By Johnny Caryopsis In Manitoba, the end of the growing season corresponds to the start of the school season. The greenery that we have grown accustomed to all summer undergoes a sudden and dramatic transformation. As leaves change colour and fall from trees they attract our attention. In the class room or at home, there are lots of ways to emphasize and take advantage of the natural phenomenon of fall colours. We've put together a few ideas for you to try out. If you have any suggestions for activities, projects or crafts related to this topic, please let us know at: email NNZ, and we'll add them in. SOME LEAF BASICS What is a Leaf? What are the basic parts of a plant? Answer: root, stem, flower and leaf. Roots anchor a plant to the ground and draw in water and nutrients. Stems and branches support the other above ground structures and contain the "circulatory system": the tubes that move water up from the roots and food and nutrients down from the leaves. Flowers allow the plant to reproduce itself and participate in the evolution of the species. Leaves are the food producing factories of plants. (Actually, all parts of plants do way more stuff than this, but let's keep it simple for now!) A leaf is usually a thin, flexible "sandwich" of plant cells. The outside, the "bread", has waxy cells, to help keep moisture inside the leaf. But it also has holes, called stomata, that allow for the exchange of gases for respiration. Plants "breath in" carbon dioxide (CO2) and "exhale" surplus oxygen (O2), through the stomata, which are mainly on the underside of the leaf. The "meat of the sandwich", between the outer waxy cells, consists of various arrangements of cells that perform photosynthesis: that miraculous process of combining water and carbon dioxide via the energy of sunlight to make simple sugars. Leaves may be the "boring" greenery around us all through the growing season, but they are the dynamos that drive the processes of life, and the basis for terrestrial food webs! Getting Ready for Winter Why do leaves change colour and fall? The short answer is that the plants are getting ready for winter. The leaves die in an orderly fashion, a process called senescence, and their contents are recycled by the plant. The long answer involves evolutionary adaptations to conserve water and avoid frost damage, photoperiod changes, and photosynthetic pigments. Check out our Fall Colours article, in this issue, for more of the technicalities of fall colours and falling leaves. ACTIVITIES Leaf Collections Collecting and preserving leaves is a class room activity with a long history. There's nothing new-age about leaf collections. But with today's new emphasis on biodiversity, it is an activity that has great merit. A hands-on, close-up experience with leaves from many different plant species is a great way to introduce kids to the variety of life within the plant kingdom. And in the fall, leaves are just lying there on the ground to pick up. You don't have to "hurt" the plants to collect them! Do you need to be able to identify all the plant species by their leaves to make leaf collecting worthwhile? I don't think so. The key to introducing the notion of biodiversity is to get kids, and people of all ages, to recognize the similarities within individuals of one species and the differences from one life form to the next. While it is rewarding to develop the skill of individual species identification and recognition, it is not something that develops overnight. It takes a lot of time and practice to become a skilled plant identifier. But that doesn't mean that you can't start teaching the basic skills of plant identification. People have to start somewhere. At the end of this article you'll find a list of some books that can help with leaf identification. Doing the Collecting Leaf collecting can be as orderly or as chaotic as you'd like, but here are some things to bear in mind. Don't collect leaves when they are wet, either after a rain or a heavy dew. Wet leaves will be too messy to deal with and more likely to rot. Don't collect too many leaves, you only need a few for all the various demonstrations, and you'll have to deal with the mess of too many, and dispose of the excess. You may want to assign certain students or groups to collect only leaves of one kind of plant - while trying to find leaves that show as much variation as possible within that species. Conversely, you could assign certain students to collect only one representative leaf from as many different kinds of plants as they can find. Collecting leaves from different locations can be revealing. Students could collect leaves from the school yard, a local park or natural area, or from near their homes. Are there different kinds of leaves in different locations? Are there more different kinds of leaves in one location versus another? Can you relate this to biodiversity within the school yard and other sites? Assessing Your Leaf Collection Comparing one leaf to another is the simplest way to address identification and biodiversity. What is different about two leaves that you have? What is the same? Are these leaves from two different species of plant or just different shaped or coloured leaves from the same species? How many different colours and shades occur in each type of leaf? What