Angiosperm Dissection Lab: Flower

 

Introduction

 

The purpose of the lab is to examine and identify various parts of an angiosperm. A flower is the reproductive portion of any flowering plant (angiosperm). Popularly, the term applies especially when part or all of the reproductive structure is distinctive in color and form. Flowers present a multitude of combinations of color, size, form, and anatomical arrangement. In some plants, individual flowers are very small and are born in a distinctive cluster (inflorescence). Each flower consists of a floral axis that bears the essential organs of reproduction (stamens and pistils) and usually accessory organs (sepals and petals); the latter may serve both to attract pollinating insects and to protect the essential organs. Flower parts are arrayed usually in whorls, but sometimes spirally. Four distinct whorls are common: the outer calyx (sepals), the corolla (petals), the androecium (stamens), and, in the center, the gynoecium (pistils). The sepals are usually greenish and often resemble reduced leaves; the petals are usually colorful and showy. Pollen is produced in the stamens. A pollen-receptive stigma rests atop each pistil. The pistil, made up of one or more carpels, encloses an ovary that contains the ovules, or potential seeds. After fertilization, the ovary enlarges to form the fruit. Hypothesis: If one were to examine and identify various structures of a flower, then a better understanding of angiosperms will be achieved.

 

 

Method

Apparatii  

 

Complete Flower (a “perfect” complete dicot flower); ie Petunia (best, Lilly, Snapdragon or Tulip)

   Optional: Supplemental Flower (for stigma, pollen, and ovule comparison) 

Compound Microscope  

Blank Slide  

Forceps

Magnifying Glass

Needle Probe

Razor Scalpel

White 8 ½ x 11 Paper

References

 

 

 

Procedure  

 

Note: Place All Answers to the Any Questions on a Separate Sheet of Paper.  

 

In this lab, you will examine the intricate structures that compose a flower. Many angiosperms have, in a single flower, both the male and the female sex organs surrounded by petals. The egg (female haploid cell) and the pollen (containing one or more haploid sperm nuclei) are contained in the same flower. Angiosperms may self-fertilize if pollen from a flower is transferred to egg cells in the same flower, or they may cross-fertilize. Carried on the wind or by other means (ie water or animals), pollen grains from other flowers may land on the sexual organs of a flower and fertilize it.

 

1.      What other ways can cross-fertilization of flowers happen (besides by the wind)?

 

The various parts of the flower help with the transfer of the pollen to the egg.

 

A.    Obtain a flower and examine it, using the diagram in Figure 1 as a reference. Look for the sepals of your flower. Note: If you are looking at a lily or tulip (see pictures), the sepals and the petals look the same; they are called “tepals”. The sepals are typically on the outside of the flower, often green, sometimes small and withered, sometimes as large as the petals. The sepals protect the bud before it opens and provides some photosynthesis

 

B.     The petals compose the next "ring" of flower structures. You can think of petals as modified leaves. Examine the texture and color of the petals using a magnifying glass. If your flower is colored, pinch a small piece of a petal between your fingers and examine how the color might rub off thus releasing the colorful pigment.

 

2.      What is a function of the sepals according to the reading?

 

3.      Why do you suppose the petals of flowers are so colorful, fragrant, uniquely shaped?

 

The structures inside of a flower produce the gametes, or ovules and pollen. The male reproductive structures of the flower, called stamens, may be T-shaped, straight or gently curved. They consist of an anther (yellowish/whitish) supported by filament. The stamens each have an anther at the top of the filament shaft. Pollen grains are released from the anther. Each stamen will produce hundreds of pollen grains. Contained inside of each pollen grain there are two (2) sperm nuclei.

C.     Carefully pull back the petal(s) of the flower to expose the stamens; if you are looking at a snapdragon they are called, “lips”. Don’t damage the stamens or pistil in any way.

4.      How many stamens do you see in your flower?

 

D.    Remove a stamen using with forceps and along its filament.  Gently tap the anther onto a slide.  Without using a cover slip, examine the pollen grains “young male gametophytes” with a microscope at the highest possible magnification.

 

5.      Describe the pollen grains: color, shape, external features, etc…

 

Making up the innermost ring of structures is one or more pistils. A pistil is a floral structure enclosing an egg in angiosperms, typically divided into stigma, style, and ovary. A flower may have one or more pistils, either single or fused. (A single pistil or a group of fused pistils is also known as a carpel.)

 

E.     To see the pistil clearly, gently separate the flower and/or petals from the green sepals and base. The stamens may stay with the flower petals and the pistil should remain attached to the base. This separation occurs naturally when a tree or plant sheds its flowers.

 

F.      Notice that the pistil has three parts: a sticky stigma at the top, a long shaft called a style, and an ovary at the bottom.

 

G.    Remove the stigma and part of the style and dab the stigma into some pollen grains.  View the stigma with pollen grains under the microscope at a lower power on the microscope.

