Objectives: By the end of this laboratory you should:

1. Know the advanced features of the Angiosperms that make them the dominant plant group.
2. Understand the functions of the sporophyte and gametophytes in the life cycle.
3. Recognize the male gametophyte (pollen grain) and female gametophyte (embryo sac).
4. Understand how ovule develops and becomes the seed, and the ovary becomes the fruit.

Angiosperms, literally "vessel seeds", are herbaceous or woody vascular plants producing flowers and fruits. There is one phylum or division, Anthophyta, with two major classes- the Monocots and Dicots.

Advanced features are :

Flowers- wind or animal pollination allows a tremendous variety of   adaptations to ensure accurate, efficient pollination.

Fruits- the mature ovary of the flower encloses the seed(s), offers superior protection and dispersal for the new sporophyte generation.

Double Fertilization and the Endosperm- triploid (3n) or higher (or 5n, 7n), tissue provides for rapid production of nutrients and growth hormones for the developing embryo.

Herb Growth Form- fast life cycle takes advantage of temporary habitats.

Vascular Tissues - with more efficient conducting cells, i.e. vessel elements in the xylem, sieve tube elements in phloem, and fibers in both xylem and phloem.

Activity 1: Pollen Grain

As in the Gymnosperms, the immature male gametophyte is the pollen grain. Obtain a prepared slide of a mature Lily anther in cross section. The anther consists of 4 microsporangia or pollen sacs. Inside, meiosis has taken place to produce dozens of haploid microspores, which have developed to become the pollen grains. Note the wall of the pollen grains. The outer wall contains chemicals which interact with the stigma of a flower and determine whether or not the pollen will be accepted (allowed to grow a pollen tube). It also has proteins which can generate an allergic reaction in our nasal passages. The pollen grains of each plant species are unique in size, shape, and wall pattern, and can be used to identify the habitat or past climate conditions from thousands of years ago.

Can you see nuclei inside each pollen grain?

How many?

Each pollen grain or immature male gametophyte consists of one cell with two haploid nuclei, a tube nucleus and a generative nucleus.

What is the function of the tube nucleus?

What is the function of the generative nucleus?

Draw your specimens below, and label the microsporangia, immature male gametophyte (pollen grain), and mature male gametophyte (germinated pollen grain) on figure 1 (last page of handout).

Activity 2: Pollen Tube Growth

Make a slide of live pollen by dabbing the anther of an Impatiens (or other) flower on a dry slide, and add a drop or two of sucrose solution. Cover, and observe every 15 minutes. Impatiens pollen looks like tiny gray rectangles; pollen tubes emerge from one or more corners after 20 to 60 minutes.

Activity 3: Ovule and Female Gametophyte

Obtain a prepared slide of a Lily ovary in cross section. Look for a megasporangium with a micropyle. Your slide may show the stage just prior to or just after meiosis, which results in formation of the haploid megaspore. The megaspore will divide several times to produce the female gametophyte, called the embryo sac. Use the circles below to draw your specimen. You may wish to use one of the circles for the slide in the next activity.

Label the stigma, ovary, ovule (megasporangium) and megaspore mother cell in figure 1.

Look at the demonstration slide of a mature female gametophyte, called the embryo sac. In Angiosperms, the female gametophyte is so reduced , it does not even form an archegonium. Why is an archegonium unnecessary?

 The embryo sac is an oval mass with seven cells and eight nuclei, and it lies in the center of the ovule. There are three antipodal cells at the far end, 2 polar nuclei in the middle, and the egg cell and 2 synergids at the micropyle end. Look at figure 1 to see the arrangement.

How many of the eight nuclei can you see?

 Locate the micropyle end of the ovule. Do you see any evidence of a pollen tube?

 Double fertilization occurs when the pollen tube, containing two sperm nuclei, enters the ovule. One sperm nucleus fertilizes the egg nucleus, to form the diploid zygote which becomes the embryo sporophyte. The other sperm nucleus fuses with the two polar nuclei to form a triploid nucleus. This triploid nucleus divides many times to form a triploid tissue called the endosperm. The endosperm, because it has extra copies of the genes, provides for the rapid production of growth hormones and nutrients used by the developing embryo.

Obtain a prepared slide of a dicot seed in longitudinal section. Unlike the Gymnosperm seed, there is no female gametophyte tissue remaining. The triploid endosperm tissue also has been "used up", converted to food now stored in the cotyledons. However, some species, particularly monocots, produce seeds in which some endosperm remains. Can you think of any?

 You should be able to identify the embryo, and the seed coat (which formed from the integuments of the megasporangium).

Questions

1. The mature female gametophyte of the Angiosperm is called the ___________________________, and consists of __________ nuclei.

2. What does double fertilization result in? 

3. How many nuclei are present in the pollen grain? Are they haploid or diploid? 

4. What does the endosperm tissue provide for the developing embryo? Is endosperm haploid, diploid, triploid?

5. What are the 2 functions of the fruit?