The fact that such forms as these can occur in the same family of plants along with typical stipules, both adnate and free, goes to show how small is the real difference between the various stipular forms. Not all stipules possess supporting tissues but, just as is the case in the ligule of most grasses, may be without any fibro vascular bundles whatever. This is the ease in Vitis, in Partheno cissus and Hydrocotyle. Vitis Labrusca L. (fig. 73) shows a somewhat thickened central streak at the base of the membranous stipule, but in Hydrocolyle Americana L. (fig. 74), the thickness is uniform and the stipule very thin. These facts give some au thority to the supposition that the pectinate interpetiolar appen dages which occur in the Composite Willouyhbya scandens (L.) Kuntze (fig. 75) are true stipules. They are hyaline in texture, without supporting tissue, and may possibly be merely of epider mal origin. To determine this point requires opportunity to ex amine their development.
It is of importance to state that the tendril of the Cucurbita cere, regarded by many as a stipule, has been determined by ana tomical examination to represent the first leaf of the axillary bud.* The spines of Xanthium spinosum L., simulating stipules in position, are degenerate pistillate flowers. As proof of this, they often bear a greater or less number of hooked prickles like those of the flowers, and there may be a spine on one side and a flower on the other, showing them to be of the same significance.f The stipules of Comptonia pereyrina (L.) Coulter (fig. 76) de nied by some to be properly so-called, do not differ anatomically from other stipules notwithstanding their peculiar morphology, and are to be included unifier the term. One of the chief reasons for their exclusion seems to have been the absence of stipules in Myrica. This is doubtless a case parallel with that of Viburnum, of which most of the species have lost their stipules by degener ation.
While it is not a generally accepted view, there is no good reason why stipules should not sometimes be distinguishable in floral parts. They are clearly present in the sepals of Rosa and Rhodo typus, and the smaller intermediate lobes of the calyx of Polentilla probably represent pairs of united stipules, one from each neigh boring calyx-lobe in the manner of interpetiolar stipules.T The teeth of the filament in Deutzia are very suggestive of stipules in stamens, and the corona of Silene may very probably represent a ligule. The glands of the leaves of Ranunculacete which have been homologized with stipules, as already stated, can often be traced up into the flowers and are familiar in connection with the petals of Ranunculus.
One of the most interesting families of plants in the develop ment of its stipules is that of the Rubiaeefe, the development being very unusual in the group of the Stellatm. Though the
foliar anomaly in this group was early remarked upon and was anatomically explained as early as 1840,* there are considerations which make its present discussion desirable.
In the greater part of the family the leaves are opposite, or oc casionally in whorls of three as in Cephalanthus occidentalis L., and are usually stipulate. The stipules are of variable character and often interpetiolar, the adjoining stipules on each side of the stem being connate. In the group of the Stellate however, com prising ten or twelve genera, the stipules usually are apparently wanting and the leaves in whorls. There is a tendency toward a verticillate arrangement of the leaves in others of the Rubiacete, as shown by the frequent occurrence of whorls of three in usually opposite-leaved species. Now an anatomical examination of the whorled leaves of Nollugo verticillata L., Silene stellata (L.) Ait. f., Leptandra Virginica (L.) Nutt. and Cephalanthus occi dentalis L. reveals the fact that in other families, as well as in the Rnbiacele exclusive of the Stellate, each leaf of any whorl receives its fibro-vascular bundles directly from the cauline cylin der. But in Gallant the case is different. Two leaves only of the whorl receive their bundles in the manner stated, and only these two produce buds in their axils. All the others receive their vascular supply from what may be termed a nodal girdle, each half of which is formed by the union of two bundles arising, one from each of the two leaf-traces in the same rummer as those supplying stipules of the ordinary form. From this girdle arise the bundles which supply the additional leaves, whether there be only one on each side, as in Gallant circtezans Michx. and G. lanceolatum Torr., two, as in G. triflorum Michx. and G. tine torium L., or even three or four, as occurs in G. Aparine L. The distribution of the vascular bundles may be seen in a cross sec tion of the node of Galium tinctorium L. (fig. 77).
This anatomical arrangement shows that the so-called addition al leaves of the whorls in Gahm arc in reality stipules and that the Stellate agree with the rest of the Rubiacem in having oppo site leaves. The tendency of the family however to produce ver ticillate leaves has been strongly felt in this group but has taken an unusual course, the increased assimilative area having been evolved through the stipules instead of by an increase in the number of true leaves. The explanation is thus made compara tively simple except in those cases where the number of stipules at a node is more than four.