The Fall of Autumnal Foliage

Abstract

MR. FRASER alludes to “the unpursued inquiry into the cause of leaves falling in autumn” (NATURE, February 26, p. 388), and I do not find it mentioned in Sach's “Text Book”; but Dr. Masters, in Henfrey's “Elementary Course of Botany,” fourth edition, p. 515, speaks of “a layer of thin-walled cells being formed across the petiole,” but does not say whence this layer is derived. Duchartre, however, gives a pretty full account of opinions up to 1877 (“El. de Bot.,” deux. éd. p. 443), which he reduces to two, viz. Schacht's, who attributes it to a growth of periderm, and that of Mohls, who recognises a special layer which he calls couche séparatrice, considering the peridermic layer as being often, but not always formed. Subsequently, M. Ledgeganck examined different plants and corroborated Schacht in regarding the periderm as the cause prédisposante, and cold to be the cause efficiente, which contracts “le tissu de la base du pétiole, spongieux, aéré, élastique à un degré beaucoup plus considérable que celui du coussinet.” From my own observations on the horse-chestnut, ash, &c., it appears to be in these clearly a continuation of periderm produced by the phellogen of the branch, which invades the base of the petiole, till it meets in the middle, cutting right through the fibro-vascular bundles of the petiole. As this suberous layer dies, the leaf necessarily falls off. But as long as a leaf is in vigorous health it would seem to resist this invasion, and last longer, as do evergreens. I inclose a figure I possess of a slide showing the process in the horse-chestnut.