THE NATURE AND ORIGIN OF STIPULES.
Though it is not part of the purpose of this paper to discuss the problem of the phylogeny of the plant world, it is nevertheless necessary in order to define our field of inquiry to make a brief statement concerning the probable relationship of the higher forms, namely of those in which foliar organs are developed, in eluding in the widest interpretation the Characem, Bryophyta, Pteridophyta and Spermatophyta.
As, in the Characem and Bryophyta, the plant body represents the gametophyte stage of development, there can be no homology of the leaves of these plants with those of the Pteridophyta and Spermatophyta in which the plant body is the sporophvte. For this reason the so-called stipules of the Charas, together with the basal lobes or saclike and straplike appendages of the leaves of many Hepaticas need not be taken into consideration.
Accepting the general theory of evolution in nature, we must admit that the origin of all the higher plants is algal, but just what the relationship of the Pteridophyta to the Spermatophyta may be is still an open question. The same is true in greater or less degree of the affinity of the Monocotyledones, Dicotyledones and Gymnospermre in the latter group.
This question of relationship is of considerable importance in connection with the problem before us as determining the homol ogy of the foliar appendages in the several groups. The evi dence in support of the doctrine of the common origin of all the Angiospermm is particularly strong and may he considered as conclusive. But the relationship of the Gymnospernue to the Anglo spermm is more remote, and that of the Pteridophyta still more so and, though there are many points of resemblance, the similar characters may be cases of parallel development rather than indi cations of a common origin. It is my present opinion, however, that the Gymnosperms; sprang from some generalized hetcro, sporous Pteridophyte,* that the early Angiospermw were ditler entiated from related forms, and that therefore, the foliar organs in the three groups may be considered as homologous. But this homology can apply to the leaves of Pteridophytes in a very general way only, namely, to such undifferentiated forms of leaves as the ancestors which gave rise to the early Gymnosperms' and Angiosperms: may he supposed to have had. While, therefore, the foliar organs in the three classes are to be considered homol ogous in their origin, they cannot be so considered in their dif ferentiation and the evolution of leaf-forms in the Pteridophyta and Gymnospermru, though analogous in many points to their evolution among the A ngiospermre, should be regarded as inde pendent. We may then consider the " stipule " of the Ophio
glossacee, Marrattiaceae and Osmundacex and the " " of Selaginella and Isoetes as special developments and as properly placed in a separate category from the appendages bearing these names among the Angiospermfe. The Gymnosperme present noth ing to represent either stipule or ligule and we have left for our special consideration the ligule, stipule and their homologues as they occur in the various groups of the Angiospermai only.
Having thus defined our field, we should have, for the consid oration of the problem before us, some conception of what sort of plant the earliest Angiosperm was. In the absence of geological evidence this conception must be purely hypothetical and, basing it on a generalization which would admit of the differentiation from it of all the varied forms of the modern group of Angio sperms, we can see that it must have been a plant of very simple organization indeed. For our present purpose we need not con cern ourselves with any other organs of this primitive Angio sperm than the leaves which, from the point of view of the pro posed generalization, must be conceived of as hardly more than the bare rudiments of leaves, mere sheathing scales at the nodes of the plant, serving slightly, if at all, the function of assimila tion which was still subserved, as in its ancestors, by the general surface of the plant, but confined chiefly to that of protection. The primitive leaf was probably parallel-veined or approximately so, giving rise in its earlier differentiation to the parallel-veined leaves of the Monocotyledones. The geological evidence indi cates that these appeared before the Dicotyledones* which must have sprung from them later at one or more unknown points, and netted-veined leaves are of a more recent evolution. Consequently the tendency of aquatic Dicotyledones to revert toward mono cotyledonous structure is rather a case of atavistic degeneration than an indication of the origin of Monocotyledones from Dico tyledones in ancient times through the effects of aquatic habit.t Now, as advance in evolution proceeded, the need of greater as similative capacity arose and, as the foliar organ was the one best *Professor L. F. Ward. Sketch of Paleobotany. Fifth Ann. Rep. U. S. Geol. Surv. 448, 1885. Professor A. C. Seward, on the contrary, does not be lieve that we have satisfactory evidence of pre-Cretaceous Monocotyledones. Notes on the Geologic History of Monocotyledones. Annals of Botany, 10 : 220. 1896.