October 2, 1895.] 



Garden and Forest. 



393 



of Oak, Beech and Sumach, as a probable waste product, 

 and, though almost colorless, is capable of oxidation into 

 bright 3rellovv, red and brown pigments. Now, it is easily 

 verifiable that all leaves red with erythrophyll contain 

 abundant tannin. Not only that, the two substances are 

 closely associated. Although erythrophyll might be formed 

 by the disintegration of chlorophyll, it often appears inde- 

 pendent of that substance, and might also be regarded a 

 derivative of tannic acid by oxygen and light. It is only 

 formed in sunlight and first appears in the exposed tips and 

 stems of leaves, as indicated by their color. Again, it is not 

 formed until leaves are in a state of low vitality. A green 

 Sorrel leaf stuck into moist sand has its under side reddened 

 if exposed to sunlight, but not if shaded. When a healthy 

 leaf of White Oak is torn or punctured by an insect or arti- 

 ficially, the wound in either case soon becomes prema- 

 turely edged with the maroon of autumn. In the case of 

 the Sorrel, sunlight opens widely the stomata, which me- 

 chanically regulate the normal escape of water from the 

 mider, shaded side of most leaves. The same uncontrolled 

 evaporation occurs from a punctured Oak leaf If evapo- 

 ration is too rapid, however, a leaf dies green without 

 assuming its autumn colors. An abnormal loss of water 

 from the leaves not being replaced fast enough by the 

 roots, implies a loss of vital power and a lessened ability to 

 counteract forces of decomposition. 



Weakened vitality, as need hardly be said, is aUvays a 

 necessary preliminary to the autumnal coloration of leaves. 

 Aside from local influences of insects, fungi, soil and other 

 things, all plants suffer a decrease of vitality from the yearly 

 depression of temperature, assisted oftentimes by continued 

 drought. We must not think of chlorophyll as formed once 

 for all in spring and remaining permanent until autumn, 

 but must consider it in constant decomposition and recon- 

 struction, its constituent pigments not only existing in 

 varying relative and absolute amount in different leaves, 

 explaining the diverse colors and shades of different plants, 

 but also occurring in varying proportion in any individual 

 leaf, according to the equilibrium between the constructive 

 and destructive forces in the plant. When this balance is 

 fatally destroyed, as by falling temperature, green pigments 

 are killed by the very solar energy which created them, and 

 previously existing yellows and reds are revealed, or may 

 also be formed in increased amount. Continued action of 

 oxygen, heat and light reduces bright pigments to brown 

 ones, by whose decay valuable mineral matters are re- 

 turned to the soil. 



I have now described in some detail the more important 

 of the many pigments found in leaves and the colors they 

 separately produce, but we must remember that almost 

 any autumn leaf contains not one, but several, distinct pig- 

 ments, the temporarily predominating one giving the leaf 

 its color. The Red Maple is a good case in point. The 

 yellow gradually creeps from the edges toward the large 

 veins, and has not gone far before the edges shade from 

 orange into crimson, which also works toward the veins. 

 These are the last to turn, often appearing green against a 

 striking yellow or red ground. Again, some Ampelopsis 

 leaves are very dark, yet reveal by the microscope a dis- 

 tinct green pigment and a separate red one. The former 

 absorbs the red rays of the solar spectrum, and the latter 

 the complementary green rays, so that little light is trans- 

 mitted to the eye, and the leaf looks almost black. Purple 

 Beech leaves shovi' the same thing, but with an excess of 

 the red pigment. In the beet, erythrophyll is a normal, if 

 useless, constituent ; but it does not appear in a Blueberry- 

 leaf until the green has given place to yellow ; then yellow 

 and red combine optically into brilliant orange and scarlet 

 "colors. Purple Beech leaves are never scarlet simply be- 

 cause the red erythrophyll decomposes before the green 

 chlorophyll does, and the same may be said of many other 

 potentially brilliant leaves ; yet we ought to be satisfied 

 with the color display nature already gives us. 



