There is a popular impression that the autumnal change of leaf color is due to the action of frost: and that early frosts conduce to a more vivid tinting of the foliage. This, however, is an error that a little thought and observation will correct. Some of our trees display red and yellow leaves in August, long before cold weather comes. The brightest red shown in all our autumn foliage is that of a Red Maple on which the leaves turn color in August.
Conceding that the intensity of color differs with the seasons, it may be said that the most brilliant coloring of our forests occurs when a rainy summer is followed by a cool, dry August and September in which there is no frost, Undoubtedly, an early host may precipitate a change by prematurely hastening the decay or death of the leal ; but the resultant color will he inferior, changing soon to the dull brown which characterizes sear, dead foliage. Moreover, when the fall months are cool and dry the leaves are persistent, and afford a longer display of autumnal tints.
Nor is frost necessary to the falling of the leaves.* Throughout our Northern States much of the foliage of the trees falls before any frost occurs, while in the South the deciduous trees denude their branches without its aid. Frost can kill, but it cannot ripen vegetation. It has been noted also by close observers that while an early host will check the development of bright leaf colors, a hot, sunny day, about the fust of October, will have the same result and loosen the leaves so that they will fall in showers when stirred by the first gentle breeze.
In preparing the colored illustrations of autumn leaves, shown in the accompanying plates, great care was taken to place the specimens as soon as possible in the hands of the artist on whom devolved the task of reproducing the exact tints, This was found to be especially necessary with the yellow leaves, as this class when taken from the tree laded quickly to some extent. With the reals there was not so much need of haste, these colors being more persistent. In order to avoid delay in this respect as far as possible, the specimens were collected mostly in Bronx Park and Northern New Jersey, although excursions for the same purpose were made to the Berkshire's and Catskills.
No attempt was made at scientific research; it was sought rather to offer in connection with the text some illustrations showing the typical color assumed in autumn by the leaves of our most common trees, illustrations which might furnish a convenient reference for naturalists it ewer needed.
The collection was made by Mr. Abraham Knechtel, F. E., a forester in the employ of this Department, whose scientific attainment and love of nature-study rendered him well qualified for the work. Ample acknowledgment is also due to Mr. William E. Bruchhauser, of New York City, the artist, whose skill and enthusiastic co-operation made the accurate reproduction of colors possible.
*Kerner and Oliver. Natural History of Plants. Vol. II, p. 355.