instance, the Mistletoe and Mesembryanthemum. This is usually the case with fleshy leaves. Among the most common modes of folding are the following, which are illustrated by the diagrams in fig. 104.

1. Conduplicate: when folded lengthways, doubled up on the midrib, as in the Rose, Bramble, Tulip Tree (fig. 104, A).

2. Plicate, or plaited: when folded like a fan on the ribs, as in the Maples, Currant, and most palmate leaves (fig. B).

3. Circinate when coiled like a crozier, as in the Ferns, Drosera, &c. (fig. C).

4. Revolute when rolled backwards from the margins, as in Azalea, Rosemary, and many plants of hot, dry places (fig. D).

5. Involute: when rolled inwards from the margins, as in Violets, Water Lilies, &c. (fig. E).

6. Equitant when the leaves are folded one over the other, as in Grasses, Sedges, Iris, and many other Monocotyledons (fig. F).

7. Convolute when rolled up from one margini.e. one margin within, the other without the coil, as in the Cherry, Peach, Pea, Vetch, &c. (fig. G).

There are sometimes considerable differences between nearly allied species; and in some cases the outer and inner leaves of the bud assume different positions.

The conduplicate arrangement seems to follow naturally from the mode of development described on p. 47. The rudiment of the leaf, spreading more or less round the growing-point, assumes a hollow form, and when it rises above the central axis would thus become folded on itself.

In pinnate leaves generally, as in those of the Rose, each leaflet is conduplicate.

In palmate leaves the folded arrangement is that naturally best calculated to enable them to fit into the bud.

The involute and revolute types perhaps have reference to the later life of the leaf, as affording a certain protection against too rapid transpiration.

The equitant type follows naturally from the mode of growth of many Monocotyledons.

Leaves of the same form may be folded in very different ways, and it is seldom that the same plan runs through a whole family, at least among Dicotyledons.

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

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.'

6

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.