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Flower Dissection Lab

The angiosperms are seed-bearing plants that produce flowers.  The seeds, which contain the plant embryo, are produced in the flower.  All the parts of a flower are actually modified leaves that are specialized for their roles in the reproductive process.   Flower parts are attached at the enlarged base of the flower, the receptacle.  Flower structures can be divided into two groups:  the essential organs and the accessory organs.  The essential organs are the reproductive structures, which include the stamens (male) and the pistils (female).  The accessory organs are the sepals and petals, which surround and protect the essential organs.

The stamen is the male reproductive organ and consists of two parts: the anther and the filament.  The anther is the enlarged structure at the top of the stamen.  Inside the anther are pollen sacs.  Special cells within the pollen sacs undergo meiosis to form pollen grains.  Each pollen grain contains two sperm nuclei.  When the pollen grains mature, the pollen sacs split open to release the dust-like pollen.  The filament is a thin stalk that supports the anther.

The pistil is the female reproductive organ and consists of three parts: the stigma, style, and ovary.  The stigma is an enlarged portion at the top of the pistil that becomes moist and sticky when mature.  The style is the middle portion of the pistil.  It can be long and slender, short, or even absent, depending upon the species.  The ovary is the enlarged structure at the bottom of the pistil.   The ovary contains one or more hollow compartments called locules.  Each locule contains one or more ovules.  Special cells within the ovule undergo meiosis to form ova (eggs) containing egg nuclei.

Procedure:

  1. Obtain a single flower and observe its parts carefully.  Flower parts are arranged in a circular pattern.  Each circle is called a whorl.  The whorls are attached at the enlarge receptacle located at the base of the flower.

  1. Obtain a blank piece of white printer paper.  You will be taping portions of the flower to this paper.

Please read this overview before you begin your flower dissection:  

As you examine your flower, you will be carefully removing parts beginning with the outer whorl and working your way in towards the pistil.  You will arrange each whorl in a circle on the plain paper, beginning with the sepals as the largest outermost circle.  As you proceed with your dissection, you will  carefully tape each whorl of flower parts into position and label them (please  use pencil!).  As each whorl is observed and removed, you will answer the corresponding question.  

  1. The sepals form the outermost whorl of the flower.  The sepals are leaf-like structures that are usually green in color.  Sometimes, the sepals are the same color as the petals, or appear to be  another set of petals of a different color.  The function of the sepals is to protect the inner part of the flower before it blossoms.  Gently remove the sepals, tape them into position onto the paper, and label them.
  1. How many sepals does your flower have?

  1. Describe the appearance of the sepals (color, markings, etc.).

  1. The petals are found directly under the sepals.  The color and odor of the petals help to attract birds and insects to the flower for pollination.  Gently remove the petals, tape them into position onto the paper, and label them.
  1. How many petals does your flower have?

  1. Describe the appearance of the petals (color, markings, etc.).

  1. The stalk-like structures inside the petals are the stamens, the male reproductive organs.  Depending on the species, the stamens may be attached  to the receptacle, to the petals, or to the pistil.  The enlarged portion at the top of the stamen is the anther.  Inside the anther are pollen sacs, which produce pollen grains.  When the pollen grains mature, the pollen sacs split open, releasing the dust like pollen grains. The filament is the thin structure that supports the anther.  Gently remove the stamens, tape them into position onto the paper, and label them.
  1. How many stamens does your flower have?

  1. To which structure(s) are the filaments attached?

  1. Have the pollen sacs opened?  How can you tell?

  1. If pollen grains are visible, describe their appearance.

  1. The central structure of the flower is the female reproductive organ, the pistil.   The top of the pistil is the stigma.  Carefully remove the pistil by cutting it from the stem just under the ovary.  Using pencil, make a life-sized sketch of the entire pistil (just the outline) in the center of the plain paper and label it.  Cut the style just at the top of the ovary, tape it next to your sketch, and label the stigma and style.  Using the ruler, measure the length of the style in millimeters.  Cut a thin cross-wise section of the ovary and tape it under the stigma and style.  Label the ovary wall, locules and ovules.   Using the dissecting needle, carefully pick the ovules out of one of the locules.  Try to estimate how many ovules are contained in one locule.
  1. What color is the pistil?

  1. Describe the appearance of the stigma.  Is the stigma mature?  How can you tell?

  1. How long is the style (in mm)?

  1. Describe the appearance of the ovary.

  1. How many locules does the ovary contain?
  2. Approximately how many ovules are contained in one locule?

  1. Put you and your partner(s) name on the paper with labeled flower parts and staple it to this packet.

 

Analysis:

  1. Which does the flower produce in greater numbers based on your observations:  ovules or pollen grains? Explain why this would be important in terms of reproductive success.

  1. What are some adaptations of flower petals to help attract pollinators?

  1. How is the stigma of the flower adapted to capture and hold pollen?

  1. Describe where pollination and fertilization occur.

  1. Explain the differences between pollination and fertilization.

  1. In which part of the male reproductive organ are the pollen grains made?

  1. In which part of the female reproductive organ are the egg cells made?

  1. By which nuclear process are these gametes formed?

  1. How do the sperm nuclei in a pollen grain reach the egg nucleus in an ovule?

  1. Which part of the flower becomes the seed?

  1. Which part becomes the fruit?

  1. Which part of the fruit contains the embryo?