Endeavoring to explain the principles which underlie 

 this complex subject I cannot do better than to reproduce 



a table by Mr. Sorby, who has made the deepest and most 

 trustworthy researches into vegetable coloring matters : 



Complete vitality j Clirysophyll (gold-yeilow). ( More or less 

 and growth. ) f bright green. 



<■ Chlorophyll (deep green). ^) More or less 



green-brown. 

 More or less 

 red-scarlet. 

 More or less 

 bright orange- 

 brown. 

 Death and de-J t- j ^ » ; j Less or more 



composition. ) H„mns rhrown.blark-V f dull brown. 



Low vitality and 

 change. 



Erythrophyll (crimson-red). 

 Xanthophyll (bright yellow) 



IMiaiophyll (brown-orange). 



Humus (brown -black). 



This table gives the shades peculiar to the more impor- 

 tant groups of leaf-pigments as classified by Mr. Sorby, and 

 the tints resulting from some of their combinations, as well 

 as the relative vitality of leaves approximately expressed 

 by their colors. 



We find all transitions between leaves which die at 

 autumn and those which are again capable of assimilation 

 after persisting dormant over winter. .Such so-called ever- 

 green leaves are always provided with an unusually thick 

 and smooth epidermis to protect the chlorophyll grains 

 within. This chlorophyll remains perfectly green during 

 a mild winter, although the leaf may appear red from an 

 abundance of erythrophyll overlying and hiding the green 

 granules, which have retreated inward; or the green may 

 fade away, as in autumn, leaving yellow granules of 

 xanthophyll, and unless vitality is further reduced and the 

 yellow oxidized into brown, such yellow leaves will again 

 become green in the warm days of early spring, and will 

 give the plant a start until the new year's buds have opened. 



Here is a good |3lace to tell why leaves fall. Sir Isaac 

 Newton was possibly not aware that his apple fell because 

 it was pushed. Of course, that was not pertinent to his 

 train of thought, although strictly true, for fruits as well as 

 leaves are severed from the plant by a "separating layer." 

 Early in summer a layer of tissue begins to form across 

 the base of each leaf-stalk where it joins the stem. This 

 layer grows down like a wedge from the upper angle of 

 leaf and stem. At first it is soft and does not resist the pas- 

 sage of water, but as winter approaches the layer becomes 

 corky like the rest of the bark of which it is a part. It re- 

 sults that when the leaf weighted with accumulated min- 

 eral matters and strained by frost, wind and rain, finally 

 falls, no raw wound is left, but a scar already healed. 



Here is an exquisite adaptation by which water passages 

 are literally "corked up" in anticipation of the fall of 

 leaves that have outlived their usefulness. As this sepa- 

 rating layer gradually shuts out nutritious fluids from the 

 leaf, the latter naturally becomes more and more of a prey 

 to decomposing agencies. Yet, before a leaf falls, all its 

 useful contents, like starch, potash and phosphoric acid, 

 travel through the stem into the bark and young wood, as 

 Sachs has clearly shown, to be stored for future need ; the 

 leaf is then rejected as of no more use, and its decomposi- 

 tion cannot affect the plant as a vv'hole. 



So the colors of autumn leaves are not adaptations for 

 beneficial purposes like the colors of flowers, but are inci- 

 dental, unaimed at and a wholesale waste of beauty, so far 

 as concerns plants themselves. We must mention, how- 

 ever, that the pigments of many, not all, petals and fruits 

 cannot be distinguished from those of autumn leaves — an 

 additional evidence of similar origin and an example of the 

 adaptation of useless structures to useful purposes, as 

 occurs in other biological fields. 



If this unsesthqtic explanation of autumn colors sacrifices 

 any poetic sentiment, we can recall the fact that no leaf 

 falls without revealing a bud, the promise of a future 

 manifestation of life. _. ,., , ,, , 



Cambridge, Mass. JuSlIlS 11 a/SO/l 1' OlSOHl. 



The frosts come to ripen late September days as they ripen 

 a persimmon. Just before sunset a thin ha/.e in the air bathes 

 the mountains on the western horizon with a purple tinge, like 

 the bloom on a fruit. It is a phenomenon which follows the 

 frost, and is an evidence that the year is ripe. — Thorcau. 



