to either end of the embryo sac. One nucleus from each group then migrates to the centre of the embryo; these become the polar nuclei. The two polar nuclei merge to form a fusion nucleus in the centre of the embryo sac. A cell wall develops around the fusion nucleus, leaving a central cell in the sac. Cell walls form around each of the chalazal nuclei to form three antipodal cells. During development, enlargement of the embryo sac leads to the destruction of most of the nucellus. This sequence of megasporogenesis and megagametogenesis, called the Polygonum type, occurs in 70 percent of the angiosperms in which the life cycle has been charted. Variations found in the remaining 30 percent represent derivations from the Polygonum type of seed development.
What type of reproduction do angiosperms have? ›
Reproduction in angiosperms. We'll look more closely at reproduction in angiosperms, which are unique among plants for three defining features: they have flowers, they have fruit-covered seeds, and they reproduce via a process called double fertilization.What are the two ways that angiosperms can reproduce? ›
- Self-pollination: Pollen falls on the stigma of the same flower. This way, a seed will be produced that can turn into a genetically identical plant.
- Cross-fertilization: Pollen from one flower travels to a stigma of a flower on another plant.
What are the steps in angiosperm reproduction? A tree produces flowers. The cells in the anther produce pollen grains. Inside the ovary, an egg cell is produced in each ovule.How do angiosperms reproduce asexually? ›
Citrus trees and many other species of angiosperms use their seeds as a method of asexual reproduction; a process called apomixis. In one form, the egg is formed with 2n chromosomes and develops without ever being fertilized.What is the reproduction and life cycle of angiosperms? ›
The angiosperm life cycle consists of a sporophyte phase and a gametophyte phase. The cells of a sporophyte body have a full complement of chromosomes (i.e., the cells are diploid, or 2n); the sporophyte is the typical plant body that we see when we look at an angiosperm.What is unique about angiosperm reproduction? ›
Double fertilization is an event unique to angiosperms. One sperm in the pollen fertilizes the egg, forming a diploid zygote, while the other combines with the two polar nuclei, forming a triploid cell that develops into a food storage tissue called the endosperm.What are the three steps of fertilization in angiosperms? ›
- Step 1: Pollination. Pollination is the process of pollen being transferred from the anther to the stigma of the carpel. ...
- Step 2: Germination. Germination occurs after the pollen grain lands on the stigma. ...
- Step 3: Penetration of the Ovule.
Sexual reproduction consists of a set of events and can be divided into three stages: Pre-fertilization, Fertilization, and Post-fertilization.What are the 5 steps of flower reproduction? ›
The 5 main plant reproduction steps involved in the life cycle are: Seed, Germination of Seeds, Seedling growing to attain maturity, Flowering, Pollinating, Seedling and Seed Dispersion; 2.What are the 7 steps of plant reproduction? ›
The major stages of the flower life cycle are the seed, germination, growth, reproduction, pollination, and seed spreading stages. Plants are able to reproduce in two different ways - sexual reproduction and asexual reproducion.
What are the steps of the angiosperm life cycle? ›
The angiosperm life cycle consists of a sporophyte phase and a gametophyte phase. The cells of a sporophyte body have a full complement of chromosomes (i.e., the cells are diploid, or 2n); the sporophyte is the typical plant body that one sees when one looks at an angiosperm.Do all angiosperms reproduce by seeds? ›
Angiosperms and gymnosperms both utilize seeds as the primary means of reproduction, and both use pollen to facilitate fertilization. Gymnosperms and angiosperms have a life cycle that involves the alternation of generations, and both have a reduced gametophyte stage.Which of the following is the first step in the reproductive process of an angiosperm? ›
Answer and Explanation: The first step in flowering plant reproduction is pollination. The stamen produces pollen grains, which are then transported to the female pistil for fertilization.What are the 4 methods of reproduction? ›
- sexual reproduction.
Final answer: In human beings, the correct sequence of events during reproduction is gamete formation, fertilization, zygote, and embryo.What are the 4 steps of reproduction? ›
- Formation of gametes through meiosis.
- Transfer of male gamete into the female body.
- Fertilisation (fusion of male and female gametes)
- Post fertilisation changes (formation of offspring from a single-celled zygote).
- Pollination. Pollen is transferred from a stamen to a stigma. ...
- Growth of Pollen Tube. A pollen tube develops to carry the pollen downward. ...
- Fertilization. The male gamete (pollen) unites with a female gamete (ovule) to form a seed. ...
