less concave, becoming slightly twisted. The axillary buds are sheltered in their axils. The terminal bud is amply protected by them. The plant forms a spreading tuft like a species of grass with coarse leaves. The same causes which have led to the length and narrowness of the leaves of grass have probably influenced Lathyrus Nissolia, as well as other species which grow under similar circumstances. The resemblance is perhaps, also an advantage in preventing its being picked out and eaten by browsing quadrupeds. CHAPTER VII ON THE SUBSIDIARY USES OF STIPULES THE general use of stipules is, as we have seen, to cover and protect the bud. This is, however, by no means the only function they perform. Though very FIG. 308.-SHOOT OF HOLLY, showing successive leaves (", ", ") and stipules (st, st). often leafy, they are generally too small to be of much use as organs of assimilation. There are, however, various gradations. Some, indeed, are quite minute, as in the Holly (fig. 308) and Hymenanthera (fig. 29, p. 24). In other cases, though smaller than the leaf-blades, the stipules assist in performing similar functions. In some the assimilation must be trifling, as in the stipulate species of Helianthemum, in Ribes, and many others; in others it is substantial, though less than in the leaf-blades, as in the Roses; in others equal to that of the leaf-blades, as in the Pansy (fig. 309), where the stipules are large, oblong, and pinnatifid, with three to six linear lobes on the external side; or Galium (Ladies' Bedstraw), where they are indistinguishable in form from the leaves. Linnæus long ago pointed out that though the Stellata (Galium, Rubia, &c.) are generally described as having their leaves in whorls of four, six, eight, or more, there are, as a matter of fact, only two leaf-blades in the ordinary sense, and the other leaf-like organs are really stipules. De Candolle expressed the same opinion, and pointed out that buds are not produced at the base of all the foliaceous appendages, but only of those corresponding to true leafblades. Where there are six leaflets, these correspond to two leafblades and their four stipules. Where there are only four leaflets, this is considered to be due to a coalescence of stipules by pairs, as in the case of the Hop. FIG. 309.-LEAF OF Acacia verticillata (fig. 310) and some nearly allied species constitute an instructive and interesting case. A. verticillata has linear, pointed, laterally compressed phyllodes, arranged in whorls, so that it has very much the look of a strong Galium. Buds only occur here and there along the stem, and the phyllodes generally have no stipules, their presence depending 1 Praelect. in Ord. Nat. Plantarum, 1792, p. 520. on whether there is or is not a bud. If there is no bud, there are no stipules, while if a bud is formed, stipules are also developed. In these and other similar cases the stipules must to some extent assist the leaves, which, however, are also well developed. In Lathyrus Aphaca (fig. 304), on the contrary, as already mentioned, the leaf-blades have disappeared as FIG. 310.-SHOOT OF ACACIA VERTICILLATA, × 13. assimilating organs, and are replaced by two large stipules, which are sessile, triangular, and provided with two large auricles, directed outwards. In some species of Adesmia (a South American leguminous genus) leaves are present only on the lower part of the long, straggling branches, the upper part bearing stipules only, which gradually pass into bracts (fig. 311). In the Urticaceae stipules are almost invariably present, and in their form and position afford good characters for the distinction of genera and larger groups. In the closely allied tropical genera, Elatostema and Procris, the leaves of succeeding pairs FIG. 311.-ADESMIA BRACTEATA. Two-thirds nat. size. become more or less separated and very unequal, one being reduced to a very small blade, or it becomes quite suppressed and is replaced by the stipules. A similar reduction of leaf and replacement by the stipule occurs in Forskohlea. |