Section VII.LEAVES.

Fig.98. A piece of the running rootstock of the Peppermint, with its node or joint, and an axillary bud ready to grow.

106. Rootstocks are more commonly thickened by the storing up of considerable nourishing matter in their tissue. The common species of Iris (Fig.164) in the gardens have stout rootstocks, which are only partly covered by the soil, and which bear foliage-leaves instead of mere scales, closely covering the upper part, while the lower produces roots. As the leaves die, year by year, and decay, a scar left in the form of a ring marks the place where each leaf was attached, that is, marks so many nodes, separated by very short internodes.

107. Some rootstocks are marked with large round scars of a differentsort, like those of the Solomon's Seal (Fig.99), which gave this name to the plant, from their looking somewhat like the impression of a seal upon wax. Here the rootstock sends up every spring an herbaceous stalk or stem, which bears the foliage and flowers, and dies in autumn. Thesealis the circular scar left by the death and separation of the base of the stout stalk from the living rootstock. As but one of these is formed each year, they mark the limits of a year's growth. The bud at the end of the rootstock in the figure (which was taken in summer) will grow the next spring into the stalk of the season, which, dying in autumn, will leave a similar scar, while another bud will be formed farther on, crowning the ever-advancing summit or growing end of the stem.

Fig.99. Rootstock of Solomon's Seal, with the bottom of the stalk of the season, and the bud for the next year's growth.

108. As each year's growth of stem makes its own roots, it soon becomes independent of the older parts. And after a certain age, a portion annually dies off behind, about as fast as it increases at the growing end, death following life with equal and certain step, with only a narrow interval. In vigorous plants of Solomon's Seal or Iris, the living rootstock is several inches or a foot in length; while in the short rootstock of Trillium or Birthroot (Fig.100) life is reduced to a narrower span.

Fig.100. The very short rootstock and strong terminal bud of a Trillium or Birthroot.

109. An upright or short rootstock, like this of Trillium, is commonly called aCaudex(93); or when more shortened and thickened it would become a corm.

110.A Tubermay be understood to be a portion of a rootstock thickened, and with buds (eyes) on the sides. Of course, there are all gradations between a tuber and a rootstock. Helianthus tuberosus, the so-called Jerusalem Artichoke (Fig.101), and the common Potato, are typical and familiar examples of the tuber. The stalks by which the tubers are attached to the parent stem are at once seen to be different from the roots, both in appearance and manner of growth. The scales on the tubers are the rudiments of leaves; the eyes are the buds in their axils. The Potato-planthas three forms of branches: 1. Those that bear ordinary leaves expanded in the air, to digest what they gather from it and what the roots gather from the soil, and convert it into nourishment. 2. After a while a second set of branches at the summit of the plant bear flowers, which form fruit and seed out of a portion of the nourishment which the leaves have prepared. 3. But a larger part of this nourishment, while in a liquid state, is carried down the stem, into a third sort of branches under ground, and accumulated in the form of starch at their extremities, which become tubers, or depositories of prepared solid food,—just as in the Turnip, Carrot, and Dahlia (Fig.83-87), it is deposited in the root. The use of the store of food is obvious enough. In the autumn the whole plant dies, except the seeds (if it formed them) and the tubers; and the latter are left disconnected in the ground. Just as that small portion of nourishing matter which is deposited in the seed feeds the embryo when it germinates, so the much larger portion deposited in the tuber nourishes its buds, or eyes, when they likewise grow, the next spring, into new plants. And the great supply enables them to shoot with a greater vigor at the beginning, and to produce a greater amount of vegetation than the seedling plant could do in the same space of time; which vegetation in turn may prepare and store up, in the course of a few weeks or months, the largest quantity of solid nourishing material, in a form most available for food. Taking advantage of this, man has transported the Potato from the cool Andes of Chili to other cool climates, and makes it yield him a copious supply of food, especially important in countries where the season is too short, or the summer's heat too little, for profitably cultivating the principal grain-plants.