- Fruit Formation. ...
The organisms reproduce in two ways: Asexual Reproduction– In this process, only a single parent is involved and no gamete formation takes place. Sexual Reproduction– In this process, two parents are involved and gamete formation takes place. Meiosis is an important step in sexual reproduction.What are the 10 stages of plant life cycle? ›
The main stages in the life cycle of a plant are seed germination, seedling formation, growth, development and differentiation leading to a mature plant, pollination and fertilisation and the formation of fruit and seeds.What are the stages of development in angiosperms before fertilization? ›
Angiosperm seed development spans three distinct generations, plus a new entity: the parent sporophyte, the gametophyte, the new sporophyte, and the new innovation—namely, the endosperm.
What are the two 2 types of reproduction? ›
There are two main forms: sexual and asexual reproduction. In sexual reproduction, an organism combines the genetic information from each of its parents and is genetically unique. In asexual reproduction, one parent copies itself to form a genetically identical offspring.What are 2 two classes of angiosperms? ›
Angiosperm diversity is divided into two main groups, monocot and dicots, based primarily on the number of cotyledons they possess.How do plants reproduce 2 ways? ›
Plants reproduce through two modes of reproduction: Asexual mode of reproduction – New plants arise from vegetative parts. Sexual mode of reproduction – New parts arise from reproductive parts of the plant.What are 2 ways plants reproduce what is the difference? ›
Plants have two ways of reproduction; sexual and asexual. . Sexual reproduction produces offspring by the fusion of gametes. This results in genetically different offspring. Asexual reproduction produces new individuals without the fusion of gametes.What are the 2 types of reproduction in flowering plants? ›
In plants there are two modes of reproduction, asexual and sexual. There are several methods of asexual reproduction such as fragmentation, budding, spore formation and vegetative propagation. Sexual reproduction involves the fusion of male and female gametes.What is the difference between budding and fragmentation? ›
Question 2: What is the difference between fragmentation and budding? Answer 2: Fragmentation refers to a form of asexual reproduction where a parent organism breaks into more than fragments, each of which can form a new organism. In contrast, budding happens when the parent cell forms a bubble-like bud.What animal reproduces asexually? ›
Animals that reproduce asexually include planarians, many annelid worms including polychaetes and some oligochaetes, turbellarians and sea stars. Many fungi and plants reproduce asexually. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in liverworts.Are all angiosperms double fertilization? ›
Double fertilization is the characteristic of angiosperms. In this process, one male gamete fuses with the egg to form a zygote, which is known as syngamy.Do angiosperms do double fertilization? ›
In angiosperms, double fertilization generates the embryo and the endosperm simultaneously, the joint development of which leads to a viable seed. Somatic embryos, however, are formed from plant cells that are not normally involved in the development of embryos (fertilization or gamete fusion).Do angiosperms reproduce with spores? ›
In angiosperms, meiosis in the sporophyte generation produces two kinds of spores: (1) microspores which develop in the microsporangium and will germinate and develop into the male gametophyte generation and (2) megaspores that develop in the megasporangium will develop into the female gametophyte generation.
What plants reproduce asexually? ›
Plants such as ferns, liverworts, and mosses can reproduce asexually via gemmae—which are small disks of green tissue that grow inside special cups. When mature, gemmae break off and scatter away from the parent plant. This separation is often due to the action of water drops or wind.How do plants reproduce without seeds? ›
Not every plant grows from a seed. Some plants, like ferns and mosses, grow from spores. Other plants use asexual vegetative reproduction and grow new plants from rhizomes or tubers. We can also use techniques like grafting or take cuttings to make new plants.How do plants reproduce step by step? ›
Pollen is carried from a male part to a female part by wind, insects or other animals (a process called pollination), where it releases male gametes that fertilise the female gametes in the ovules. The ovules develop into seeds from which new plants will grow.What is reproduction process in plants? ›
Once pollen gets transferred to the stigma the male gametes from pollen grains release and fuse with the egg in the ovule to form a zygote. This process of fusion of gametes is called fertilization. The zygote thus formed, divides and develops into an embryo, and later into a seed. The ovary develops into a fruit.What is the process of reproduction in flowering plants? ›
Pollen is carried by insects or blown by the wind from one flower to another. This process is called pollination. Pollen reaches the new flower and travels to the ovary where it fertilises egg cells (ovules) to make seeds. This is fertilisation.