Leaves are dentate when the edge is intersected with acute, Pointed teeth, as in the Chestnut, Fiig. 100. Crenate when the Margin presents sail/ant parts, as in the Saxifrage, Fig. 101. They ate lobed when the leaf is more deeply indented, as in the Gingho,

Fig. 102, They are cleft when their division embraces one half of

Fig. 101.

Fig. 100.

Fig. 99.

Fig. 98.

Fig. 102.

Fig. 103.

Fig. 98.—Leaf of Robinia. Fig. 99.—Leaf of Gleditschia. Fig. 100.—Chestnut. Fig. 101—Leaf of Saxifrage.

Fig. 102.—Leaf of Gingko. Fig. 103.—Leaf of Bauhinia.

the leaf, as in the Bauhinia, Fig. 103, the leaf of which gives a very good idea of the cleft leaf. The leaf of Castor-oil tree (Ricinis communis), Fig. 104, is cleft in eight sections. Finally, leaves are partite when the separation penetrates thearly to the petiole, or reaches the centre of the limb, as in Fig. 105, a bipartite

a leaf, and again in Fig. 106, which represents the leaf of Cannabis 3:P4 sativa, the Hemp; as Fig. 107 does Echinops spherocephalus, and 3 as Fig. 108 does the leaf of Scholymus hispanicus, in which the

q divisions of the leaf are more numerous still. The leaf is, as we have said, a flattened organ having two surfaces, and border, the whole of which constitutes the /amina, or blade. The blade of every leaf is traversed by prominent lines or ridges called veins, which are, however, more saillant on the lower

face than on the upper. These are formed of woody tissues, spiral vessels and cellular tissues. They form the nervure of the leaf, and are retained in their positions, and the intervening space is filled up, by cellular tissue. The tissues of the veins are brought into closer proximity in the petiole, which is a small stem. Having passed into the stem, one part ef the food of the plant enters the bark, while the other traverses the wood and vay penetrates to the medullary sheath at the centre of the stem. Every leaf is thus in communication with {‘ the stem, and not this only: itis,in {* fact, aprolongation of the pith, spiral \ vessels, and wood of the system. The disposition of the nervure, or veins, differs very little in the three principal types. In the Chestnut, whose leaf is represented in Fig. 100, the nervure runs from the base ny to the summit of the blade, sending _Fig- 109—Leaf of the Marsh Mallow. out to the right and left a secondary set of veins parallel to each other, disposed like spray of a feather. In the Mallow (Malva sylvestris), Fig. 109, five principal nerves run from the base of the leaf to the apex, and radiate in the blade, like the foot of a web-footed bird, In the Iris, of which leayes are represented in

Fig. 30, p. 30, a great number of delicate veins run from the base G

Fig. 104.

PHl. ne CastorOil Tree, _Fig. 105.—A bipartite leaf. _Fig. 106.—Leafofthe Hemp:

Fig. 107.—Leafof Echinops. Fig. 108.—coLaat ofUeboipma.

of the leaf towards the summit, all being parallel to each other. The petiole, sometimes long, sometimes short, and sometimes absent altogether, is often cylindrical, sometimes arched and inflated as in the Water Caltraps (Zrapa natans), or compressed, as in the Birch-tree (Carpinus betulus), and many poplars, in which the surface is large, in place of being a continuation of the. blade, it abuts upon it at right angles. In such cases the petiole gives little support to the leaf, but presents its two largest sides to the wind, which cause it to oscillate and tremble, producing the rushing sound which distinguishes the Aspen-tree (Populus tremula). Tn some cases the petiole fails; the blade even may be defective;. the leaf is then reduced to its petiole. But in such cases, in obedience to the laws of compensation, which intervene when any organ p bortive,, the neichlg i g parts take a greater development; the petiole is enlarged and assumes a ribbon-like form, a sort of blade which was long taken for a leaf. It is distinguished by the position of its veins, and also by the fact that in place of being compressed in such a manner as to present the usual upper and under surface, it is set edgeways, and its two faces are lateral. This form of petiole is termed a Phyllodium, and is applied to so called leafless plants, where the petiole performs the functions of one. The Acacia heterophylla, Fig. 110, is full of instruction in this respect. We find in it all the intermediate steps between a perfect compound leaf and a phyllode. The petiole is there seen to flatten and enlarge in exact proportion as the leaf decreases; they bear leaflets at the earliest stage of their development, and have parallel veins, although occurring in exogenous plants. This transformation’of the petiole, which is frequent in the Acacias of Australia, occurs also in many other plants, as in Dionea muscipula, Venus — Fly-trap, in which the petioles extend laterally, and resemble true leaves; in the tendrils of the pea, and in some others belonging to the leguminous plants, as well as among the umbelliferous plants and the ranunculacew, the petiole grows longitudinally.