Fig.101. Tubers of Helianthus tuberosus, called "artichokes."Fig.102. Bulblet-like tubers, such as are occasionally formed on the stem of a Potato-plant above ground.

Fig.101. Tubers of Helianthus tuberosus, called "artichokes."

Fig.102. Bulblet-like tubers, such as are occasionally formed on the stem of a Potato-plant above ground.

111.The Corm or Solid Bulb, like that of Cyclamen (Fig.103), and of Indian Turnip (Fig.104), is a very short and thick fleshy subterranean stem, often broader than high. It sends off roots from its lower end, or rather face, leaves and stalks from its upper. The corm of Cyclamen goes on to enlarge and to produce a succession of flowers and leaves year after year.That of Indian Turnip is formed one year and is consumed the next. Fig.104represents it in early summer, having below the corm of last year, from which the roots have fallen. It is partly consumed by the growth of the stem for the season, and the corm of the year is forming at base of the stem above the line of roots.

Fig.103. Corm of Cyclamen, much reduced in size: roots from lower face, leaf-stalks and flower-stalks from the upper.Fig.104. Corm of Indian Turnip (Arisæma).

Fig.103. Corm of Cyclamen, much reduced in size: roots from lower face, leaf-stalks and flower-stalks from the upper.

Fig.104. Corm of Indian Turnip (Arisæma).

112. The corm of Crocus (Fig.105, 106), like that of its relative Gladiolus, is also reproduced annually, the new ones forming upon the summit and sides of the old. Such a corm is like a tuber in budding from the sides, i. e. from the axils of leaves; but these leaves, instead of being small scales, are the sheathing bases of foliage-leaves which covered the surface. It resembles a true bulb in having these sheaths or broad scales; but in the corm or solid bulb, this solid part or stem makes up the principal bulk.

Fig.105. Corm of a Crocus, the investing sheaths or dead leaf-bases stripped off. The faint cross-lines represent the scars, where the leaves were attached, i. e. the nodes: the spaces between are the internodes. The exhausted corm of the previous year is underneath; forming ones for next year on the summit and sides.Fig.106. Section of the same.

Fig.106. Section of the same.

113.The Bulb, strictly so-called, is a stem like a reduced corm as to its solid part (or plate); while the main body consists of thickened scales, which are leaves or leaf-bases. These are like bud-scales; so that in fact a bulb is a bud with fleshy scales on an exceedingly short stem. Compare a White Lily bulb (Fig.107) with the strong scaly buds of the Hickory and Horse-chestnut (Fig.72 and 73), and the resemblance will appear. In corms, as in tubers and rootstocks, the store of food for future growth is deposited in the stem; while in the bulb, the greater part is deposited in the bases of the leaves, changing them into thick scales, which closely overlap or enclose one another.

114.A Scaly Bulb(like that of the Lily, Fig.107, 108) is one in which the scales are thick but comparatively narrow.

Fig.107. Bulb of a wild Lily. 108. The same divided lengthwise, showing two forming buds of the next generation.

Fig.109. A ground leaf of White Lily, its base (cut across) thickened into a bulb-scale. This plainly shows that bulb-scales are leaves.

115.A Tunicated or Coated Bulbis one in which the scales enwrap each other, forming concentric coats or layers, as in Hyacinth and Onion.

116.Bulbletsare very small bulbs growing out of larger ones; or small bulbs produced above ground on some plants, as in the axils of the leaves of the bulbiferous Lilies of the gardens (Fig.110), and often in the flower-clusters of the Leek and Onion. They are plainly buds with thickened scales. They never grow into branches, but detach themselves when full grown, fall to the ground, and take root there to form new plants.

Fig.110. Bulblets in the axils of leaves of a Tiger Lily.

