Page:An introduction to physiological and systematical botany (1st edition).djvu/250

 220 just above their insertion. The European Rubiaceæ have whorled leaves, as Asperula, Galium, Rubia, &c.; but Asperula cynanchica, ''Engl. Bot. t.'' 33, has sometimes two of its four leaves so small as to look like stipulas, seeming to form an intermediate link between such as have whorled leaves and such as have opposite ones with stipulas. The next step from Asperula is Diodia, and then Spermacoce. In the two last the bases of the stipulas and footstalks are united into a common tube.

Some stipulas fall off almost as soon as the leaves are expanded, which is the case with the Tulip-tree, Liriodendron tulipifera; in general they last as long as the leaves.

The absence or presence of these organs, though generally an indication that plants belong to the same natural order and even genus, is not invariably so. Some species of Cistus have stipulas, others none, which is nearly the case with grasses. The stipula in this, one of the most distinct of all natural orders, is peculiar, consisting of an internal white membrane crowning