On the morphology of leaves

Abstract

Until about 30 years ago all authorities regarded leaves as appendages, completely distinct, except for a circumscribed area of attachment, from the axes which bear them. Actually a leaf is only a special type of shoot of determinate growth; therefore it contains vascular bundles and other parts representing stem structures. The problems involved in location go far back in evolution, far beyond the evolution of higher plants, indeed, beyond the evolution of leaves themselves and are inherent in every form of branching which depends upon apical growth. There are many reasons for believing that a leaf is closely identified physiologically with a portion of the stem immediately around and below its point of insertion, and that a similar demarcation into leaf fields and primordium fields exists at the apex (11). A typical leaf is dorsi-ventral and possesses a leaf lamina in the form of a thin plate of tissue. Radial and bilateral leaves are very common in Australia and are found in the most different families; there are also transitions between dorsi-ventral and bilateral leaves. The range of morphological variation is much greater among leaves than among stems. The green foliage leaf is assumed to be the ideal form, and all other leaf organs are supposedly metamorphosed foliage leaves (4). Whatever formal definition we assume, it will be possible to find some cases which appear to contradict it (10). Arber (2) was particularly impressed by the amount of evidence carried by leaves. It seemed to her that any one who made a sufficiently detailed comparative examination would find it possible to identify, not only the family but even in many cases the genus