117.Consolidated Vegetation.An ordinary herb, shrub, or tree is evidently constructed on the plan developing an extensive surface. In fleshy rootstocks, tubers, corms, and bulbs, the more enduring portion of the plant is concentrated, and reduced for the time of struggle (as against drought, heat, or cold) to a small amount of exposed surface, and this mostly sheltered in the soil. There are many similar consolidated forms which are not subterranean. Thus plants like the Houseleek (Fig.91) imitate a bulb. Among Cactuses the columnar species of Cereus (Fig.111,b), may be likened to rootstocks. A green rind serves the purpose of foliage; but the surface is as nothing compared with an ordinary leafy plant of the same bulk. Compare, for instance, the largest Cactus known, the Giant Cereus of the Gila River (Fig.111, in the background), which rises to the height of fifty or sixty feet, with a common leafy tree of the same height, such as that in Fig.89, and estimate how vastly greater, even without the foliage, the surface of the latter is than that of the former. Compare, in thesame view, an Opuntia or Prickly-Pear Cactus, its stem and branches formed of a succession of thick and flattened joints (Fig.111,a), which may be likened to tubers, or an Epiphyllum (d), having short and flat joints, with an ordinary leafy shrub or herb of equal size. And finally, in Melon-Cactuses, Echinocactus (c), or other globose forms (which may be likened to permanent corms), with their globular or bulb-like shapes, we have plants in the compactest shape; their spherical figure being such as to expose the least possible amount of substance to the air. These are adaptations to climates which are very dry, either throughout or for a part of the year. Similarly, bulbous and corm-bearing plants, and the like, are examples of a form of vegetation which in the growing season may expand a large surface to the air and light, while during the period of rest the living vegetable is reduced to a globe, or solid form of the least possible surface; and this protected by its outer coats of dead and dry scales, as well as by its situation under ground. Such are also adapted to a season of drought. They largely belong to countries which have a long hot season of little or no rain, when, their stalks and foliage above and their roots beneath early perishing, the plants rest securely in their compact bulbs, filled with nourishment and retaining their moisture with great tenacity, until the rainy season comes round. Then they shoot forth leaves and flowers with wonderful rapidity, and what was perhaps a desert of arid sand becomes green with foliage and gay with blossoms, almost in a day.

118.Stemsbear leaves, at definite points (nodes,13); and these are produced in a great variety of forms, and subserve various uses. The commonest kind of leaf, which therefore may be taken as the type or pattern, is an expanded green body, by means of which the plant exposes to the air and light the matters which it imbibes, exhales certain portions, and assimilates the residue into vegetable matter for its nourishment and growth.

119. But the fact is already familiar (10-30) that leaves occur under other forms and serve for other uses,—for the storage of food already assimilated, as in thickened seed-leaves and bulb-scales; for covering, as in bud-scales; and still other uses are to be pointed out. Indeed, sometimes they are of no service to the plant, being reduced to mere scales or rudiments, such as those on the rootstocks of Peppermint (Fig.97) or the tubers of Jerusalem Artichoke (Fig.101). These may be said to be of service only to the botanist, in explaining to him the plan upon which a plant is constructed.

120. Accordingly, just as a rootstock, or a tuber, or a tendril is a kind of stem, so a bud-scale, or a bulb-scale, or a cotyledon, or a petal of a flower, is a kind of leaf. Even in respect to ordinary leaves, it is natural to use the word either in a wider or in a narrower sense; as when in one sense we say that a leaf consists of blade and petiole or leaf-stalk, and in another sense say that a leaf is petioled, or that the leaf of Hepatica is three-lobed. The connection should make it plain whether by leaf we mean leaf-blade only, or the blade with any other parts it may have. And the student will readily understand that by leaf in its largest ormorphologicalsense, the botanist means the organ which occupies the place of a leaf, whatever be its form or its function.

121. This is tautological; for foliage is simply leaves: but it is very convenient to speak of typical leaves, or those which serve the plant for assimilation, as foliage-leaves, or ordinary leaves. These may first be considered.