characteristics let you distinguish between certain species of leaves, and which are diagnostic of a certain species? Here are some characteristics to look for in each kind of leaf. Shape There are many formal names for the various shapes of leaves. You may want to consult one of the plant guides listed below, or just make up your own terms. Size The size (length and width) of the leaves will vary a lot from plant to plant, especially in young trees! Leaf Margin The edge of leaf may be toothed, smooth, wavy, or have other recognizable characteristics. Compound or Simple Some leaves are subdivided into what looks like several smaller leaves or leaflets - an example would be black ash, while others are clearly just one entire leaf - an example would be elm. Texture There may be hairs, spots or bumps on the leaf. The leaf surface may be rough or smooth. Colour The colour and patterning of a leaf in both the growing season and in fall is often characteristic, but can be highly variable, too. Techniques For Pressing And Preserving Leaves Leaves and entire plants can be pressed and preserved indefinitely, as long as they are kept cool, dry and out of strong light. Pressed, dried leaves can also be used to make attractive decorations. A plant press for use in pressing leaves or entire plants is easy to make. Collect some corrugated cardboard and cut it into sheets about the size of a folded piece of newspaper. Gather some newspapers, too. Place a sheet of cardboard on a flat, firm surface in an open, airy location. Open a sheet of newspaper and place it onto the cardboard. Place some leaves, or an entire plant, onto the newspaper. Arrange them so they do not overlap. (Do not use wet leaves or plants!) Fold the newspaper over top of the leaves and place another sheet of cardboard on top of the newspaper. Set another piece of newspaper on top of this and repeat the process, building up a series of cardboard and newspaper "sandwiches" containing the leaves. Finally, place a flat board on top of the pile and place some heavy object on top of this to press down on the whole array. Or, you can use two boards, the size of the cardboard sheets, on either side of the "multi-layered sandwich" and use some string or belts to cinch the whole pile together. Keep your plant press somewhere dry and airy, but not too warm. Don't leave it in a sunny window. If the temperature is too high, the leaves may rot before they are thoroughly dry. Leave the press for at least one week, then check to see if the leaves are dry. Open the plant press and examine the leaves in the top sandwich. If they are thoroughly dry, you can disassemble the press and use the leaves for further examinations or in projects. Another way to preserve leafs, which is particularly good for maintaining fall colours is to iron them between sheets of waxed paper. Cut some sheets of waxed paper to whatever size you need. Place some leaves between 2 sheets and iron them gently, on top of a smooth, hard surface - that can take the heat of an iron! Make sure the leaves are fairly dry to begin with - don't iron wet leaves! The wax sheets will fuse together and make a sheath for preserving the leaves for a long time. The wax paper helps to reinforce the brittle leaves and allows you to handle the sheets without directly handling the leaves. For the best long-term preservation of fall colours you must still keep the sheets of leaves out of strong light. Observing Leaf Senescence and Explaining Fall Colours Demonstration 1: Plant Pigments Demonstration You can demonstrate the concept of plant pigments, how different pigments can interact to produce various colours, and even how one colour can swamp others using food colouring. (Refer to Fall Colours, in this issue for more background on plant photosynthetic pigments.) First of all you can show that some substances are not pigments by adding sugar or salt to water. They produce no colour effects at all; they are not pigments. Try adding a small amount of yellow food colouring to a test tube or jar of water until there is a noticeable yellow colour. Then add green colouring until the yellow is no longer visible, thus demonstrating that various pigments can be present while one may tend to dominate and swamp out the others. Measure the amount (drops) of green it takes to swamp out the yellow. In another (control) tube, add only green colouring and note the point at which the two are indistinguishable, despite having different combinations of pigments. Try adding red to the yellow to make various shades of yellow and orange or finally red. Add some blue to make various purples. A few plant pigments can create a wide range of fall colours, just by having different ratios. This is what really happens in nature! During the growing season plants can have leaves of varying shades of green. You can produce many different shades of green, too, with just the 4 basic pigments (green, yellow, blue and red). Again, this is what happens in nature. Plants that have red or purple leaves during the growing season, like the now-common Schubert's chokecherry trees, contain purplish anthocyanin pigments that mask the underlying green photosynthetic pigments. Demonstration 2: Collecting Over Time You can document leaf senescence (the orderly death of leaves) and demonstrate some aspects