Leaves transform themselves into other organs with wonderful facility. It is, in fact, by modifications of the leaves that nature produces many essential organs in the life of plants. They are

changed into scales, a transformation of which we have an example in the Asparagus plant, Fig. 113; into tendrils, as we have already observed in the Pea, Fig. 111; and into spines, as in the Barberry (Berberis), Fig. 115, for spines would have become branches under other evolutions of the axillary leaf-bud. Now what is the source of this disposition of leaves upon the _ stems and branches which carry them? Are they thrown at random upon the axes of the vegetable? The most superficial examination suffices to satisfy us that the leaves are always placed in

( %, = A n

Fig. 110.—Acecia heterophylla.

the same manner and in a fixed order for every species of plant. 10 other words, that their relative distance and time are rigorously: fixed by nature. A more profound examination leads to the conviction that this order is subject to certain laws, and may expressed by an arithmetical formula. If we examine a branch of we observe the Elm (Fig. 116), of the Willow, or the Cherry-tree, at once that the leaves are all inserted at different heights. In

Fig. 114.

Fig. 112.

Fig. 111.

Fig. 113.

Fig. 111.—The Pea. Fig. 112.—The Nettle. Fig. 113.—Asparagus. Fig. 114.—Rose Laurel.

[illegible artist signature]

this case they are said to be alternate. In the Willow, the Nettle (Urtica), Fig. 112, on the contrary, the leaves are grouped in pairs all at the same height. These leaves are said to be opposite. In the Loose-strife (Lysimachia vulgaris), and the Rose Laurel, Fig. 114, three leaves are grouped at the same height round the stem. In this case, as also in cases where many leaves are grouped i. the same manner, we say the leaves are verticillate.

. )

Fig. 115.—The Barberry. Fig. 116.—Branch of the Elm. »

What gives to plants their peculiar physiognomy and appeal ance, is, that the elements which constitute the verticillate feature correspond with the intervals which separate the pair or gToUP above or below, and it may be added that the: placed immediately elements of a similar pair or verticille are always equidistant. bs. return, however, to the alternate leaves. Let us take a branch 0 the Peach or the Plum-tree, Fig. 117, and examine any leaf what- | ever. We shall find that higher up, the branch carries another P| leaf immediately above it, and that in the interval between these a

two leaves there are four others diversely placed. All these leaves are placed upon the line of an ideal spiral, which commences in one leaf and terminates in the leaf on the line

unit of the spiral may consist of one or more revolutions of the spiral. round the axis. Fixing on any leaf and ascending the branch till the

is found, by counting all the intermediate ones the number constituting the unit or cycle is arrived at. In the Peach and Plum-tree the cycle embraces / five leaves, and the spiral goes twice round the @y branch. This is expressed in botani lane guage by the Ay ye

| ‘| fractional for- )

ifmula a the nu- Fig. 117.—Branch of the Peach ‘| merator indicating the number of turns the spiral makes on the cycle, the denomimator the number of leaves which constitute the cycle, as in Fig. 118. In the Alder (Alnus glutinosa), Fig. 119, three leaves constitute the cycle;and the spiral only describing a single turn on the stem, the disposition of its leaves is represented by the fraction }, as exhibited in Fig. 120. fi In the Elm (Udnus), as we have seen in Fig. 116, and Big.us,he in the Lime-tree, two leaves only constitute the cycle,in Peach leaves. which case they are represented by the fraction 4. Let us write these three fractions in a single line,4, 3,3, and we find the fraction 3 is the sum of the two first fractions. Let us add the _

term of the 4 and 2, we obtain $. Do the same for 2 and 3, and we obtain;‘;, In this manner the terms 4, 4, 2, 3, i%s, $4, are obtained. Singularly enough these fractional formulas express precisely the disposition of leaves which nature realises. The denominators of these fractions, in giving the leaves of each cyele, indicate at the same time the vertical lines following which the are ranged. leaves Thus in the Elm and the in the Lime-tree, are disposed leaves and are in two rows, said to be distichous, in which it agrees with the Ivy (Hedera and the Yew helix), (Tazus banata). In the Alder, the Meadow Saffron (Colchicum autumnale), the leaves are disposed in three rows, and said to be tristichous. In the Peach-tree, where the leaves are disposed in five rows, they are said to be quincuncial. “The distribution of leaves upon the branches,” says De Candolle, “is in accord with their functions, which are almost exclusively determined by the action of the sun. [| In order that this action should exercise itself properly,4. it is necessary that the leaves be wide apart one from & the other. We have seen that in all the systems by Eteeag which the positions of the leaves are arranged, it results ee ofeethe that the leaves which bear immediately above others are j never covered. In cases the least favourable, the third only covers — the first, and the fourth the second. In another case it is the ©

Page 11 of 93