pass either from right to left or from left to right. It sometimes follows a different direction in the branches from that pursued in the stem. When it follows the same course in the stem and branches, they are /zomodromozis; when the direction differs, they are heterodramaus. In different species of the same genus the phyllotaxis frequently varies.”

All modifications of leaves follow the same laws of arrangement as true leaves-a fact which is of importance in a morphological point of view. In clicotyledonous plants the first leaves produced (the cotyledons) are opposite. This arrangement often continues during the life of the plant, but at other times it changes, passing into distichous and spiral forms. Some tribes of plants are distinguished lvl

by their opposite or verticillate, others by their

alternate, leaves. Labiate

plants have decussate

leaves, while Boraginaceae

have alternate

leaves, and Tiliaceae usually

have distichous

leaves; Rubiaceae have

opposite leaves. Such

arrangements as § , § , T55

and -ff are common in

Dicotyledons. The first

of these, called a quincunx,

is met with in the

apple, pear and cherry

FIG. 34.-Cycle of thirteen leaves (fig. 32); the second, in

the bay, holly, Plantago

media; the third, in the

cones of Picea alba (fig.

35); and the fourth in

those of the silver fir.

In monocotyledonous

plants there is only one seed-leaf or cotyledon, and hence the arrangement is at first alternate; and it generally continues so more or less, rarely being verticillate. Such arrangements as é, é and § are common in Monocotyledons, as in grasses, sedges and lilies. It has been found in general that, while the number 5 occurs in the phyllotaxis of Dicotyledons, 3 is common in that of Monocotyledons. In the axil of previously formed leaves leaf-buds arise. These leaf-buds contain the rudiments of a shoot, and consist of leaves covering a growing point. The buds of trees of temperate climates, which lie dormant during the winter, are protected by scale leaves. These scales or protective appendages of the bud consist either of

FIG. 33.-Diagram of a phyllotaxis represented by the fraction § .

placed closely together so as to form a rosette, as in Seinpervivzim. A is the very short axis to which the leaves are attached. The leaves are numbered in their order, from below upwards. The circles in the centre indicate the five turns of the spiral, and show the insertion of each of the leaves. The divergence is expressed by the fraction Tfgths.

F IG. 35.~Cone of Picea alba

with the scales or modified

leaves numbered in the order

of their arrangement on the

axis of the cone. The lines

indicate a rectilinear series of scales and two lateral secondary spirals, one turning from

left to right, the other from

right to left.

the altered laminae or of the enlarged petiolary sheath, or of stipules, as in the Hg and magnolia, or of one or two of these parts combined. These are often of a coarse nature, serving a temporary purpose, and then falling off when the leaf is expanded. They are frequently covered with a resinous matter, as in balsam-poplar and horse chestnut, or by a thick downy covering as in the willow. In plants of warm climates the buds have often no protective appendages, and are then said to be naked.

The arrangement of the leaves in the bud is termed 'uernation or prefoliation. In considering vcrnation we must take into account both the manner in which each individual leaf is folded and also the arrangement of the leaves in relation to each other. These vary in different plants, but in each species they follow a regular law. The leaves in the bud are either placed simply in apposition, as in the mistletoe, or they are folded or rolled up longitudinally or laterally, giving rise to different kinds of venation, as delineated in figs. 36 to 45» where the folded or curved lines represent the leaves, the thickened part being the midrib. The leaf taken individually is either folded longitudinally from apex to base, as in the tulip-tree, and called reclinate or replicate; or rolled up in a circular manner from apex to base, as in ferns (ng. 36), and called circinate; or folded laterally, con duplicate (fig. 37), as in oak; or it has several folds like a fan, plicate or plaited (fig. 38), as in vine and sycamore, and in leaves with radiating venation, where the ribs mark the foldings; or it is rolled upon itself, con volute (fig. 39), as in banana and apricot; or its edges are rolled inwards, involute (fig. 40), as in violet; or 5 ig 55 E §

FIG. 36. FIG. 37. FIG. 38.

V V f .ag

Y ' J I ini' -=iT:"= /fri A

I '% "fi § 1*'°' if

fa; 5' T2 li

f7;;;i.;zf. FIG. 39. FIG. 40. FIG. 41 FIG. -Circinate venation.

FIG -Transverse section of a con duplicate leaf . FIG. -Transverse section of a plicate or plaited leaf FIG. -Transverse section of a con volute leaf. FIG. -Transverse section of an involute leaf. FIG. -Transverse section of a revolute leaf outwards, revolute (fig. 41), as in rosemary. The different divisions of 3 cut leaf may be folded or rolled up separately, as in ferns, while the entire leaf may have either the same or a different kind of venation. The leaves have a definite relation to each other in the bud, being either opposite, alternate or verticillate; and thus different kinds of venation are produced. Sometimes they are nearly in a circle at the same level, remaining Hat or only slightly convex externally, and placed so as to touch each other by their edges, thus giving rise to valvate venation. At other times they are at different levels, and are applied over each other, so as to be imbricated, as In lilac, and in the outer scales of sycamore; and occasionally the mar in of one leaf overlaps that of another, while it in its turn is ovefalapped by a third, so as to be twisted, spiral or contoftive. When leaves are applied to each other face to face, without being folded or FIG. 42. FI 43. FI . 44. I

FIG. 42.-Transverse section of a bud, in which the leaves are arranged in an ac cum bent manner. FIG. 43.-Transverse section of a bud, in which the leaves are arranged in an equitant manner. FIG. 44.-Transverse section of a bud, showing two leaves folded in an obvolute Inanner. Each is con duplicate, and one embraces the edge of the other.

FIG. 45.-Transverse section of abud, showing two leaves arranged in a super volute manner.

rolled together, they are oppressed. Vl/hen the leaves are more completely folded they either touch at their extremities and are ac cum bent or opposite (fig. 42), or are folded inwards by their margin and b€COII1€ in duplicate; or a con duplicate leaf covers another similarly folded, which in turn covers a third, and thus the venation 15 equitant (fig. 43), as in privet; or con duplicate leaves are placed so that the half of the one covers the half of another, and thus they b€C0lTl€ half-equitant or obvalute (fig. 44), as in sage. When in the C2156 of con volute leaves one leaf is rolled up within the other, it 15 § “P@f" volute (fig. 45). The scales of a bud sometimes exhibit one kind of venation and the leaves another. The same modes of arrangement occur in the flower-buds.

Leaves, after performing their functions for a certain time, w1tl}€r and die. In doing so they frequently change colour, and hence M158

the beautiful and varied tints of the autumnal foliage. This change