The Nature and Origin of Stipules

Prunus Cerasus L. gives a very good morphological series, but the venation is obscure. A view of the several forms can be had by an examination of the tenth, thirteenth, fifteenth, sixteenth and seventeenth leaves (figs. 57-61). They show the transition from the simple primitive scale to the mature condition in which the stipules are rendered entirely free. The series is similar in Rebus occidentalis L., Pyrus Malus L. and Pyrus communis L. In Rubus villosus Ait. (figs. 62-66), the basal degeneration is not carried quite so far and the stipules in the adult leaf-forms re main adnate for some distance from the base of the leaf. The tips of the stipules have taken a larger comparative development than in Agrimonia. Anatomically, however, Rubus villosus A it. resembles the latter in having a vein which enters the petiole, neighboring to the main stipular bundle much as in Viola oblique Hill (fig. 18). The venation in Pyrus Malus L. (fig. 67) is still more like that in Agrimonia.

The

stipules of Fragaria and Rosa show the highest degree of adnation and little, if any, basal degeneration seems to have taken place, though the lateral leaf bundles curve in toward the median one at but a short distance from the stem. This arrangement of the bundles is probably secondary in these forms for the purpose of giving a firmer support to the leaf by an axial concentration of the vascular tissue in the sheath and a corresponding thicken ing of the surrounding tissues, a firmer support than could be given by only three bundles if they did not converge till they ap proached the point of their entering the petiole. The venation of the stipules is also peculiar. In Fragaria Virginiana Duchesne (fig. 68), there is a single strong bundle running out into the free tip of the stipule. From this are sent out one or two weak veins above, and below there is a faint vascular network confined mostly to the region of the tip and extending in a long curve toward the outer portion of the base, where it gradually fades out without forming any connection with other vascular tissue below. This condition seems to indicate a former basal connection of these stipular bundles, either with the lateral bundle of the leaf or pos sibly with those of the stem, forming an additional leaf-trace bun dle distributed to the stipules only. The former case is far more likely. A probable explanation of this degeneration of the basal stipular bundles can be found by a consideration of the conditions of the environment. All the leaves being basal, the stipules are clustered together and are supported by one another and by the surrounding soil. They are more or less fleshy, destitute of chlo rophyll, and in their moist surroundings loss of water by evapo ration is comparatively slight. All these circumstances lessen the necessity of the supply of fresh sap. The rapidly conducting vascular tissue has come into disuse, and its degeneration and disappearance is the natural consequence. The same arrangement in forms with leafy stems is not so readily explainable except by the supposition that the arrangement is ancestral. This seems

rather evident in the case of Agrimonia striata Michx. (figs. 53— 56), where the same condition of the bundles occurs, for the earliest leaves representing the ancestral forms develop under the same conditions as the adult leaves of Fragaria. But in Itosa it would be by no means clear did we not have such intermediate types as ..4grimonia. Rosa humilis Marsh. (fig. GO) may be taken as typical of the genus. The venation of the tip of the stipules is nearly like that in Fragaria, but with a little larger develop ment above the main bundle. The vascular network below is much more extensive and is reenforced by several small branches from the lateral bundle which enters the petiole, below the main stipnlar branch. This additional supply of vascular tissue is evi dently rendered necessary by the exposure of the stipules to the light and air and the development of chlorophyll. It seems to be of secondary introduction.

The nearest approach to the stipnlar conditions occurring in Fragaria and Rosa which I have observed among the Legumi nom is found in the adnate stipules of Trifolium pratense L. ( fig. 70 ). There are two sets of stipular bundles. One of these supplies the tip of the stipule and consists of three veins of which the lowest corresponds to the single large bundle of the tip of the stipules of Fragaria and Rosa. The other has its ori gin as branches from the lateral bundle of the leaf-trace at the base of the leaf, the usual point of origin of the veins of free stip ules. This set of veins is distributed to the lateral and basal parts of the stipules and apparently corresponds to the lower net work of the stipules in Fragaria. These stipules are mainly protective in function. Their meshes are filled with hyaline tis sue, but there is some green parenchyma along the veins.

Two very interesting cases in the family of the Rosacem are those of ClTortia graminea L. f. of South Africa ( fig. 71 ) and Potentilla fruticosa L. ( fig. 72 ). In the former the leaves very closely simulate those of grasses with the linear lamina sessile upon a sheathing petiole. They differ in having the tips of the lateral portions ( stipules ) free instead of turning in across the insertion of the lamina to form a ligule. In the latter the con ditions are very closely similar to those of the ochrea of Polygo num. There is a short sheathing petiole, above the apex of which the tips of the stipules rise. Each of them is supported by a strong vein which has its origin at the base of the true petiole. But instead of being free from one another as in Rosa, the stip ules are connected back of the petiole by a hyaline ligular tissue. The lateral portions of the sheathing petiole are also united to one another on the opposite side of the stem, at least in young leaves, to a considerable degree. Thus an ochrea is formed, not quite a typical one it is true, yet more nearly so than that of Polygonum sagittatum L. ( fig. 32 ).