of the appearance of fall colours by collecting and comparing leaves at different times. Select certain individual plants (trees or shrubs) and collect a few green leaves in early September, before senescence sets in. Press these in a plant press and keep them out of direct light. Collect more leaves from the same plants when the fall colours are at their peak and press these as before. (Be sure to put the date on each collection!) Finally, collect some leaves near the end of autumn, either any remaining on the plant or on the ground nearby and press these. After a time compare the series of leaves, from the same tree or shrub and compare the differences in colour. In the absence of light, which speeds the decay of pigments, the colours will persist for a long time. Green leaves removed from the plant before the onset of senescence will still be green, despite being dead much longer than the last leaves you collected! Leaves collected at the height of fall colours should still be more brightly coloured than those allowed to age and fall from the trees naturally. This demonstration also helps to explain why many introduced varieties of plants don't show attractive fall colours. Senescence is triggered by photoperiod changes that are specific to the latitude where the plant evolved. Plant species rapidly adapt, through evolution, to the conditions of their immediate environment. As plants spread north after the last ice age they had to adapt to the shorter growth seasons and begin senescence earlier than they would further south. An introduced (exotic) plant that originates too far south, or from another geographical range where the climate is less severe (Remember, Vancouver is about the same latitude as Winnipeg!) will be overcome by the cold before it can go through its orderly shut-down for winter. So the leaves are killed by frost rather than being reclaimed by the plant, and they flutter to the ground, still green. Demonstration 3: Fading Colour To demonstrate how sunlight plays a role in the production of fall colours by fading the green chlorophyll pigment, try the following. Collect a few leaves from several different kinds of native trees or shrubs (exotic plant species may not work as well with this demonstration) while they are still green, before mid-September in southern Manitoba, earlier further north. Divide the leaves into two batches, with some of each kind in each batch. Take two sheets of plastic wrap or thin paper and tape the leaves from each batch to one of the sheets. Tape one sheet of leaves to a south-facing window, with the leaves facing the glass, so they are exposed to lots of sunlight. Place the other sheet in a well covered cardboard box (so that no light gets in) that you keep near the window with the other sheet of leaves. This way both sheets experience similar conditions, except for the sunlight. After a time (wait at least 2 weeks) you will notice that the leaves in the window have lost most of their green colour and, probably, now resemble fallen leaves. Take out the other leaves that you kept out of the light and compare them. These should still be green, though by now they will be dry and brittle, too. These two sets of leaves have experienced near similar conditions (though you could argue the ones in the window will experience greater temperature changes), yet now look quite different. This difference is due to the decaying effect of strong sunlight on the chlorophyll pigment in the leaves. Demonstration results An elm leaf from the window (left) vs. one from the press (right). Demonstration 4: Fun With Fading If you really want to get fancy, try this. In early autumn, locate a tree or shrub where there are leaves low enough for you to work with, and that is in a safe enough location, so they won't be tampered with or damaged. The leaves must still be green! Cut out some series of matching small shapes from some box board (cereal-box-type cardboard). Select some large leaves and staple the matching shapes to the front and back of the leaf, making sure the shapes align accurately. The idea is to prevent sunlight from reaching the part of the leave covered by the cardboard shapes. Watch for the fall colours to develop in your selected leaves, then carefully remove the leaves and take off the cardboard shapes. The absence of sunlight should have resulted in the covered areas still being green, or at least not the same colour as the rest of the leaf. Use you imagination to make this demonstration of the cause of fall colours memorable. Hint: try cutting out letter or number shapes and spelling words! If you have enough large leaves, there's no telling what you could do! This same technique could work with the "Fading Colour" (leaves in the window) demonstration, as well.   Reference Books for Leaf Identification A Guide to Field Identification: Trees of North America. 1968. By C. F. Brockman. Golden Press, New York, Western Publishing Co., Inc. A Field Guide to Trees and Shrubs: The Peterson Field Guide Series, No. 11. 1972. By G. Petrides. Houghton Mifflin Co., Boston. ISBN: 0-395-17579-8 Native Trees of Canada, 8th Edition. 1990. By R. C. Hosie. Fitzhenry & Whiteside Ltd. ISBN: 0-88902- 572-X Identification Guide to the Trees of Canada. 1989. By J. Lauriault. Fitzhenry & Whiteside Ltd. ISBN: 0- 88902-564-9