 

6.      Why do you think the stigma is sticky? (hint: pollen)

 

H.    Carefully lay the pistil down onto your slide (may still have pollen which is alright). Use a razor blade to cut open the ovary “lengthwise”.  Observe the opened ovary a compound microscope (you may also want to try a stereoscope) and observe the haploid eggs or ovules.  The ovules will turn into seeds when fertilized by sperm nuclei traveling down a pollen tube from the stigma.

 

7.      What will the ovules eventually turn into if fertilized?

 

There are two classes of angiosperms: the monocots and the dicots. The seeds in monocots have only one cotyledon, while in dicots, the seeds contain two cotyledons. You can typically characterize a flower as monocot or dicot by looking for identifying physical characteristics. The floral parts in a monocot generally occur in multiples of three and the leaves have parallel veins. In dicots, floral parts are usually in multiples of four or five and leaves are net-veined or branchy.         

                                                                                                                                             

Once a pollen grain has become stuck onto a stigma, it begins to grow a tube through which the sperm nuclei travel down to the ovary. There, the haploid sperm nuclei from the pollen unite with the haploid egg cells to produce diploid zygotes. When the plant sheds its flower, the fertilized egg develops into a seed. The ovary wall surrounding the seed often develops into a fruit.

 

8.      Is your plant a monocot or a dicot?

 

9.      Once the ovules are fertilized; the “matured” ovary will develop into what, which surrounds the newly formed seed(s)?

 

In the flower (sporophyte) you examined, you were able to see both male and female structures. If a flower contains both male and female parts, botanists call them perfect flowers. Most flowers (roughly 85%) are perfect flowers and are able to self-fertilize. Imperfect flowers contain either male or female parts, but not both, and therefore, cannot self-fertilize.

 

10.  What type of flower do you have, a perfect or imperfect flower (choose)?

 

11.  By using a 8 ½ x 11 piece of plan paper, separate, lay out, and label each of the following structures: When completed, call to be “checked off”:   

 

 

 

 

 

 

 

 

 

Flowering Plant Vocabulary

 

angiosperms - flowering plants.

anther - in flowers, the end part of the male stamen that produces grains of pollen.

carpel - a structure that encloses an egg in angiosperms, composed of ovary, style, and stigma.

cotyledon - the endosperm in a seed that develops into a seedling.

cross-fertilization - the fertilization of a female egg by pollen which did not come from the same plant.

dicot - the larger of the two classes of angiosperms characterized by having two cotyledons, floral parts that occur in multiples of four or five, and net-veined leaves. Examples of dicots are maple trees and tomato plants.

egg - the female gamete (haploid sex cell derived by meiotic division).

endosperm - triploid (containing three sets of chromosomes) nourishing tissue in the seeds of angiosperms.

filament - the long shaft of a stamen atop which the anther is attached.

fruit - the ripe, mature ovary containing seeds.

gametes - specialized sex cells in plants, such as eggs and pollen. Gametes are haploid. A male gamete and a female gamete fuse and give rise to a diploid zygote, which develops into a new individual.

imperfect flowers - flowers containing either male or female parts, but not both; therefore, imperfect flowers are incapable of self-fertilization. The same plant may contain both male and female flowers, however. The corn flower is an example of an imperfect flower.

monocot - the smaller of the two classes of angiosperms characterized by having one cotyledon, floral parts which occur in multiples of three, and leaves that are parallel veined. Examples of monocots include lilies and trillium flowers.

ovary walls - the tough walls surrounding the egg in a developing fruit or vegetable.

perfect flowers - flowers containing both male and female parts. Examples of perfect flowers are tulips and hibiscus.

petals - specialized leaf structures that protect the sex organs of a flower and direct insects and birds to assist in fertilization.

pistil - a single carpel or a group of fused carpels; the female parts of a flower composed of ovary, stigma, and style.

pollen - the male gamete of a flowering plant which typically contains two sperm nuclei capable of fertilizing an egg.

receptacle - structure at the base of a flower which attaches the flower style to the stem. The receptacle will often break down in a fertilized, mature flower to aid in dispersal of seeds.

seed - the product of a fertilized egg of a seed plant, generally consisting of an embryo with its food reserves encased in a protective coat.

self-fertilization - the fertilization of a female egg by pollen which come from the same flower.

sepal - the green leaf-like structures that cover a bud and later open up to reveal the petals of a flower.

sperm nuclei - haploid male nuclei which are encased in a pollen grain.

stamen - the male reproductive structure in a flower; consists of anther and filament.

stigma - the sticky top of a carpel or a pistil that serves as a pollen receptacle.

style - a structure leading from the ovary of flowering plants to the stigma.

zygote - the diploid cell that results from the fusion of an egg and a pollen cell.