178 



SCIENCE-GOSSIP. 



THE AUTUMN WOODLANDS. 

 By Dr. P. Q. Keegan. 



'THERE are few spectacles in nature more 

 impressive than the autumn woodlands. 

 The tints or shades exhibited by the leaves are 

 singularly varied and beautiful, when on favour- 

 able occasions they are sun-lit in a way that is 

 most efficient in the development of their highest 

 attractiveness. The sullen sombreness of the 

 atmosphere, the shadowing forth, so to speak, of 

 approaching decay, followed closely by the chilling 

 advent of winter, is only intensified by the last fitful 

 gleams of sunlight. Who would think that the 

 golden and crimson glories of the autumn scene 

 are due to the self-same or closely allied bodies 

 that help to paint the spring and summer flowers 

 and render them richly dight in hues and tints 

 of azure, scarlet, purple, orange or yellow ? Yet it 

 is so. I shall endeavour in the following remarks 

 to explain the subject. 



The poets or close observers of the sylvan 

 enchantments have recognized two phases of 

 colour in the autumn leaves of most of our in- 

 digenous trees. Thus we have " the rowan scarlet 

 and yellow ; the broad gold pieces of the aspen ; 

 the crimson leaf hanging loose on the cherry ; the 

 jewels of gold in the hair of the birch tree ; the 

 beech leaf yellowing, the oak leaf reddening ; the 

 maple yellow-leafed." It must be understood, 

 however, in the way of science, that as soon as 

 the chlorophyll green has faded through incipient 

 loss of vitality, then the leaves of our oaks, beeches, 

 poplars, elms, ashes and others turn in the first 

 instance to a bright yellow. This change is 

 brilliantly exhibited, under favourable conditions, 

 in the case of the beech, whose " lucid leaves, vary- 

 ing in hue from auburn to gold colour, reflect back 

 the level rays of the descending sun and thus burn 

 with pre-eminent lustre, like a sudden illumina- 

 tion." The blaze is especially lustrous in the 

 autumn woodlands for precisely the same reason 

 that the buttercups or other yellow and orange 

 flowers present a brilliant, almost metallic, lustre 

 in the lap of spring meadows. In both cases the 

 pigment is carotin dissolved, perhaps, in the oil of the 

 leaf, a degenerate product of the dying protoplasm. 

 The effect is enhanced by a background of starch, 

 which subserves the function of a reflector. Let 

 anyone who doubts the fact extract and prepare 

 the colouring principle of yellow petals and of 

 autumn yellow leaves. He will see that they 

 respond to the same chemical tests and exhibit 

 chemical reactions in a precisely similar manner. 

 The only palpable difference in the two cases is, 

 that while in the petals the tint is frequently 

 deepened into orange-red or brick-red, as in many 

 garden flowers, such as marigolds and zinnias, in 



the autumn leaves the colour due to carotin never 

 passes further than the fiery gold so vividly dis- 

 played in the beech, birch or aspen. 



On the other hand, the glorious pageantry of the 

 scarlet maple, the crimson cherry and rowan, or 

 the ruddy oak leaf, proceed from a different 

 principle altogether. They appear only when the 

 yellow tint has disappeared, but before the leaf is 

 quite dead and utterly decayed. Sometimes these 

 ruddy splendours do not appear at all, or are but 

 feeble, as in large towns where the air is vitiated 

 by smoke, when we have only a dry rusty brown. 

 In other cases the influence of the season, whether 

 dry or wet, or occasionally frosty, tells very 

 decidedly on the production of these particular 

 tints. A medium amount of moisture and late 

 frost seem to be the conditions most favourable to 

 the greatest brilliancy of autumnal colour effect. 

 Exceptionally dry seasons occurring in a climate 

 generally moist serve also to call forth distinctive 

 crimson in leaves that would otherwise rapidly 

 pass to a dull brown and muddy shade. On what 

 does all this characteristic or exceptional glow 

 depend ? It results from a powerful chromogen 

 called tannin, which exists in red and blue flowers 

 as well as in the leaves in greatest quantity in the 

 autumn. I must explain that when a solution of 

 certain kinds of tannin undergoes concentration in 

 the presence of dehydrating acids, or of certain 

 salts, there are produced a series of anhydrides, 

 that is to say, the tannin has given up, or lost, the 

 elements of water, with the result that various 

 coloured substances are produced, of which the 

 lowest or first formed are crimson, and soluble in 

 slightly acid water, while the last formed are red- 

 brown and insoluble. The former constitute the 

 colouring matter of the red, the latter that of the 

 brown and russet autumn leaves. Such being the 

 case, as praved by artificial experiments, we must 

 endeavour to exhibit how the process is carried 

 out in nature under the natural conditions. There 

 must, it is obvious, be a sufficient concentration of 

 the cell sap in the living leaf as it hangs upon the 

 tree. This may be brought about, as is palpably 

 obvious, by a deficiency of the water-supply from 

 without, when the soil and atmosphere are un- 

 usually dry. Moreover, this deficiency will chiefly 

 be felt in the case of those leaves which possess, in 

 relation to their size, the largest number of 

 stomata, which, as everyone knows, are the organs 

 of transpiration, that is, the passages in the leaf 

 which permit of water vapour being ejected into 

 the surrounding air. 



With regard to the first point, the unusual 

 dryness of the season, I have already hinted that 



