FOSSIL PLAl TS AND THEIR USE I

TEACHI G HIGH SCHOOL BIOLOGY

IlY

DAVID L. DILCHER

Reprinted from School Science and Mathematics

pri], 1967

Fossil Plants and Their Use in Teaching High School Biology*

David L. Dilcher Dept. of Botany, Indiana University, Bloomington, Ind. 47401

Most high school students have at some time been intcrested in Gnding and collecting fossils or simply reading about prehistoric forms of life. The word prehistoric is an attention getter that conjures within the imagination a vision of some massive carnivorous animal hungrily searching for his prey. This common interest il prehistoric life can be used to great advantage in introducing and carrying through a unit in the study of the evolution of the life we have in the world today.

Almost everyone likes fossils. They are fun to collect, interesting to Jook at, and provide a stimulus for a responsive classroom discussion about past forms of life. Both plant and animal fossils can be used. Both should be considered in the light of where they lived at t e time they were alive on earth. Most animal fossils found in the Midwest wcre deposited during past invasions of ancient seas and are thus marine organisms. The plant fossils, on the other hand, are usually found in terre trial deposits or brackish water deposits near land and represent organisms which lived in shallow fresh water or land en­vironments. The environments in which most fossil plants grew are similar to various modern terrestrial environments with which most high school students are familiar.

When using fossil plants as an aid to or as a basi for a classroom discussion of prehistoric life it is best to start with kind of plants which are well known and which can be recognized easily by the studen ts. If your lass lack a good co eetion of plant fossils a Satu r­day or an afternoon field trip by orne of the students especially jn­terestcn in biology should be arranged to a near Jy fossil locality. I say nearby bccause there are numerous areas in Indiana, illinois, Mich­igan, Missouri and Iowa which have very good (museum quality) plant fossil. There are many other states which also have good plant fossils, in fact it would ue easier to name those states which lack good fossils than those in which they can be collected. So when you are traveling throughout the United States you can collect with very little efIort an assortment of various types of plant fossils.

To do this you must know where to look and what to look for. The first good place to oak is in a rock s lOp. See what they have available In fossils and ask if there is any good collecting in the area. Any state geological survey, university or college paleontologist can usually give

• Paper presc"ter at the Annual CASMT Convention, Fren h Lick, Indiana, November 1966.

you good leads on proffilsmg fossil-collecting sites. The Indiana Geological Survey publishes a small bookl listing several plant fossil sites in the state and iJJustrating typical plant fossils found at them. The state geological surveys of West irginia2 and Kansas3 also have special publications describing fossil localities and kinds of fossil plants that can be collected within their states. Jay Ellis Ransom~ has written a book for the amateur showing techniques of fossil collecting and indicating, state by state, localities where they can be or have been collected. However, his localities are old and not well described. Turtox, Macalaster Scientific and Ward's sell an assortment of plant fossils and fossil lab kits if you prefer to purchase rather than collect plant fossils.

Active participation by the students should be encouraged. Have them collect some of their own fossils or bring to class fossil ollections which they may already have. The students should be encouraged to handle the fossils and make observations concerning the plant fos il material. After inspecting several types of fossils they should be able to draw some conclusions concerning the way in which plants can be preserved. They will see that sometimes plants are compres ed in layers of sediments either leaving an imprint or remaining pres rved as arbon film. These are called impressions and compres ions. This type of preservation is ery common and constitutes the majority of fossils found. Occasionally a stem or root of a plant may be covered up by sediments, then decay, and the resulting mold be tilled in by more sediments forming a cast of the original. The e replicas f the original stems or roots are termed molds or casts. Undoubt dly the students will notice that in some fossils only evidences of past plants are preserved (impressions, molds, casts) while in others the cellular detail is still preserved. This is the case in petriti d wood and in coal ball material in which numerous plant parts are petrified. Here the original cell wall may still be present embedded in rock.

The impressions, casts and molds should be examined in further de­tail by the students and they should be encouraged to relate the fossil plants to similar modern plants. They should be able to see the fern or fern-like affinities of several of the leaves and the hor etail-like affinities of some of the segmented, striated stems.