122.The Parts of a Leaf.The ordinary leaf, complete in its parts, consists ofblade,foot-stalk, orpetiole, and a pair ofstipules.

123.First theBladeorLamina, which is the essential part of ordinary leaves, that is, of such as serve the purpose of foliage. In structure it consists of a softer part, thegreen pulp, calledparenchyma, which is traversed and supported by a fibrous frame, the parts of which are calledribsorveins, on account of a certain likeness in arrangement to the veins of animals.The whole surface is covered by a transparent skin, theEpidermis, not unlike that which covers the surface of all fresh shoots.

124. Note that the leaf-blade expands horizontally,—that is, normally presents its faces one to the sky, the other to the ground, or when the leaf is erect the upper face looks toward the stem that bears it, the lower face away from it. Whenever this is not the case there is something to be explained.

125.The framework consists ofwood,—a fibrous and tough material which runs from the stem through the leaf-stalk, when there is one, in the form of parallel threads or bundles of fibres; and in the blade these spread out in a horizontal direction, to form theribsandveinsof the leaf. The stout main branches of the framework are called theRibs. When there is only one, as in Fig.112,114, or a middle one decidedly larger than the rest, it is called theMidrib. The smaller divisions are termedVeins; and their still smaller subdivisions,Veinlets. The latter subdivide again and again, until they become so fine that they are invisible to the naked eye. The fibres of which they are composed are hollow; forming tubes by which the sap is brought into the leaves and carried to every part.

Fig.112. Leaf of the Quince:b, blade;p, petiole;st, stipules.

126.Venationis the name of the mode of veining, that is, of the way in which the veins are distributed in the blade. This is of two principal kinds; namely, theparallel-veined, and thenetted-veined.

127. InNetted-veined(also calledReticulated) leaves, the veins branch off from the main rib or ribs, divide into finer and finer veinlets, and the branches unite with each other to form meshes of network. That is, theyanastomose, as anatomists say of the veins and arteries of the body. The Quince-leaf, in Fig.112, shows this kind of veining in a leaf with a single rib. The Maple, Basswood, Plane or Buttonwood (Fig.74) show it in leaves of several ribs.

128. Inparallel-veinedleaves, the whole framework consists of slender ribs or veins, which run parallel with each other, or nearly so, from the base to the point of the leaf,—not dividing and subdividing, nor forming meshes, except by minute cross-veinlets. The leaf of any grass, or that of the Lily of the Valley (Fig.113) will furnish a good illustration. Such parallel veins Linnæus calledNerves, and parallel-veined leaves are still commonly callednervedleaves, while those of the other kind are said to beveined,—terms which it is convenient to use, although these "nerves" and "veins" are all the same thing, and have no likeness to thenervesand little to the veins of animals.

129.Netted-veinedleaves belong to plants which have a pair of seed-leaves or cotyledons, such as the Maple (Fig.20,24), Beech (Fig.33), and the like; whileparallel-veinedornervedleaves belong to plants with one cotyledon or true seed-leaf; such as the Iris (Fig.59), and Indian Corn (Fig.70). So that a mere glance at the leaves generally tells what the structure of the embryo is, and refers the plant to one or the other of these two grand classes,—which is a great convenience. For when plants differ from each other in some one important respect, they usually differ correspondingly in other respects also.

Fig.113. A (parallel-veined) leaf of the Lily of the Valley. 114. One of the Calla Lily.

130. Parallel-veined leaves are of two sorts,—one kind, and the commonest, having the ribs or nerves all running from the base to the point of the leaf, as in the examples already given; while in another kind they run from a midrib to the margin, as in the common Pickerel-weed of our ponds, in the Banana, in Calla (Fig.114), and many similar plants of warm climates.

131. Netted-veined leaves are also of two sorts, as in the examples already referred to. In one case the veins all rise from a single rib (the midrib), as in Fig.112,116-127. Such leaves are calledFeather-veinedorPenni-veined, i. e.Pinnately-veined; both terms meaning the same thing, namely, that the veins are arranged on the sides of the rib like the plume of a feather on each side of the shaft.

