PBIO 111: Spring, 2003 Lab 8:

Fossil Plants, & Life Cycles of Vascular Plants

INTRODUCTION AND OBJECTIVES

This labaoratory will serve as an introduction to the fossil evidence used by paleobotanists to interpret the history and evolution of life, and also as an opportunity to briefly pause in our survey of plant life to reemphasize similarities and differences that are reflected in life cycles.

A. Fossil Plants of Ohio: Ohio's tropical forests grew from about 310 to 280 million years ago, during the Pennsylvanian

period. Most of these florished in wetland habitats when central North America was located on the equator. Three groups of homosporous and heterosporous plants (ferns, lycophytes & sphenophytes) grew among primitive seed plants called seed ferns, and conifers grew in dryer areas of the surrounding hills. Many species of all of these groups were large trees. These plants are featured on pages 433, 456-457 & 466 of your text book.

When plants die, they tend to fall apart, so that we don't find whole plants as fossils. Rather, fossils tend to consist of isolated leaves or leaf fragments, stems, roots, sporangia, seeds, pollen, etc. Plant parts may be fossilized and preserved in several different ways. When plant parts are flattened in rock layers, the resulting compression fossils show external morphology from the surface of the coalified (literally turned to coal) organic residue. When this residue is lost, the resulting impression fossils also faithfully preserve the outer form of the plant organ on the rock surface. Some plant parts become filled with sediment or surrounded by sediment that later solidifies to produce either a mold or a cast of the plant part. In contrast to compressions and impressions, which are flat, molds and casts show the external form of extinct plants in three dimensions. Still other plant parts become imbedded in a mineral matrix, and have their internal anatomy faithfully preserved.

In this laboratory we will look at a variety of Pennsylvanian age plant fossils from the tropical forests of Ohio, to get a glimpse of this lush and magnificent vegetation. Please be extremely careful with the fossils, they are easily damaged and can not be replaced!

Examine and Identify: fossil specimens that are on demonstration. You can use the Key for Identifying the Fossils if you wish. This key will be available in lab.

Can you determine which fossils represent ferns, which represent sphenopsids, which represent lycopods, and which represent conifers?
What characters of each allow you to make decisions about the systematic affinities of each?

Using data sheets, make lists of which fossils belong to each major group of vascular plants, and also indicate (1) what plant organ they represent and (2) by which mode they are preserved (compression, mold, etc.).

an you figure out which fossils may go together to produce whole plants?Make a coal ball peel: and examine the internal anatomy of the plant parts. Your laboratory instructor will show you how to make a coal ball peel. Be sure to bring a copy of the coal ball peel technique (print off of the web site) if you think you can't remember the steps. If you take care and are patient, you will have a fine fossil that shows lots of information about Ohio's tropical forests. (When you are finished with this laboratory, you may keep your peel if you wish.)

Make a coal ball peel as demonstrated by your instructors.
Examine the peel under a dissecting microscope. Put a white sheet of paper under your peel, and light it from the top. Look at the shiny side.

Can you identify any of the plant organs, any of the tissues, any of the cell types? These will become more familiar to you after we have studied the internal anatomy of living plants.
Can you relate any of the anatomy you see in the peel to the compression/impression and mold/cast fossils you have already studied?

Examine the piece of coal on demonstration. Do you know from what coal was formed and how it formed? When we burn coal, we are liberating energy from past life. Can you imagine how that energy was captured and stored?

B. Life Cycles of Vascular Plants (Division Tracheophyta): By now we are all aware that vascular plants have a sporic life cycle, and that there are several variations on the general theme of this life cycle. The homosporous life cycle is ancestral (primitive) among vascular plants. The heterosporous life cycle is a modification of the homosporous life cycle, and further modifications in the heterosporous life cycle lead to seed plants (including flowering plants). Therefore, the key to understanding structures and relationships of seed plants lies in understanding which of their features are equivalent to (or derived from) those of homosporous and heterosporous vascular plants. Throughout this laboratory ask yourself the following questions:

1. Is this haploid or diploid?
2. Is this sporophyte or gametophyte?
3. What and where are the gametophytes of seed plants?
4. What structures of seed plants represent sporangia and gametangia?
5. Where does fertilization take place?
6. Where does the embryo develop? In this exercise we are comparing and contrasting vascular plants with different types of life cycles, and asking ourselves what is the same about them? Establishing concepts of "sameness" allows us to determine how organisms are related to each other structurally, functionally and evolutionarily. Homology and Analogy are two concepts of "sameness" with radically different implications. We say that featurse or characters of two organisms are homologous if they are inherited from a common ancestor that had those features. This is true even if the features are somewhat modified in one or both of the living organisms. Homology implies common ancestry.

