creasing in size, decrease in number. At first there may be three leaflets, and then only a single large leaflet (fig. 38, G).

Finally, towards the

floral end of the branch the leaflet becomes smaller and smaller until it disappears altogether (fig. 38, H and 1). The apical leaves, like the basilar leaves, are eventually reduced simply to the hypopod. 2. Serratula centauroides. The foliage leaves of this species are close to the ground, their bases are ensheathed, and the long petioles terminate in rhomboidal blades.

FIG. 40.-Sagittaria sagittifolia. A, winter bud with membran ous leaves enclosing the swollen portion and ro led leaves protecting the bud; B C, submerged leaves; D,E, floating leaves; F, aërial foliage leaf; G, leaf protecting the flower-bud; H, seedling grown from a seed planted in the water on the surface of the mud. I, seedling from seed planted in the water under a layer of mud. In figures H and I, c is the cotyledon, I the first leaves.

In passing on from these assimilative leaves to the basilar and apical leaves, a double adaptation becomes apparent, as in Rosa rugosa. In Rosa rugosa, however, both adaptations were protective, while in Serratula centauroides this

is only the case with the basilar leaves, which surround the winter buds.

The modifications of the basilar and apical leaves of Serratula centauroides are similar to those of Rosa rugosa, and it is therefore unnecessary to repeat the description.

Those of the apical leaves tend towards their protection from rain. When rain falls upon the radical leaves, they are flattened out upon the ground, which prevents them from being torn. This is not so with the apical leaves, and had their primitive structure remained unchanged, they would have been constantly exposed to destruction. M. Stahl has shown how these leaves have protected themselves by shortening their petioles and fringing their edges. In this way the leaf has acquired greater resistance, and the water is better enabled to run off it. This modification, however, although one advantageous to the plant, has involved the partial degeneration of the petiole and the blade.2

3. Sagittaria Sagittifolia.—The Sagittaria is an 1 E. Stahl, Regenfall und Blattgestalt (Ann. du Jard. bot. de Buitenzorg, vol. xi., 1893).

2 In the leaves (fig. 39, в and c), immediately above the foliage leaves, the petiole gradually disappears altogether, while the blade has become deeply serrated, thus providing for the water drainage while retaining an assimilative surface.

Further up the stem the leaf blades are more and more reduced, until eventually they are no longer differentiated into hypopod and epipod, but arise directly from the growth of the primordial leaf (fig. 39, D to F).

aquatic plant, the foliage leaves of which emerge from the water. The blade of these leaves (fig. 40, F) is shaped like an arrow-head, and the barbs facilitate rain-water drainage. The petiole is very long, and terminates in a sheathed base derived from the hypopod.

The apical leaves (fig. 4, G) are represented by transparent membraneous scales, which serve as a protection to the floral buds.

The basilar leaves, which either float on the surface of the water or else are entirely submerged, have undergone both development and degeneration. Degeneration is more noticeable on glancing downwards from the foliage leaves to those which protect the winter buds.1

1(a) The leaves nearest to the foliage leaves float upon the surface of the water (fig. 40, E). The lateral barbs, being no longer required, have begun to degenerate. (b) The floating leaves immediately beneath exhibit barbs which are still more reduced (fig. 40, D), and there are no stomata except those upon the upper surface. (c) The next leaves are completely submerged and lengthened out into long ribbons which broaden into flat blades at their upper ends, which are not separated from the hypopod by petioles (fig. 40, c). In this case the blade is very much smaller, and the petiole has disappeared. (d) The next leaves to these are submerged, and are ribbonlike in shape. They spring immediately from the primordial leaf without differentiating into hypopod, mesopod, and epipod.

(e) After these leaves follow leaves consisting solely of a hypopod, which is much reduced in size (fig. 40, A). These leaves are folded round, and serve as a protection to the bud.

Lathyrus Aphaca.-The foliage leaf (fig. 4, leaves 4 and 5), which most nearly resembles the primitive type,1 consists of two stipules and a petiole which terminates in a point and is provided with a pair of lateral leaflets.

Each individual plant is furnished with from one to three of these foliage leaves.

Above them are leaves (fig. 41, leaf 6) consisting of only a hypopod which forms two large stipules that partially enclose a small point arising from the epipod. The entire function of assimilation is exercised by these stipules, and consequently both (f) Finally, there are some scaly leaves which also consist merely of a hypopod. These protect the tubers (fig. 40, A), and are practically devoid of chlorophyll. These various leaves follow one another in the course of the growth of the plant, in the inverse order to that in which they are here described. As winter approaches, some upright stems appear, the inflated ends of which are stored with reserve nutrition (fig. 40, A). It is round these tubers that the scaly leaves are to be found. The bud which does not develop until the following Spring is at the top, surrounded by folded leaves. The ribbon-like leaves which grow under water and also those broadened out at the ends make their appearance in the course of the Summer. Next come the floating leaves with slightly developed barbs, then the leaves which rise well above the surface of the water, and finally the scaly leaves which grow out of the flowering stalk.

1 The foliage leaves of Lathyrus Aphaca may be regarded as the best representatives of the primitive leaf, for in most species of the genus Lathyrus, each leaf is furnished with leaves and with one or more tendrils. L. Aphaca is probably specialized from other species, for the majority of the leaves have lost their lateral leaflets.

the petiole and the lateral leaflets have atrophied.

FIG. 41.-Seedling of Lathyrus Aphaca. 1, 2, 3, very rudimentary leaves; 4, 5, foliage leaves; 6, leaf composed only of a pair of stipules. 7, 8, terminal leaves turned into tendrils.

Higher up on the stem (fig. 41, leaves 7 and 8) are some leaves consisting of two stipules and a nonbranching tendril. The latter represents the mesopod and the epipod from which the lateral leaflets have disappeared. Below the foliage leaves, there are usually three leaves which have considerably degenerated (fig. 41,leaves 1 to 3). The hypopod is represented by two very small stipules, and the epipod by the tiny point in the centre. The portion of the stem out of which the basilar leaves would grow does not, under normal conditions, emerge from the soil, which accounts for the way in which these leaves have degenerated.

5. Nymphaea dentata. -Here the foliage leaves are large, floating, fleshy.