Gymnosperm Lab

Materials

1. Fresh pine branch with cluster of pollen cones (demonstration)

2. Fresh pine, fir, and other conifer branches for examination of leaves

3. Pine seed cones with seeds on the scales 4. Conifer pollen

5. Prepared slides of longitudinal sections through a pine ovule

6. Demonstrations of specimens of cycads, Gnetum, Ginkgo, Ephedra, and Welwitschia, if available

Some Suggested Learning Goals

1. Understand the difference between the two types of leaves produced by pines, and how pine leaves differ from those of other conifers.

2. Know the life cycle of a pine tree, and be able to indicate within the life cycle where events such as meiosis, fusion of gametes, development of an embryo, and production of sperms take place.

3. Understand the differences between male (pollen) and female (seed) pine cones.

4. Know the locations and functions of a pine micropyle, integument, pollen chamber, and nucellus.

5. Know the function of the bladders or wings on pine pollen grains.

6. Be able to distinguish a pine, a cycad, Ginkgo, Gnetum, Ephedra, and Welwitschia from one another (if they are available for examination).

Introduction

Although there are some exceptions among the algae and certain relatives of ferns, most of the plants studied thus far each produce just one kind of asexual spore. Beginning with the gymnosperms, however, the higher plants produce two distinct kinds, each within its own distinctive structure. Also, in Kingdoms Archaea, Bacteria, and Protista, and the bryophytes' of Kingdom Plantae, the primary means of dispersal is a spore. In gymnosperms and angiosperms (flowerng plants), however, dispersal is primarily by means of seeds, which are considerably more complex and larger than spores. The word gymnosperm comes from two Greek words that mean "naked-seeded," in reference to the fact that gymnosperm seeds are produced out in the open on cone scales, while the seeds of flowering plants are produced completely enclosed within fruits.

In contrast to the branching patterns of most broadleaf trees, the growth of most conifers is excurrent (i.e., the trunk of the tree does not divide unless the terminal bud is removed). Also, with the exception of Ginkgo, larch (Larix), and dawn redwood (Metasequoia), most gymnosperms have evergreen leaves. Unlike deciduous trees that seasonally lose all their leaves, most conifers lose a few leaves at a time. Nearly all conifers do have a complete change of leaves every two to five years. Bristlecone pines are an exception; they retain their leaves for about 30 years.

A. Conifers-Phylum Pinophyta

Carefully examine the youngest part of a stem of a pine branch. Note that two kinds of leaves are present. The most conspicuous leaves are needlelike and in fascicles (clusters) of 2, 3, or 5. With a sharp razor blade, cut one of the leaves and examine the cut surface with your dissecting microscope. How many vascular bundles (veins) can you see? If you cut all the leaves in one fascicle and hold the cut remnants tightly together, do they form a complete cylinder? Are there small, inconspicuous, brownish scale leaves also present on the stem? How do pine leaves differ from those of the other conifer leaves provided? Are any of the other conifer leaves scalelike? Are the other conifer leaves arranged opposite one another or in a spiral on the stem?

Examine an open woody seed cone of a pine tree. Notice the paired, winged seeds at the base of each cone scale. The seeds in your particular cone may be missing; if so, the slight depressions in which the seeds were produced should be discernible. The seeds develop from ovules, in which a megasporocyte has undergone meiosis, producing megaspores. One megaspore develops into a female gametophyte that contains two or more archegonia, each with a large egg cell and a little nutritive tissue; the nucellus, above the archegonia; and other gametophyte tissue surrounding and below the archegonia. The female gametophyte is itself surrounded by a massive integument that later develops into the seed coat of a seed. Above the nucellus is a space called the pollen chamber. A somewhat tubular micropyle is located in the integument directly above the pollen chamber. A sticky fluid oozes through the micropyle to the outside, where it forms a pollination drop. A pollen grain may catch in the pollination drop and be slowly drawn into the pollen chamber as the fluid evaporates. Note that unless your slide happens to be a specially selected median section, parts of one or both archegonia and/or the micropyle may have been cut off during manufacture. Once a pollen grain

163 comes to rest above the archegonia, it may produce a pollen tube and two sperms. By the process of fertilization, one sperm unites with the egg, forming a zygote, which then develops into the embryo of a seed.

