are basifugal (fig. 98), as in the Pea, Ailanthus, Mahonia, &c. A similar difference in the mode of growth may be shown to occur even when there are no leaflets. Thus, Steinheil made a nick with a knife on a young growing leaf of Mesembryanthemum, half way up, and found that in the full-grown leaf it was much nearer the apex than the base. M. deltoideum has a number of small fleshy points, especially one at the summit, and two at each side. These are in proportion much nearer FIG. 97.-TO ILLUSTRATE FIG. 98.-TO ILLUSTRATE the base in the young than in the full-grown leaf. Again, in Urtica biloba the notch at the apex of the leaf, from which the species takes its name, reaches in the young leaf to the centre, but in the full-grown leaf only a third or a quarter of the length. In pinnate leaves the insertions of the pinnæ are, as Steinheil says, marks written by Nature on the leaves, and when, as in many cases, they are nearer together at the base, the upper leaflets are the older and the lower ones younger. Thus, a growing leaf of Asclepias syriaca may have six pairs of lateral nerves in the upper half of the leaf and eighteen in the lower. In the perfect leaf, on the contrary, each half bears thirteen. In some cases, however, the growth of the leaf as a whole and that of the separate leaflets follows different lines. Trecul, moreover, has shown that the development of leaves cannot in all cases be brought under these two categories. In Centaurea, for instance (fig. 99), the central lobe is formed first, and others appear successively, both upwards and downwards. There has been much difference of opinion whether any, and, if so, what, other modes of development exist in addition to those already mentioned, but from my special point of view it is not necessary to into this question. go FIG. 99. - YOUNG LEAF OF CENTAUREA larger leaf embraces a younger one at its base. GALIUM The following figures, after Eichler, illustrate the development of the leaf in the Ladies' Bedstraw (Galium Mollugo). Fig. 100 gives a vertical view of the tip of a shoot. Round the central growing-point is a nearly circular ridge. Immediately below this is another ridge, which shows a number of small eminences- -two larger, right and left, which are the rudi ments of the leaf-blades; and four smaller, at the four corners, which represent the rudiments of the four leaf-like stipules, two belonging to each leaf. Fig. 101 represents a rather more advanced stage, in which two other knobs (one of which is already indicated at the top of the left-hand figure) have made their appearance, which complete the ring of LEAF DEVELOPMENT IN GALIUM MOLLUGO. Highly magnified. eight so-called leaves. This figure is taken from a lower node, the leaves of which will be at right angles to those of the previous node. Hence the leaf-rudiments which were to the right and left in the first figure now stand above and below, while the last added rudiments stand right and left. The four true stipulerudiments still occupy the corners. Some leaves have what is known as anocrea.' This is a sheath which, taking its origin at the base of the leaf, surrounds the stem above it for a greater or less distance. Ocreas occur in the Plane, Dock (Rumex), Polygonum, Pondweed (Potamogeton), &c. THE PLANE (PLATANUS) The leaf of the Plane originates in a ridge which FIGS. 102, 103.-LEAF DEVELOPMENT IN PLATANUS OCCIDENTALIS. The upper figure shows the growth of the first leaf-ridge (01) round the growingpoint, A. The ridge is not yet complete, being still open towards the front. The outermost row of cells is the inner surface of the base of the protecting leaf-stalk. The lower figure (transverse section) shows three leaf-ridges (o', 02, 03) surrounding the growing-point, A. The youngest ridge (o') is still incomplete. runs round the whole axis immediately below the growing-point. At one place it raises itself more than elsewhere, and develops two side processes-the two primary leaflobes. These soon develop two side lobes. In the meanwhile the conjoined stipules gradually close over the growing-point, and finally cover it entirely. They do not, however, coalesce in the centre, but only touch. At a later period the edges turn outwards and assume a leaf-like form (fig. 136). The account given by Benjamin (10) is somewhat different, but I believe that of Eichler, confirmed by Mikosch, is substantially correct. POLYGONUM In Polygonum also the leaf arises as a small projection just below the growing-point, and gradually extends round it. This ring, which originates the primordial leaf, is higher at one place, which will develop into the leaf-blade. The rest of the ring grows and extends across the base of the leaf until it entirely surrounds the growing-point, forming the ocrea, and often projects some way beyond the growing-point. It completely covers the bud, which, however, eventually forces its way through. VERNATION The attitude of leaves in their very young stage is termed by botanists 'vernation.' |