Angiosperms are the most progressive plant species inhabiting the land. Reproduction in flowering plants starts with the division of fertilized egg to form an embryo. The plants follow a haplodiplontic pattern that alternates between gametophytic and sporophytic phases. Thus it is important to have clear knowledge between the two generations. Gametogenesis occurs in gametophytes followed by a union of gametes to form a diploid zygote. The fertilization results in the formation of diploid sporophytes, producing haploid spores through meiosis. The spores so formed undergo mitotic division to yield multicellular haploid gametophyte. So in Plantae, a single genome creates two distinct morphologies. The most basic haplodiplontic life cycle is shown below in the diagram.
It is the nonsexual phase containing two sets of chromosomes containing spore-producing organs.
The organography of sporophytes shows complex organization. In angiosperms, their body is differentiated into roots, stems, and leaves. While in bryophytes sporophytes show simple organization consisting of a single unbranched seta that bears a single sporangium.
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In vascular plants the sporophyte phase is dominant over the gametophyte phase. The gametophyte is minimized and is represented by a germinated pollen and embryo sac.
Sporocytes produce spores by the process of sporogenesis. Microsporogenesis gives rise to male gametophytes while megasporogenesis produces female gametophytes.
In bryophytes sporophyte is dependent on the gametophyte for nutrition hence is non-photosynthetic and parasitic. On the other hand in seed-bearing plants like gymnosperm and angiosperm sporophytes survive independently.
A sporophyte contains spore-producing organs called anthers and ovaries. The process of formation of spores is called sporogenesis and is divided into microsporogenesis and megasporogenesis.
Microsporogenesis is the formation of microspores or pollen grains. The anther is the fertile part of the stamen. A typical anther showing important features is shown below in the diagram.
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Megasporogenesis is the formation of megaspores within the ovule. The typical ovule is shown below in the diagram.
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Inside the nucellus, a single hypodermal cell enlarges and gets modified to form a primary archesporial cell. This cell divides perclinally to form primary parietal cells and primary sporogenous cells. The primary sporogenous cell functions as a megaspore mother cell.
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Fertilization occurs when pollen grain is transferred from mature anther to stigma. This results in the germination of pollen grain on stigma to produce two male gametes. The pollen pistil interaction results in the formation of a pollen tube within the style. The pollen tube carries two male nuclei which enter the embryo sac for fertilization. Two events occur in the embryo sac.
Syngamy − first male gamete fuses with the ovum to produce a zygote(2n)
Triple fusion − second male gamete fuses with a central cell to form endosperm(3n)
The double fertilization thus leads to the formation of a diploid sporophyte.
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The haploid multicellular phase which produces gamete is gametophytic generation.
The body organization of gametophytes in bryophytes bears structure known as antheridia producing male gamete and archegonia producing female gamete. These structures together are called Gametangia. In angiosperm gametophytes are referred to as pollen grain and embryo sac.
Gametophyte is the dominant phase in bryophytes while in seed-bearing vascular plants the gametophyte is significantly reduced to few cells.
Gametophytes are haploid multicellular plants, they undergo mitosis to form male and female gametes in both bryophytes and angiosperm plants.
In bryophytes gametophytes are photosynthetic and live independently in moist regions. Their body is attached to the substratum through roots like rhizoids. In angiosperms, they are dependent on sporophytes.
The male gametophyte − The figure below shows the formation of two-celled gametophytes from microspores.
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The female gametophyte − The monosporic embryo sac undergoes three successive mitotic divisions and gives rise to 7 celled 8 nucleate embryo sac. The complete flow chart below shows the sequence of events that form female gametophytes.
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It is a unique life cycle occurring in plants and algae. Plants alternate between the multicellular diploid phase and the multicellular haploid phase. To have a clear vision let us understand by taking the example of a fern. The green leafy fern represents a diploid sporophytic generation. The fronds or leaves from sori develop into spores by meiotic division. These spores get shed away and germinate to form haploid male and female gametophytes. The male gametophyte has antheridium while the female gametophyte is composed of the archegonium. The antheridia and archegonia undergo mitotic division to produce sperm and egg respectively. The sperm is carried in water droplets to the egg where fertilization occurs. This leads to the formation of diploid sporophytes.
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| Sporophyte | Gametophyte |
|---|---|
| Diploid phase | Haploid phase |
| Union of male and female gamete results in the formation of diploid sporophyte | Gametophyte produces male and female gametes. |
| Dependent phase in bryophytes and is non-photosynthetic | Independent and dominant phase in bryophytes and are photosynthetic |
| It is the dominant photosynthetic phase in seed-bearing plants like gymnosperm and angiosperm | In seed-bearing plants it comprises few cells; pollen grain and embryo sac. Thus dependent on sporophyte. |
| asexual phase | sexual phase |
| Produces microspores and megaspores | Produces male and female gametes |
| This phase is the outcome of fertilization of male and female gametes | This phase is generated from the germination of spores. |
In this tutorial, we have studied the alternation of generation which is a unique feature of plants and algae. The two phases are the sporophytic phase which is diploid and the dominant phase in vascular plants while the gametophytic phase is haploid and remains dominant in bryophytes and pteridophytes.
Q1. What do we mean by double fertilization?
Ans. In flowering plants, one male nucleus fuses with an egg to produce a zygote. This event is called syngamy. The other male nuclei fuse with the central cell to form a triploid endosperm referred to as triple fusion. Hence, syngamy and triple fusion together are called double fertilization.
Q2. Does alternation of generation exist in animals?
Ans. In animals, multicellular diploid organisms produce male and female gametes. The sperm from the male parent fuses with ova in females to form a diploid organism. No spores are produced in animals therefore multicellular haploid organisms do not exist. Thus animals do not show alternation in a generation.
Q3. Why do Pollen grains survive in extreme climatic conditions?
Ans. Pollen grain has distinctive pollen wall composition. The pollen wall is encapsulated by sporopollenin which consists of oxidative polymers of carotenoids and carotenoid esters. This makes sporopollenin resistant to various chemicals and extreme climatic conditions.
Q4. What is the role of synergids and antipodal cells present in the embryo sac?
Ans. Synergids guide the pollen tube growth by secreting chemotropically active substances. Antipodal cells are known to provide positional information on the embryo sac.
Q5. Why do bryophytes not grow tall?
Ans. Bryophytes lack the xylem and phloem responsible for the transport of water and nutrients. These plants remain attached to the substratum with help of rhizoids.
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