The leaves of water-plants are as a rule flat or rolled, not folded.

The consistence of the leaf exercises much influence, for thick, leathery, or fleshy leaves, such as those of Mistletoe (Viscum), Aloe, many Crassulas, Saxifragas, &c., naturally are as a rule flat; sometimes rolled; but rarely folded. Pinnate and trifoliate leaves are generally folded.

In Monocotyledons the leaf rises as an elevation immediately below the growing-point, gradually surrounding part of the stem, and highest at the point opposite the opening. The ridge extends until it forms a sheath more or less completely surrounding the stem.

The leaves of Palms and Aroids develop in a manner very unlike that usual among Dicotyledons. The leaf originally forms a continuous blade, which is much folded, and subsequently divides. According to Eichler, this is effected by a process of disorganisation at the edge of the fold. A similar process occurs in the segmentation of the originally entire thallus of species of Laminaria, a common genus of brown seaweed. Naumann (11), on the contrary, maintains that the cells along the line of the fold detach themselves and diverge from one another. I believe, however, that Eichler's view is correct.

F

CHAPTER IV

ON THE PROTECTION OF BUDS

YOUNG leaves are extremely delicate.

They require

protection from too great heat, cold, dryness, moisture, and light, as well as against the attacks of animals.

For this purpose they are often provided with a covering of more or less woolly hair, which generally drops off when it has served its purpose.

In other cases the outer envelopes of the bud are specially modified for the purpose.

As Grew (12) quaintly says: 'According to the Form and Foulding of every Leaf or Germen, is its Protection order'd; about six ways whereof may be observed; sc. by Leaves, Surfoyls, Interfoyls, Stalks, Hoods, and Mantlings. To add to what we have above given, one or two Instances. Every Bud, besides its proper Leaves, is covered with divers Leafy Pannicles, or Surfoyls; which, what the Leaves are to one another, are that to them all. For not opening except gradually, they admit not the Weather, Wet, Sun or Air, to approach the Leaves, except by degrees respondent, and as they are gradually inur'd to bear them. Sometimes, besides Surfoyls, there are also many Interfoyls

set betwixt the Leaves, from the Circumference to the Center of the Bud; as in the Hasel.'

Here he uses the term 'Surfoyls' for the outer scales, whatever their true character may be. His Interfoyls' are stipules.

Besides the protection afforded by actual coverings, the opening buds in many cases guard themselves, to some extent, at any rate, by assuming either an erect position, as in the Whitebeam (Pyrus Aria) (Pl. II. fig. 5), Acer platanoides (Pl. II. figs. 2–4), and, I believe, most shrubs with opposite leaves; or a drooping attitude, as in the Lime (Tilia) (Pl. I. figs. 1–3), Beech (Fagus) (Pl. IV.), Hazel Nut (Corylus), Elm (Ulmus) (Pl. III.), &c. The great leaves of Palms are at first vertical.

One advantage thus gained is that the radiation is much less than it would be if the leaves were to assume at once their permanent horizontal position. Darwin has shown that this position really does tend to check the effect of radiation. Our experiments, he says, 'show that leaves compelled to remain horizontal at night suffered much more injury from frost than those which were allowed to assume their normal vertical position.' 1

'We exposed on two occasions during the summer to a clear sky several pinned-open leaflets of Trifolium pratense, which naturally rise at night, and of Oxalis Movements of Plants, p. 286.

purpurea, which naturally sink at night (the plants growing out of doors), and looked at them early on several successive mornings, after they had assumed their diurnal positions. The difference in the amount of dew on the pinned-open leaflets and on those which had gone to sleep was generally conspicuous, the latter being sometimes absolutely dry, whilst the leaflets which had been horizontal were coated with large beads of dew. This shows how much cooler the leaflets fully exposed to the zenith must have become than those which stood almost vertically, either upwards or downwards, during the night.

From the several cases above given there can be no doubt that the position of the leaves at night affects their temperature through radiation to such a degree that, when exposed to a clear sky during a frost, it is a question of life and death.'1

It is probable, however, that in some cases, at any rate, this position is assumed as a protection against too intense light.

are:

The principal modes by which buds are protected

1. By the expanded base of the preceding leaf.
2. By scales which represent modified outer leaves,
as in Willows, Ash.

3. By the outer leaf-stalks, as in Maples, Ash.
4. By stipules, as in the Beech, Oak.

Loc. cit. p. 293.

5. By the two connate stipules of a leaf, as in the Elm, Spanish Chestnut.

6. By two connate stipules belonging to different leaves, as in the Hop.

7. By spines.

8. By furry hair.

9. By gum, resin, or mucus.

In considering the protection of buds in their youngest stages we must distinguish between two different cases: the first, where the young bud as a whole is protected by older organs; and the second, when the future leaves are protected by the outer budscales.

The cases in which the young bud is protected by older organs may be divided into four principal categories:

1. Those in which the protection is mainly afforded by leaf-blades.

2. Those in which the protection is mainly afforded by the petiole or by the leaf-base.

3. Those in which the protection is mainly afforded by stipules.

4. Those in which the protection is mainly afforded by hairs.

PROTECTION BY LEAVES

In hot countries, where the young bud principally requires protection from the scorching rays of the sun, this is often effected by their simply lying under the