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It isn't known, for sure, where the Robins and other birds get the coloring for their eggs -- nor, indeed, just why they color them, although there are some interesting theories about it (A Year in the Wonderland of Birds.) -- but it is known where the gorgeous coloring of the leaves in the fall comes from; and if you believe, as I do, that one purpose of Nature is to make life beautiful, we can see why she makes her annual art display in the autumn woods. Here, as everywhere, in the Grand Plan of things, use and beauty go together; that is to say, it is in the very act of doing something useful to the life of the tree that the leaves sing their glorious color song of farewell, when the year and their work are drawing to an end. The leaves are, among other things, the food factories of the trees. (This is one of the things I meant when I said, at the beginning of this chapter, that "strictly speaking, trees grow on leaves.") In the autumn when all these little factories close down for the season, most of the manufactured product in the leaves at the time -- starches, sugars, oils, and so on -- is transferred into the buds and twigs for winter storage, but some of it is left behind and the sun changes it into the autumn colors -- the red and yellow and orange of the Maples, the scarlet flame of the Sumacs, the bronze of the Oaks, the riot of color in the Sweet-Gums -- purplish blue, crimson, lavender and pink, burnt sienna, indigo, cobalt, ochre. The greenness of the leaves, which is so conspicuous in the growing season, is really made up of four kinds of pigments (coloring matter), two of which are bright green, of different shades, and two of them yellow; the same yellow coloring matter that Nature uses in flowers and in yellow fruits -- the orange, for example. As you know, a young leaf is not so deep a green as an older one; it has much more of a yellow tinge also. This is because it has a relatively small amount of the green coloring matter (chlorophyll) and a relatively large amount of the yellow pigment. In the sunlight the green coloring increases very fast and reaches its highest point about the time the leaf reaches full size. In the natural course of events, when the leaf's life-work is over in the fall, the green color disappears, while the yellow tints remain behind. This accounts for the yellow and gold in fall leaves in their various shades and combinations. The reds are due to the kind of pigment that gives it distinctive color to the Copper Beech and to many buds and shoots. You can see it in the spring foliage of the Cherry. The purple and mauve and orange tints come from the different degrees of acidity in the sap which is combined with the red and yellow coloring materials. The particular shades of these colors also depends on the character of the previous summer. If it has been rainy, keeping the veins full of sap and the leaf skin thin and smooth, the colors are brilliant and show through; but, if it has been dry, the colors are less brilliant and are made to look still duller through the thicker and more wrinkled skin. Much the same difference, by the way, to be seen between the pink cheeks of a young girl and the wrinkled and weathered face of the Old Salt who has long faced sun and wind! A COLOR INDEX TO THE ART GALLERY IN THE AUTUMN WOODS Trees have their characteristic autumn colors. These colors, together with the Picture Index of Leaf Forms in Section III of this chapter, will help you identify them for your tree book. In using the following grouping of trees according to autumn coloring1 (Compiled from Parkhurst's Trees, Shrubs, and Vines, and Keeler's Our Native Trees.) take as a basis of comparison in each group the leaf of some tree you know well; the Black Walnut for the "Bright Yellow" group, the Hickories for the "Pale or Rusty Yellow" group, and so on. Bright Yellow: Black Walnut, Tulip Tree, Cucumber Tree, Ailanthus, Striped Maple, Judas Tree, Kentucky Coffee Tree, Yellowwood, Wild Red Cherry, Wild Black Cherry, Scarlet Thorn, Shadbush, Mulberry, Chestnut, Common Aspen, Large-Toothed Aspen, Balsam Poplar. Pale or Rusty Yellow: The Hickories, Basswood; Silver, Norway, and Ash-Leaved Maples; Common, Honey, and Clammy Locusts; Mountain-Ash, Spindle Tree, Hop Tree, Choke-Cherry, Papaw, Persimmon, Silver-Bell Tree, Fringe Tree, Slippery Elm, Butternut, Nettle Tree, the Birches, Hop-Hornbeam; Burr, Chestnut, Swamp White Spanish, and Willow Oaks; the Poplars; Beech, Cottonwood. Red: Red, White, and Pin Oaks. Red and Yellow: Horse-Chestnut, Mountain-Maple, Sassafras, Post-Oak, Angelica Tree. Orange, Crimson, and Yellow (more or less combined in the same leaf): Red and Sugar Maples; Sweet-Gum, Cockspur Thorn, Dotted Haw; Black and Scrub Oaks; Hornbeam. Scarlet: Scarlet Oak, Sour-Gum, Black Haw, Sorrel Tree. Brown: American Elm, Buttonwood; White, Red, Blue, and Black Ashes. Many of the Oaks also show brown and reddish-brown (bronze) colorings in various shades and combinations. In the case of the evergreens the leaves or needles may not reach their full color until they are two years old or more. No doubt it's because it's their particular business to be evergreen, and so it takes them longer to manufacture all their greenness! But they take an additional precaution; they shed their leaves in "shifts," so to speak, somewhat as birds shed their feathers,1 (A Year in the Wonderland of Birds.) and for a similar reason -- because they can't go on doing business without them. In other words, they shed their leaves at intervals, just as other trees do, but they always retain enough to "carry on" and so they always look green. |
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