However, their most active laboratory work will probably be done with the petrified plant material. A ery simple technique, the peel technique, allows thin sections to be made from the urface of many

,an.right, J. E.. "Fossil Plants of Indiana." Tndiana Department of Conservation, Geological Sunrey, Re­port 01 Progress No. 14 (1959), pages 1-45, plates 1-5.

2 Gillespie, W. IT., 1. R. Latimer Jr., and J. A. Clendening, "Plant Fossils of West Virginia." West Virginia L"logical and Economic Survey, Educational Series (1966), pages 1-40, plates 1-43. , Baxter, R. W., "Pennsylvanian Fossil Plant. from Kansas Coal Balls." A Field Conference Guidebook for

tbe Annual Meetings, Goo\. Soc. f Am. and Assoc. Societies, Kansas City (1965), pages 1-34, figures 1-23. • Ransom, J. E., "Fossils in America." Harper and Row, New York (1964), page. 1-402.

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petrified plants. These thin-section peels are something that each student can make and then examine under a dissecting or compound microscope to see the cellular detail of the preserved plants. Peels can easily be made from balls of petrified plant material often found as­sociated with coal deposits. Water, rich in carbonates, percolated around and through some of the egetation on the floor of the coal forests or swamps and deposited calcium or magnesium carbonates which actually embedded various plant fragments and the cells of these plant parts. The spherical masses of petrified plant material formed are commonly called coal balls. They can be awed by a rock saw with a diamond impregnated blade to expose the numerous em­bedded plant parts (leaves, stems, roots) ill section on the cut surface of the coal ball. Pre-cut coal balls can often be obtained from a paleo­botanist at a university. He ",ill probably be happy to send you a cut slab which he would otherwise discard becau e it does not contain the specific sorts of plant fragments necessary for his research. Also Macalaster Scientific Corporation sells coal ball peel kits which con­tain a cut coal ball slab, plate glass, no. 600 carborundum and cellulose acetate sheets.

To prepare the cut surface of the coal ball [list polish it on a lap wheel with no. 400 carborundum and then further polish it on plate gla s with no. 600 carborundum. Then the carbonates embedding the cell walls of the petrified plants are etched away with dilute hydro­chloric acid. Usually a 5% hydrochloric acid solution is u ed. The polished surface of the coal ball is held in the dilute acid (etched) for about 20-30 seconds. (The acid is dilute and will not harm hands but should not be spilled on clothing.) Next wash the etched surface of the coal ball in running water. Care should be taken here not to touch the etched surface of the coal ball because the cell walls are now standing in relief on the surface, and they are no longer embedded in the carbonate matrix. Place the coal ball in an upright position and allow the etched surface to dry (a few drop of acetone will speed up dry­ing). Then pour a thin layer of acetone on the etched surface and carefully roll on, from one side to the other, a piece of cellulose ace­tate. 5 This sheet i dissolved by the acetone and flows around the cell walls and embeds them. Allow the peel to dry for 20 to 30 minutes. Loosen the peel along one edge of the coal baU, being careful not to tear the peel, and then slowly pull it free across the entire cut surface of the coal ball. The cell wall which were etched into relief will have been embedded in the acetate sheet and will be peeled away from the surface of the coal ball with it. Thus a thin section of the coal ball containing petrified plant parts which show excellent cellular detail will have been made. This process can be repeated to make additional

, Cellulose al:etate sheets c~n be I,urcha.-,ed from the Colonial Kolonitc Coo. 2232 \Vest Armitage, Chicago 4i I Illinois. A. 004 inch lhickness is recnmmended [or sludent use.

peels by repolishing the peeled surface with no. 600 carborundum on a plate glass and repeating the procedures outlined above.

The students can easily learn how to make good peels and will en­joy preparing fossil plant material for their own examination. They .hould he encouraged to examine their peels closely under a dissecting microscope and then, if they wish, to look at them in more detail un­der a compound micro cope. Have the students observe what sort of cells and plant parts are preserved. Illustrations of reconstructions of the sorts of plants the students will see in sectional view will help them to vi ualize what part of the plant they are looking at in cellular (letail as well as what the entire plant looked like. The peels could be kept by the students as each year the new students in biology should hu\"e an opportunity to prepare their own thin sections of fossil plants. A small sample of coal ball material (1"-1 til thick) is enough to make several hundred peels.