In contrast to homology, analogy is sameness due to separate evolution of a character that is displayed by two organisms. Unlike homologous characters, that help us establish evolutionary and systematic relationships, analogous characters shed no light on relationships among organisms.

We have already discussed all of the following plants in lecture, and you have already seen all of them in laboratory. Therefore, concentrate on the questions posed above, satisfy yourself you know the answers, and then move on to the next plant. If you understand life cycles of vascular plants, this exercise should not take a great deal of time. If you can't answer the above questions confidently, you need to carefully review life cycles of the vascular plants. Don't hesitate to ask you laboratory instructors and your lecturer for assistance. This understanding is crucial for your success in the remainder of the course!

I. Free Sporing Pteridophytes (lycophytes, psilotophytes, sphenophytes and pterophytes)

Exercise A - The Homosporous Life Cycle: The lycophyte Diphasiastrum, and the fern Polypodium. You will recall both the fern Polypodium and the lycophyte Diphasiastrum from the last laboratory. This time we are going to concentrate on their life cycles.

The lycophyte Diphasiastrum (as you proceed through the observations of this plant, use a data sheet to record your observations, and make drawings that you arrange as a life cycle)

Examine the sporophyte plant of Diphasiastrum and refresh your memory about stems, leaves, roots and cones. Can you find sporangia and spores in the cones?
Shake spores out of the cone onto a microscope slide. Add a drop of water and a cover slip, and observe under the compound microscope. Do the spores show the mark that demonstrates that they were produced by meiosis (a trilete mark in this plant)?
Now examine the life cycle of Lycopodium (a closely related genus) on p. 436-437 of your text book. Identify the parts of the plant in the life cycle that you have seen up until now. The remaining features of the life cycle (gametophyte, archegonia with egg, antheridia with sperm, zygote and embryo) are similar to those of ferns, and will be illustrated by the fern Polypodium.

The fern Polypodium (as you proceed through the observations of this plant, use a data sheet to record your observations, and make drawings that you arrange as a life cycle).

Examine a sporophyte plant of Polypodium and refresh your memory about its stems, leaves (fronds), and roots. Can you find sori that consist of groups of sporangia on the leaves?
Crush a sporangium on a microscope slide, add a drop of water and a cover slip, and view the preparation under the compound microscope. Can you find spores and fragments of the sporangium? Are the spores round like those of Diphasiastrum, or are they bean shaped? Do the spores show the mark that demonstrates they were produced by meiosis (a monolete mark in this plant)?
Now examine your fern minimarsh for young gametophytes. Do they look like those in the fern life cycle on p. 460-461 of your text book?
Examine the archegonia and antheridia of sexually mature gametophytes that are on demonstration, and draw one of each in the life cycle you are constructing on your data sheet.
Examine a fern minimarsh from last quarter that has sexually mature gametophytes and young sporophytes (sporlings). Draw a relatively large sporling that is still attached to the gametophyte to complete your life cycle.

Exercise B - The Heterosporous Life Cycle. The heterosporous life cycle is derived from the homosporous life cycle by modification of the gametophytes. In these plants, the gametophytes are reduced to tiny plants that remain within the spore wall through out their entire life span. Also, each gametophyte is unisexual (either male or female), and produces either sperm or eggs only. We will use the heterosporous lycophyte Selaginella to demonstrate this life cycle (see p. 440-441 in your text book).

The Lycophyte Selaginella(as you proceed through the observations of this plant, use a data sheet to record your observations, and make drawings that you arrange as a life cycle).

Examine the plants of Selaginella that are on demonstration in the laboratory. Identify stems, leaves, roots and cones.
Under the dissecting microscope, examine a cone of Selaginella that has been cut in longitudinal section to reveal sporophylls attached to the cone axis. Each sporophyll has one sporangium that produces eithermicrospores(microsporangia) or megaspores (megasporangia). Can you tell which is which? Draw the cone so as to show megasporangia and microsporangia with either megaspores or microspores inside.
Now examine the microgametophytes and megagametophytes within the spore walls that are illustrated in the text book (p.460-461). Identify the vegetative and fertile parts of the gametophytes. Draw one gametophyte of each type into your Selaginella life cycle. Be sure to label all of the important parts. Now examine and draw the megagametophyte in the life cycle that contains both large and small embryos. This will fill out your heterosporous life cycle.

