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Art connection

 Illustration depicts the embryo sac of an angiosperm, which is egg-shaped. The narrow end, called the micropylar end, has an opening that allows pollen to enter. The other end is called the chalazal end. Three cells called antipodals are at the chalazal end. The egg cell and two other cells called synergids are at the micropylar end. Two polar nuclei are inside the central cell in the middle of the embryo sac.
As shown in this diagram of the embryo sac in angiosperms, the ovule is covered by integuments and has an opening called a micropyle. Inside the embryo sac are three antipodal cells, two synergids, a central cell, and the egg cell.

An embryo sac is missing the synergids. What specific impact would you expect this to have on fertilization?

  1. The pollen tube will be unable to form.
  2. The pollen tube will form but will not be guided toward the egg.
  3. Fertilization will not occur because the synergid is the egg.
  4. Fertilization will occur but the embryo will not be able to grow.

Sexual reproduction in gymnosperms

As with angiosperms, the lifecycle of a gymnosperm is also characterized by alternation of generations. In conifers such as pines, the green leafy part of the plant is the sporophyte, and the cones contain the male and female gametophytes ( [link] ). The female cones are larger than the male cones and are positioned towards the top of the tree; the small, male cones are located in the lower region of the tree. Because the pollen is shed and blown by the wind, this arrangement makes it difficult for a gymnosperm to self-pollinate.

 The conifer life cycle begins with a mature tree, which is called a sporophyte and is diploid (2n). The tree produces male cones in the lower branches, and female cones in the upper branches. The male cones produce pollen grains that contain two generative (sperm) nuclei and a tube nucleus. When the pollen lands on a female scale, a pollen tube grows toward the female gametophyte, which consists of an ovule containing the megaspore. Upon fertilization, a diploid zygote forms. The resulting seeds are dispersed, and grow into a mature tree, ending the cycle. Both the male and female cone are made up of rows of scales, but the male the female cone is round and wide, and the male cone is long and thin with thinner scales.
This image shows the life cycle of a conifer. Pollen from male cones blows up into upper branches, where it fertilizes female cones. Examples are shown of female and male cones. (credit “female”: modification of work by “Geographer”/Wikimedia Commons; credit “male”: modification of work by Roger Griffith)

Male gametophyte

A male cone has a central axis on which bracts, a type of modified leaf, are attached. The bracts are known as microsporophylls ( [link] ) and are the sites where microspores will develop. The microspores develop inside the microsporangium. Within the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores. Further mitosis of the microspore produces two nuclei: the generative nucleus, and the tube nucleus. Upon maturity, the male gametophyte (pollen) is released from the male cones and is carried by the wind to land on the female cone.

Watch this video to see a cedar releasing its pollen in the wind.

Female gametophyte

The female cone also has a central axis on which bracts known as megasporophylls ( [link] ) are present. In the female cone, megaspore mother cells are present in the megasporangium. The megaspore mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure. The female gametophyte is contained within a structure called the archegonium.

 Part a shows a cross section of a male cone, which is oval with a flattened bottom. A stem-like structure runs up the middle, and oblong microsporophylls radiate from either side. Migrograph b shows a blowup of a microsphorphyll, which is filled with round pollen grains. Micrograph C shows a blowup of a pollen grain, which is oval with two lobes and resembles Mickey Mouse. Part D shows a cross section of a female cone, which is similar in shape to the male cone but with a wider central structure. Megasporophylls radiate from either side of this structure. At the base the megasprophylls are narrow, and the widen out into a roughly diamond shape. Part E shows a blowup of the megasprophyll, which has an oval ovule at its base. Part F shows a blowup of the ovule. The megaspore mother cell is at the center. An opening called a micropyle allows entry of the pollen tube from the base.
These series of micrographs shows male and female gymnosperm gametophytes. (a) This male cone, shown in cross section, has approximately 20 microsporophylls, each of which produces hundreds of male gametophytes (pollen grains). (b) Pollen grains are visible in this single microsporophyll. (c) This micrograph shows an individual pollen grain. (d) This cross section of a female cone shows portions of about 15 megasporophylls. (e) The ovule can be seen in this single megasporophyll. (f) Within this single ovule are the megaspore mother cell (MMC), micropyle, and a pollen grain. (credit: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Questions & Answers

what is the mean of pair of chromosomes
Kazula Reply
how are you studying in this quarantine? .. how are you keeping yourselves motivated?
what is the important of sex
Aremu Reply
why did human being need sex?
Aremu
because he/she have feelings
Chripine
reproduction...to make more
Yazi
due to active harmon
Manish
One important of sex is to reproduce
Emma
to ensure the countinuty of life
Yusuf
all of you are right
Edith
what is momentum
Asiya Reply
The strength or force that allows something to continue or grow stronger or faster as time pass
Emma
What is Centripetal Force?
Justin
centrepital force is the inward force required to keep a body moving with constant speed in a circular path
Yusuf
what is the test for protein
Takii Reply
List four condition necessary for seed germination
Tedeka Reply
water,air(oxygen),light,temperature
Hassan
water, light, oxygen and temperature
lilchris
importance of biology
Alabina Reply
importance of boilogy
Alabina
what is soil
Amina Reply
soil is the upper part of the earth
Alabina
what is importance of studying biology
Alabina
soil is the uppermost layer of the earth on which plant grows
Yusuf
soil is defined as the thin surface of the upper most layer of the earth crust on which plants grow
Thanni
soil is the upper part of the earth which plants grow on
Emma
differences between euglenoid and amoeboid
Grace Reply
what are the difference between aerobic and anaerobic respiration?
Maxwell Reply
Aerobic respiration involves the use of oxygen whiles anaerobic respiration does not involve the use of oxygen
Quabena
what is assmilation
Lucy Reply
what is cell
Manish Reply
cell is the structural and functional unit of life or living things
hamid
where anaerobic respiration occurre?
Manish
in cell
Manish
in cells?
Manish
where anaerobic respiration occurre in cell?
Manish
what's phloem?
Gift Reply
what is different between Latin name and common names
mary Reply
hey
Gift
what's enzyme
Gift
what structure help root hair cells to take up water.
Jackson Reply
which two part of a plant cell are affected when the is immersed in sucrose solution?
Jackson
which two part of a plant cell are affected when the is immersed in sucrose solution?
Jackson
the xylem cells absorbs water and mineral salt from the soil to all part of the plant.
Nana
And the phloem cell take up food prepared from the leaves to all part of the plant
Nana

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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