By this time the students' interest and curiosity should have been built up through examining and working with the various fossil plants to the level where an active classroom discussion should follow. With minimal direction by the teacher the class should try to compare the sorts of fossil plants they have looked at in class with plants gro\'.ring today. They should be able to recognize fern leaves from the coal age (Pennsylvanian) depo its and with help to relate some of the com­pressed and petrified remains they have looked at to the lycopods (modern Lycopodium), sphenopsids (modern Equisitum) , ferns and gyrrmosperms. Angiosperm leaves will be easily recognized and some such as oak, sassafras, liquid amber and beech are easily identified by students.

Once told the age of the various fossils they have looked at the students will quickly realize that plants have changed a great deal through geologic time. The oldest forms are similar to modern plants which we consider more primitive in their anatomy and reproduction while fossil forms of the angiosperms with their advanced anatomy and reproduction do not appear until much later. Given this fossil plant material evidence to consider, let the students do some thinking and they can draw quite logical conclusions for themselves concerning the evolution which ha ta.ken place in the land plants. At this point the students can be challenged by questions concerning why some plants become extinct and others continue to live and how this pro­cess might have occurred. Here the morphology of the plants will cer­tainly be discussed but also the physiological capa.city of these ex­tinct plants should be considered.

After considering the fossil evidence for organic evolution it might be wise to turn the classroom discussion to some of the other uses of fossil plants. Many paleoecological studies have been based upon fossil plant remains. By studying the environments and environmen­

tal tolerances of their modern plant relatives the past environments of fossil plants can be reconstructed. Both water (swamp vs. dry land) and temperature (tropical, temperate, arctic) eem to have played an important role in the migration and perhaps evolution of past and recent plants. When looking at the roots of Psaroni1~S in coal ball material the students will certainly notice the large air spaces which are characteristic of several semi-aquatic plants living today. The presence of growth rings in woody tissue of petrified stems is the re­sult of growth cycles probably caused by water (wet-dry) or tempera­ture (warm-cooL) cycles.

It is interesting to note that the ecological niches occupied today by trees, shrubs and herbaceous plants were also occupied in the coal age although by very different sorts of plants. Then arborescent lyco­pods, sphenopsids and primitive gymnosperms were common with shorter tree ferns and herbaceous sphenop ids and ferns growing be­low them.

Although there is no end to the diverse directions to whi h such discussions can lead the two major areas to be considered are those discussed above: 1) the fossil evidence of evolution and 2) the uses of fossil plants in interpreting past environments. It is best if the ideas come from the students and simply are restated or directed by the teacher so that the students must draw upon their own observations of the fossil plants they worked with to make conclusions about the evolution and ecology of these fossil plants. Used in this way even a small collection of various types of fossil plants can make an impor­tant contribution to a student's under tanding of the changes which have occurred in organisms throughout geologic time.

REFERENCES

DEPARTMENT OF ~ATURAL RESOURCES, Geological Survey., "Geologic Publica­tions of Indiana." (1966) page 1-26.

SHAVER, R. H., "Adventure with Fossils." Indiana Department of Con ervation, Geological Survey, Circular No.6 (1959), pages 1-52.

TAYLOR, T. N., "The Coal Ball Peel Technique." Fast Journal (April-i\lay, 1962). WIER, C. E., "Our Indiana Coal." Indiana Geological Survey, pages 1-10.

BRAI SURVIVES DEPRIVATIO

The brain may survive oxygen deprivation if the body temperature is lowered. It is well known that deficiency of oxygen to the cerebral tissues inhibits the

intensity of metabolism in the brain, and the Russian experiments explain the mechanism, using as an example the synthesis of phospholipids. (Phospholipids, or phosphatides, are founei in all living cells in connection with fat storage.)

Using male albino rats of the \Vistar strain, Drs. D. A. Chet\'erikov and S. \'. Gasteva proved their point by four experiments-all in a cooled barochamber with lowered oJl:ygen for two hours. When the floor of the chamber was heated, however, half of the rats died; examination indicated that it was the hypothermia, or cooling, that diminished the oxygen requirements of tissues by reducing the rate at \vhich metabolic processes use energy.