THE ANGIOSPERMAE : LEA\ES ,;y3 



base carries a number of paired projections, which are usually tipped with 

 glands and might be taken for true stipules if gradations did not occur between 

 them and the nectar}- glands further up the petiole. Both sets of structures 

 are modified leaf teeth and belong to the laminar portion of the leaf which 

 runs down the petiole to the base in the form of two parallel ridges. Similar 

 examples may be found in other species. Stipules and pseudo-stipules may 

 both exist together in the same plant, as in Lotus, where the foliolar pseudo- 

 stipules cover and hide the glandular stipules. 



Plants with opposite leaves often have stipules which are placed between 

 the leaf bases and ensheath the young leaves of the next node above. Such 

 interpetiolar stipules are particularly striking in Galium, where only two of 

 the apparent leaves at each node are true foliage leaves, the rest, two to eight 

 in number, being stipules which closely resemble the leaves but do not receive 

 trace bundles directly from the stem stele as do the true leaves. 



True stipules may often be found united together. When the union is 

 along their inner margins they may form a single organ bridging across the 

 leaf axil. If, on the other hand, they fuse by their outer margins, the united 

 structure may be antidromous, that is, apparently opposite to the leaf. 

 Stipules belonging to opposite leaves may also unite to form partial or complete 

 sheaths around the node, as in the Rubiaceae. 



The ochrea, mentioned before, is a membranous sheath, arising from the 

 leaf base and surrounding the axillary bud and the stem for a short distance 

 above the node. It is a family character in the Polygonaceae, and also occurs 

 in some Ranunculaceae, such as Caltha. It has been interpreted as derived 

 from the axillary fusion of two stipules, but it develops as a single sheathing 

 organ, and seems to be better regarded as a tubular upgrowth of the leaf base 

 and not of stipular nature. 



Whatever be their morphological origin, stipules and pseudo-stipules 

 definitely function as protections for the leaf rudiment, which thev often 

 precede in development, and for the axillary buds. In manv cases they are 

 caducous, that is, they are shed when the leaf approaches full development, 

 thus show-ing that their chief importance is then past, but in others thev are 

 retained, and, by carrying on photosynthesis, they contribute something, 

 though in most cases probablv not much, to the nutrition of the plant. 



Three types of leaf structure remain to be mentioned. The first are called 

 intravaginal scales because they appear in the axil within the leaf sheath or 

 vagina of certain Monocotyledons, especially members of the Helobiae. 

 They take the form of small tooth-like scales, generally linear or lanceolate, 

 and they actively secrete mucilage (Fig. 980). They are not stipules, since 

 they may occur in association with stipules or even, as in Hydrocharis, in 

 their axils, and they are generally regarded as trichomes. They may be single 

 or serial, and according to Arber they arise either from the surface of the axis 

 or from the dorsal tissues of the base of the leaf immediately above them. 



Secondly, the leaf base may be enlarged into a wide sheath, or vagina, 

 more or less encasing the stem (see Fig. 970, p. 979). This is commonest in 

 Monocotvledons, but is also characteristic of certain dicotyledonous families, 



