Let us now take up, in the light of the foregoing conclusions the consideration of the destiny of the primitive foliar organ as it has been modified and developed in the course of evolution. For convenience in making our inquiry, I would divide the primitive leaf into the central-basal, axial, apical and lateral por tions. Each of these figures prominently in the evolutionary his tory of foliar organs, for from the original condition there has been progressive development along several lines of varying de grees of relationship and the morphological result of the develop ment of the several parts has been quite different in the divergent groups, so much so as to render the question of homology a doubtful one to many minds. We shall endeavor to establish its reality.
The lamina of the leaf, as we shall see, has been developed chiefly from the apical portion, usually from scarcely more than a mere point, though it may also include the axial and lateral por tions. The true petiole, when present, is developed from the axial portion,* the sheathing petiole from the central basal to gether with the lateral portions, stipules and structures of the same signification from the lateral portions. It is with the lateral portions, therefore, that we are chiefly concerned. .
With reference to the formation of stipules there are three principle types of leaf-development : that in which the several portions of the primitive leaf have developed together into a sim ple unappendaged blade, that in which a sheathing petiole is formed with or without a ligule or ochrea, and that in which stipules properly so-called are present.
together gives rise to such leaf-forms as are found in Vacciniunt and Sassafras, the principal portion of the lamina being formed by the development of the apical portion, but including at the base the lateral, central-basal and axial portions which are con tracted below into a short petiole.
If we observe the development of the leaf in Sassafras the relative growth of the several parts can be readily traced. The first four leaves (fig. 1) are very simple. In the fifth (fig. 2) con siderable development has taken place. The apical portion, now forming about half of the organ, is provided with the three typi cal veins as they appear in the adult leaf, but starting out sepa rately from the very base. The lateral portions have reached their highest development and each is furnished with a pair of veins. In the sixth leaf (fig. 3) there is a very close approach to the adult form. The upper part has expanded and the lower parts have elongated, removing the point of separation of the three principal veins of the leaf to a considerable distance from the stem. At the same time there has been a basal contraction look ing toward the formation of the petiole with a considerable de generation of the lateral portions, one of the veins having disap peared from each of them, while the other has become associated with the midvein. The seventh leaf (fig. 4) represents the nu lobed adult form and differs but little from the sixth.
A similar condition is observable in Ailanthus glandulosa Desf. (figs. 5-10), but resulting in this case in the final separation of the lateral portions as small gland-bearing fngacious stipules, com parable to those at the base of the leaves of many of the Ranun culacece. The comparison of Sassafras and Ailanthus shows how small a difference in development may determine a leaf as stipu late or exstipnlate.
The case of Syringa vulgaris L. is like that of Sassafras, though more difficult to trace, owing to the larger number of veins in the leaf, but the homologies of parts may be followed more or less distinctly from the second leaf up to the sixth, the first adult leaf (figs. 11-14). The lateral portions are seen to have degen erated almost entirely and, their bundles having disappeared, they remain only in the margin of the petiole.
The Composite furnish examples of a similar course of develop ment but often with a closer approach to the true stipular condi tion, as the lateral portions are supplied with vascular tissue by small branches coming off at the base of the leaf from the main lateral bundles.
In Erigeron annuunt (L.) Pers., for example, there are three fibro-vascular bundles in the leaf-trace which pass up through the central portion of the petiole, converging as it narrows. But almost immediately on their departure from the stem each of the lateral bundles gives off a branch in the same manner as when true stipules are present. This branch forks at once and supplies the wings of the petiole. In the cauline leaves (fig. 15) its branches can be distinctly traced into the lower lobes of the leaf. The basal leaves of Aster undulatus L. show a condition very closely similar to that found in Erigeron annum (L.) Pers., but in the cauline leaves there is a considerable modification by which the large lobes of the base of the petiole (fig. 1G) are formed. The stipular bundle curves outward through the lobe giving off branches which form a net-work supporting its paren chyma. It then passes up through the wing of the petiole and into the basal part of the leaf. In Solidago juncea A it., there are eleven bundles in the leaf-trace and a stipular bundle is given off on each side, supplying the margins of the petiole. Artemisia vulgaris L. affords a very interesting variation. The lateral portions of the primitive leaf have branched in a very curious manner (fig. 17), forming several small leaflet-like appendages to the base of the petiole. That they belong to the lateral portions and are stipular in their character is shown by the fact that they are supported by branches of the stipular bundle which is given off a little higher up than in Erigeron, passes on through the wings of the petiole after giving off the branches and enters the base of the blade as in other cases. This is the nearest approach to the true stipular condition that I have observed among the Compositors.