Be sure you understand how the reproductive structures of Selaginella relate to those of the homosporous plants you studied earlier.II. Seed Plants (cycads, Ginkgo, conifers, gnetophytes and angiosperms). The life cycle of seed plants is derived from the heterosporous life cycle by further modification and reduction of the gametophytes, and by the megagametophyte remaining within the megasporangium for its entire life. Also, the gametophytes of seed plants are totally dependent on the sporophyte for both nutrition and support. This is roughly the opposite of bryophytes, where the sporophyte is the dependent phase of the life cycle. We will introduce the seed plant life cycle using the genus Pinus as an example.

Exercise C - Conifer Life Cycle (Pine, the genusPinus;as you proceed through the observations of this plant, use a data sheet to record your observations, and make drawings that are arranged as a life cycle).

Examine the small pine plant (sporophyte of the genus Pinus) on demonstration. Can you identify stems, leaves and roots on this plant?

Examine the pine branches that bear pollen cones and seed cones. These are illustrated on p. 477 of your text book. Note that the pollen cones have sporophylls that produce microsporangia. Are these microsporangia homologous to those of Selaginella?

Microgametophytes: The microgametophytes of seed plants develop entirely within the spore wall, just like those of Selaginella. Make a microscope slide of pine pollen by shaking a pollen cone over a drop of water on a microscope slide, and then adding a cover slip. Compare what you see under the compound microscope with the microgametophytes of pine that are included with the life cycle on p. 478-479 of your textbook.

Megagametophytes: Seed cones and seeds (or ovules; both terms are used) of pine are also available for examination. Compare what you see with the illustrations on p. 480 of your text book, and be sure that you can recognize the cone axis, the ovuliferous scales, and the seeds. There are lots of homologies to be considered with conifer seed cones, but in this course we will focus only on those of the seeds.

Examine the demonstration of the young seed cone, and compare what you see to the photograph on p. 480 and the figures of the ovuliferous scale at the top of p. 478-479. What is ovuliferous scale and what is seed? Be sure to draw and label diagrams of these.

Now you are ready to consider what a seed (or ovule) represents in relation to what we have seen in the heterosporous life cycle. The main structures to consider are the seed coat (or integument), the nucellus, and the megaspore mother cell. Of these, only the integument is new. The others are homologous to structures you saw in the homosporous and heterosporous life cycles. Find all of these in figure 20-24(b) on p. 480. Now find them on the demonstration slide of the young ovuliferous scale, and on the life cycle diagrams. As these structures develop, the megaspore mother celle undergoes meiosis to produce megaspores within the nucellus. Use your knowledge of plant life cycles to identify the homologies of the nucellus. What do we call the homologous structure in Selaginella?

In seed plants, only one of the megaspores within each ovule will be functional, and develop into a megagametophyte that has archegonia, each with an egg.

After the ovule is pollinated, the egg will be fertilized by the sperm to form the zygote. The zygote will develop into an embryo, and a seedling will eventually emerge from the seed coat to grow into a new pine tree. As you complete your pine life cycle, be sure that you can answer all of the questions at the beginning of this laboratory for the genus Pinus.

Exercise D - Angiosperm Life Cycle (as you proceed through the observations of this plant, use a data sheet to record your observations, and make drawings that you arrange as a life cycle).

As we all know, the fertile parts of flowering plants are contained within the flowers (is this a strobilus?). In this lab, we will use flowers of the monocot Yucca to interpret the homologies of these fertile parts. Yucca flowers are quite similar to the lily flowers on p. 499 of your text book.

Together with your partner, select a Yucca flower for dissection under the dissecting microscope. First review your knowledge of flowers by identifying the basic floral structures; the receptacle (stem tip), sepals and petals, stamens (microsporophylls) consisting of a filament that bears anthers (microsporangia; p. 504), and ovary at the base of the stigma and style.
Crush an anther on a microscope slide, add a drop of water and a cover slip. Use your knowledge of pine and of homologies to identify what is microsporangium, and what is microspores and/or microgametophytes in this preparation.
Now examine the contents of the ovary. Do you find seeds here? These seeds differ somewhat from those of pine, but the parts are homologous. Identify what is megasporangium, what is seed coat and what is female gametophyte. Later in the quarter we will look at mature seeds with embryos of flowering plants, but for now you should be able to answer all of the questions at the beginning of this lab for the flowering plants. Try it with the life cycle of the bean plant on p. 512-513.