132. In the other case (as in Fig.74,129-132), the veins branch off from three, five, seven, or nine ribs, which spread from the top of the leaf-stalk, and run through the blade like the toes of a web-footed bird. Hence these are said to bePalmatelyorDigitatelyveined, or (since the ribs diverge like rays from a centre)Radiate-veined.

133. Since the general outline of leaves accords with the framework or skeleton, it is plain thatfeather-veined(orpenni-veined) leaves will incline to elongated shapes, or at least to be longer than broad; while inradiate-veinedleaves more rounded forms are to be expected. A glance at the following figures shows this.

Fig.115-120. A series of shapes of feather-veined leaves.

134.Forms of Leaves as to General Outline.It is necessary to give names to the principal shapes, and to define them rather precisely, since they afford easy marks for distinguishing species. The same terms are used for all other flattened parts as well, such as petals; so that they make up a great part of the descriptive language of Botany. It will be a good exercise for young students to look up leaves answering to these names and definitions. Beginning with the narrower and proceeding to the broadest forms, a leaf is said to be

Linear(Fig.115), when narrow, several times longer than wide, and of the same breadth throughout.

Lanceolate, orLance-shaped, when conspicuously longer than wide, and tapering upwards (Fig.116), or both upwards and downwards.

Oblong(Fig.117), when nearly twice or thrice as long as broad.

Elliptical(Fig.118) is oblong with a flowing outline, the two ends alike in width.

Ovalis the same as broadly elliptical, or elliptical with the breadth considerably more than half the length.

Ovate(Fig.119), when the outline is like a section of a hen's egg lengthwise, the broader end downward.

Orbicular, orRotund(Fig.132), circular in outline, or nearly so.

Fig.121, oblanceolate; 122, spatulate; 123, obovate; and 124, wedge-shaped, feather-veined, leaves.

135. A leaf which tapers toward the base instead of toward the apex may be

Oblanceolate(Fig.121) when of the lance-shaped form, only more tapering toward the base than in the opposite direction.

Spatulate(Fig.122) when more rounded above, but tapering thence to a narrow base, like an old-fashioned spatula.

Obovate(Fig.123) or inversely ovate, that is, ovate with the narrower end down.

CuneateorCuneiform, that is,Wedge-shaped(Fig.124), broad above and tapering by nearly straight lines to an acute angle at the base.

Fig.125, sagittate; 126, auriculate; and 127, halberd-shaped or hastate leaves.

136.As to the Base, its shape characterizes several forms, such as

CordateorHeart-shaped(Fig.120,129), when a leaf of an ovate form, or something like it, has the outline of its rounded base turned in (forming a notch orsinus) where the stalk is attached.

Reniform, orKidney-shaped(Fig.131), like the last, only rounder and broader than long.

Auriculate, orEared, having a pair of small and blunt projections, orears, at the base, as in one species of Magnolia (Fig.126).

Sagittate, orarrow-shaped, where such ears are acute and turned downwards, while the main body of the blade tapers upwards to a point, as in the common Sagittaria or Arrow-head, and in the Arrow-leaved Polygonum (Fig.125).

Hastate, orHalberd-shaped, when such lobes at the base point outwards, giving the shape of the halberd of the olden time, as in another Polygonum (Fig.127).

Fig.128-132. Various forms of radiate-veined leaves.

Peltate, orShield-shaped(Fig.132), is the name applied to a curious modification of the leaf, commonly of a rounded form, where the footstalk is attached to the lower surface, instead of the base, and therefore is naturallylikened to a shield borne by the outstretched arm. The common Watershield, the Nelumbium, and the White Water-lily, and also the Mandrake, exhibit this sort of leaf. On comparing the shield-shaped leaf of the common Marsh Pennywort (Fig.132) with that of another common species (Fig.130), it is at once seen that a shield-shaped leaf is like a kidney-shaped (Fig.130, 131) or other rounded leaf, with the margins at the base brought together and united.