Pollen grains are developed from microspores produced when diploid microsporocytes in the microsporangia (sacs) at the base of the pollen cone scales undergo meiosis. Pollen cones are much smaller than their woody seed cone counterparts; they are usually produced in clusters at the tips of the lower branches of a tree. Mount a small amount of conifer pollen in a drop of water on a slide, and examine it with the aid of your compound microscope. Note the wings or bladders on each pollen grain. These give the pollen greater buoyancy in the wind.

When a seed germinates, the embryo within it develops into a new tree that constitutes the sporophyte.

B. Other Gymnosperms (Phyla Ginkgophyta, Cycadophyta, and Gnetophyta)

Examine the other representative gymnosperms on display. Note that Ginkgo has distinctive fan-shaped leaves with dichotomously forking veins. Ginkgo trees are dioecious; the male strobili (pollen cones) are produced only on male trees. The female trees, whose seeds have a fleshy covering with a nauseating odor, are produced singly instead of in cones. Unlike the sperms of pines, those of Ginkgo have flagella.

Cycads are slow-growing plants that have a trunk and large, palmlike leaves. Like Ginkgo, the species are dioecious; the sperms have thousands of spirally arranged flagella. Both the pollen and the seed cones of cycads can be very large, with some seed cones being more than a meter long and weighing over 200 kilograms at maturity.

There are about 70 known species of gnetophytes distributed among three genera. Gnetum species have broad leaves and are mostly vines and trees of the tropics. Half the species of gnetophytes are in the genus Ephedra, native to drier regions of southwestern North America. Most photosynthesis takes place in the stems of the shrubby Ephedra plants, which have tiny scalelike leaves. There is only one species of Welwitschia, a bizarre-looking plant confined to temperate desert regions of southwest Africa. Welwitschia produces two large straplike leaves from a somewhat cupshaped "trunk." The leaves, which grow continuously from the base, flap in the wind and become split. Most of a Welwitschia plant's water supply comes from condensate of fog that rolls in from the ocean at night.

Drawings to Be Submitted

1. Draw a small branch of a pine, showing the needlelike leaves. Also draw the cone(s) present. Label FASCICLE, SCALE LEAVES, SEED CONE, and POLLEN CONE.

2. Draw a short part of a branchlet of one of the other conifers provided. Show not only the shapes of the leaves but how they are arranged on the stem.

3. Diagram a section through a pine OVULE with the aid of the prepared slide provided. Label ARCHEGONIUM, NUCELLUS, POLLEN CHAMBER, MICROPYLE, and INTEGUMENT.

4. Label the following on the drawings of the life cycle of a pine tree provided: SPOROPHYTES, CLUSTER OF POLLEN CONES, MICROSPORANGIUM, MICROSPOROCYTES, MICROSPORES, POLLEN GRAINS, SEED CONE, CONE SCALE, MEGASPOROCYTE, MEGASPORE, DEVELOPING FEMALE GAMETOPHYTE, ARCHEGONIA, INTEGUMENT, NUCELLUS, POLLEN CHAMBER, POLLEN TUBE(S), SEED, EMBRYO SPOROPHYTE, and SEEDLING SPOROPHYTE.

5. Draw a pine POLLEN GRAIN, showing the WINGS or BLADDERS.

6. Draw a habit sketch of any one of the other gymnosperms available.

Questions 1. What does the term gymnosperm mean, and in what sense does it apply to pine trees?

2. Apart from size differences, how can you distinguish a cone scale of a pine seed cone from that of a pine pollen cone?

3. Of what does the female gametophyte of a pine consist? What other tissues surround it?_______________

4. What is the function of a nucellus?

5. What constitutes the sporophyte in a pine?_________________________________________________

6. Where, specifically, are pine pollen grains produced?_________________________________________

7. What structure of a pine ovule develops into a seed coat?______________________________________

8. What do the pollen grains of pine trees have that aid in their dispersal by the wind?__________________

9. Could all the representatives of gymnosperms mentioned in this exercise be differentiated by their leaves alone? If not, why not and if so why?

1. What is the difference between a gymnosperm and an angiosperm?

2. How many seeds are produced at the base of each pine seed cone scale?

3. What specific cells of pine undergo meiosis?

What do these cells then become?

4. Through what passage is a pollen grain of pine drawn prior to its full development into a mature male gametophyte?

5. What is the space above the nucellus in a pine ovule called?

6. From what specific cell does the embryo of a seed develop?

7. Where on a pine tree are pollen cones usually produced.

8. From what structure does the seed coat of a pine seed develop?

9. Specifically, where are pine sperms produced?

10. Which of the gymnosperms discussed has two straplike leaves?

pine cone