When the ligule has developed sufficiently to require special support, it is supplied by the introduction of vascular bundles. These bundles have their origin most frequently as tangential branches of the main leaf-bundles at their point of passage from the sheathing petiole into the true petiole, or, where the latter is undeveloped as in the grasses, into the blade. This mode of ori gin of the ligular bundles is seen in some of the tropical grasses and in the ligular portion of the stipule of the Naiadacea; and the ochres of the Polygonaceac. Rich ardia shows au exceptional ve nation of the ligule.
The best marked examples of the sheathing petiole among the Monocotyledones are found in the Araeea3, the Cyperacew and the GranainetB. If we examine a developing plant of the common hot-house calla (Richardia iifricana Knuth.), the first leaf (fig. 25) is seen to be a short, broad sheath, the second (fig. 2G) has in creased to a considerable length and the apical and axial tissues have developed into a minute blade and petiole. The third leaf is of the adult form, but smaller, though all the parts have increased very much in size. This is contrary to what is observed in Ranunculus where the sheathing petiole degenerates while the other parts advance. The margins of the sheathing petiole of Richardia curve inward at their apices and meet in the middle line of the leaf as in the case of Thalictrum polygamum L., but they are much broader and form a distinct ligule which is sup ported by the incurving and union of the marginal veins of the sheath instead of by tangential branches. In Ariseema triphyl lum (L.) Torr., the transition is not so well marked, owing to the small number of leaves the first of which is but a sheath as in Richardia, while the second bears a mature lamina.
Scirpus polyphyllus Valli. (fig. 27) will serve well to illustrate the ligule in the Cyperacere. It is but little developed as a slight hyaline outgrowth upon the ridge at the union of the sheath and lamina, but the sheath is closed, as is typical in the family. and a little farther development of marginal tissue would produce an ochrea. Typical of the ligule in our common grasses is that of Phalaris arundinacea L. (fig. 28). It consists of a considerable outgrowth of hyaline tissue which is continuous laterally with the marginal hyaline tissue of the sheath. This continuity strongly
supports the position taken as to the origin of the ligule. The purpose of the ligule is evidently to prevent the flow of water from the upper parts of the leaf down between the sheathing pet iole and the stem which together with the axillary bud it invests and protects, and neither the ligule nor the primitive ridge which bears it are found in those cases where the sheathing petiole does not closely invest the stem, at least in the early stages of growth, and its purpose could not be in any considerable measure ful filled.
The usually axillary position of the " stipule " in the Naiada cere has occasioned considerable discussion as to its real re lation to the ligule of grasses and to stipules proper. That it is in reality a development of the lateral portions of the primi tive leaf, and that it corresponds to the ligule together with the margins of the sheathing petiole of grasses and is rendered more or less nearly axillary by the degeneration of the central basal portion, becomes clear from the fact that in some species of N.aiadacere the sheathing petiole retains a considerable degree of what should be regarded as its ancestral development, and a con dition approaching that which occurs in grasses is found. Po tamogeton crispus L. is one of our species which will serve well for an illustration. The first leaves do not develop a blade, but the lateral and central-basal portions are well developed. In the adult leaves there is present a true sheathing petiole (fig. 29). The fibro-vascular bundles of the central-basal portion pass into the blade, giving off tangentially, at the point of transition from sheath to blade, the bundles of the ligular part of the stipule. The bundles of the lateral portions do not in this case curve about to join those entering the blade but are prolonged upward, re maining parallel and supplying the lateral portions of the stipule with supporting tissue directly. In illthenia filiformis Petit. (fig. 30), the conditions are more primitive in the larger relative development of the lateral and central-basal portions. In Ruppia the ligule is not developed, and the tips of the lateral portions are free as in ordinary adnate stipules.