137.As to the Apex, the following terms express the principal variations:—

Acuminate,Pointed, orTaper-pointed, when the summit is more or less prolonged into a narrowed or tapering point; as in Fig.133.

Acute, ending in an acute angle or not prolonged point; Fig.134.

Obtuse, with a blunt or rounded apex; as in Fig.135, etc.

Truncate, with the end as if cut off square; as in Fig.136.

Retuse, with rounded summit slightly indented, forming a very shallow notch, as in Fig.137.

Emarginate, orNotched, indented at the end more decidedly; as in Fig.138.

Obcordate, that is, inversely heart-shaped, where an obovate leaf is more deeply notched at the end (Fig.139), as in White Clover and Wood-sorrel; so as to resemble a cordate leaf inverted.

Cuspidate, tipped with a sharp and rigid point; as in Fig.140.

Mucronate, abruptly tipped with a small and short point, like a mere projection of the midrib; as in Fig.141.

Aristate,Awn-pointed, andBristle-pointed, are terms used when this mucronate point is extended into a longer bristle-form or slender appendage.

The first six of these terms can be applied to the lower as well as to the upper end of a leaf or other organ. The others belong to the apex only.

Fig.133-141. Forms of the apex of leaves.

138.As to degree and nature of Division, there is first of all the difference between

Simple Leaves, those in which the blade is of one piece, however much it may be cut up, and

Compound Leaves, those in which the blade consists of two or more separate pieces, upon a common leaf-stalk or support. Yet between these two kinds every intermediate gradation is to be met with.

Fig.142-147. Kinds of margin of leaves.

139.As to Particular Outlines of Simple Leaves(and the same applies to their separate parts), they are

Entire, when their general outline is completely filled out, so that the margin is an even line, without teeth or notches.

Serrate, orSaw-toothed, when the margin only is cut into sharp teeth, like those of a saw, and pointing forwards; as in Fig.142.

Dentate, orToothed, when such teeth point outwards, instead of forwards; as in Fig.143.

Crenate, orScalloped, when the teeth are broad and rounded; as in Fig.144.

Repand,Undulate, orWavy, when the margin of the leaf forms a wavy line, bending slightly inwards and outwards in succession; as in Fig.145.

Sinuate, when the margin is more strongly sinuous or turned inwards and outwards; as in Fig.146.

Incised,Cut, orJagged, when the margin is cut into sharp, deep, and irregular teeth or incisions; as in Fig.147.

Lobed, when deeply cut. Then the pieces are in a general way calledLobes. The number of the lobes is briefly expressed by the phrasetwo-lobed,three-lobed,five-lobed,many-lobed, etc., as the case may be.

140. When the depth and character of the lobing needs to be more particularly specified, the following terms are employed, viz.:—

Lobed, in a special sense, when the incisions do not extend deeper than about half-way between the margin and the centre of the blade, if so far, and are more or less rounded; as in the leaves of the Post-Oak, Fig.148, and the Hepatica, Fig.152.

Cleft, when the incisions extend half way down or more, and especially when they are sharp; as in Fig.149, 153. And the phrasestwo-cleft, or, in the Latin form,bifid,three-cleftortrifid,four-cleftorquadrifid,five-cleftorquinquefid, etc., ormany-cleft, in the Latin form,multifid,—express the number of theSegments, or portions.

Parted, when the incisions are still deeper, but yet do not quite reach to the midrib or the base of the blade; as in Fig.150, 154. And the termstwo-parted,three-parted, etc., express the number of such divisions.

Divided, when the incisions extend quite to the midrib, as in the lower part of Fig.151, or to the leaf-stalk, as in Fig.155; which really makes theleaf compound. Here, using the Latin form, the leaf is said to bebisected,trisected(Fig.155), etc., according to the number of the divisions.

