Journal of the
Washington Academy of Sciences 2: 218-220 (1912)
Jointed leaves of Amygdalaceae.
O. F. COOK
Bureau of Plant Industry.
The leaves of the plum, peach, and apricot have a joint at the base, just above the insertion of the stipules, as do many leguminoseae. The basal section of the leaf, below the joint is in all cases short, but is often as long as broad, and is the part to which the stipules are attached. It does not fall off with the petiole but remains alive and serves as a supplementary bud-scale. The stipules are deciduous at an early stage, a fact which may have allowed the persistance of the base of the leaf to remain overlooked.
It is not to be supposed, of course, that this specialization has remained entirely unnoticed by preceding observers, but Sargent and other authorities do not allude to it further than to state that the stipules are distinct from the petiole, and the botanical significance of this fact is not pointed out. The reader is left to infer that the stipules are attached directly to the branch, for nothing is said of another foliar element below the base of the petiole.
Without attempting to determine whether this joint in the leaves of the Amygdalaceae is truly homologous with the leaf-articulations of leguminoseae, it is at least to be considered as a vegetative character that sets the Amygdalaceae one stage farther away from the Malaceae and true Rosaceae. Botanists have long recognized a very close approximation between the rosaceous and leguminous series. Such types as Chrysobalanus have often been passed back and forth between the two series by different authorities on classification. Some botanists may not consider the attachment of the stipules as a sufficient indication that the persistent base is really a part of the leaf instead of an outgrowth from the internode. But it is believed that a comparison of the stone fruits with other rosaceous types will convince most observers on this point.
Reduced to the simplest terms of morphology, the leaves of the higher plants, including the monocotyledons, consist of two structural elements, a more or less sheathing base and a more or less expanded blade. The petiole is a secondary specialization, in some eases representing an elongation of the upper part of the basal sheath, in others a narrowing of the lower part of the blade. In the fan-palms the petiole is a part of the leaf sheath, with the ligule at the end, while in Desmoneus and other related genera of Cocaceae, the ligule is below the petiole which evidently represents only the naked base of the rachis.
In most of the dicotyledonous families the sheathing character of the basal element has been lost, but in this respect the Rosaceae, Ranunculaceae and related families are less specialized, so that there can be little question of the homology of the stipular portions of the leaves with the bud-scales and sheaths of Artocarpaceae, Piperaceae, aroids and palms.
The leaf-bases of the Amygdalaceae are able to persist because of the joint that allows the petiole to separate and fall off with the blade at the end of the season. At San Antonio, Texas, where these facts were first noticed, the persistent leaf-base of the peach remains alive for a year or for two but finally dies and withers away. In Maryland peach trees the petiole base lives through the winter and separates when the buds start in the spring, leaving a fresh green leaf-scar.
The apple and its relatives do not share these specialized leaf characters of the stone fruits. There is no joint above the attachment of the stipules and basal section of the leaf falls off with the rest. The stipules of the quinces are much closer to the base of the leaf than those of the apple, and might be supposed to have a separate attachment to the branches. But on ''water shoots'' of Chinese quinces at San Antonio some of the stipules were of very large size and had strongly dentate margins. In these cases there could be no question regarding the attachment of the stipules to the petiole.
Another fact that may indicate greater complexity of leaf structure among the ancestors of the Amygdalaceae is the presence of small oblong or spatulate leafy organs on the upper part of the petiole, taking the place of nectaries. In some varieties of apricots these small accessory blades are of frequent occurence. They suggest the possibility that the nectaries of the petioles of Amygdalaceae may correspond to the marginal glands of the blade and may represent rudiments of divisions of compound leaves. If this be true the petiole in this group may correspond to the rachis of a compound leaf rather than to the more specialized petioles of some of the simple-leaved families.