Fig.148, pinnately lobed; 149, pinnately cleft; 150, pinnately parted; 151, pinnately divided, leaves.Fig.152, palmately three-lobed; 153, palmately three-cleft; 154, palmately three-parted; 155, palmately three-divided or trisected, leaves.

Fig.148, pinnately lobed; 149, pinnately cleft; 150, pinnately parted; 151, pinnately divided, leaves.

Fig.152, palmately three-lobed; 153, palmately three-cleft; 154, palmately three-parted; 155, palmately three-divided or trisected, leaves.

141.The Mode of Lobing or Divisioncorresponds to that of the veining, whetherpinnately veinedorpalmately veined. In the former the notches or incisions, orsinuses, coming between the principal veins or ribs are directed toward the midrib: in the latter they are directed toward the apex of the petiole; as the figures show.

142. So degree and mode of division may be tersely expressed in brief phrases. Thus, in the four upper figures of pinnately veined leaves, the first is said to bepinnately lobed(in the special sense), the secondpinnately cleft(orpinnatifidin Latin form), the thirdpinnately parted, the fourthpinnately divided, orpinnatisected.

143. Correspondingly in the lower row, of palmately veined leaves, the first ispalmately lobed, the secondpalmately cleft, the thirdpalmately parted, the fourthpalmately divided. Or, in other language of the same meaning (but now less commonly employed), they are said to bedigitately lobed,cleft,parted, ordivided.

144. The number of the divisions or lobes may come into the phrase. Thus in the four last named figures the leaves are respectivelypalmately three-lobed,three-cleft(ortrifid),three-parted,three-divided, or better (in Latin form),trisected. And so for higher numbers, asfive-lobed,five-cleft,etc., up tomany-lobed,many-cleftormultifid, etc. The same mode of expression may be used for pinnately lobed leaves, aspinnately 7-lobed,-cleft,-parted, etc.

145. The divisions, lobes, etc., may themselves beentire(without teeth or notches), orserrate, or otherwise toothed or incised; or lobed, cleft, parted, etc.: in the latter cases makingtwice pinnatifid,twice palmatelyorpinnately lobed,parted, ordividedleaves, etc. From these illustrations one will perceive how the botanist, in two or three words, may describe any one of the almost endlessly diversified shapes of leaves, so as to give a clear and definite idea of it.

146.Compound Leaves.A compound leaf is one which has its blade in entirely separate parts, each usually with a stalklet of its own; and the stalklet is oftenjointed(orarticulated) with the main leaf-stalk, just as this is jointed with the stem. When this is the case, there is no doubt that the leaf is compound. But when the pieces have no stalklets, and are not jointed with the main leaf-stalk, it may be considered either as a divided simple leaf, or a compound leaf, according to the circumstances. This is a matter of names where all intermediate forms may be expected.

147. While the pieces or projecting parts of a simple leaf-blade are calledLobes, or in deeply cut leaves, etc.,Segments, orDivisions, the separate pieces or blades of a compound leaf are calledLeaflets.

148. Compound leaves are of two principal kinds, namely, thePinnateand thePalmate; answering to the two modes of veining in reticulated leaves, and to the two sorts of lobed or divided leaves (141).

Fig.156-158. Pinnate leaves, the first with an odd leaflet (odd-pinnate); the second with a tendril in place of uppermost leaflets; the thirdabruptly pinnate, or of even pairs.

149.Pinnateleaves are those in which the leaflets are arranged on the sides of a main leaf-stalk; as in Fig.156-158. They answer to thefeather-veined(i. e.pinnately-veined) simple leaf; as will be seen at once on comparing the forms. Theleafletsof the former answer to thelobesordivisionsof the latter; and the continuation of the petiole, along which the leaflets are arranged, answers to the midrib of the simple leaf.

150. Three sorts of pinnate leaves are here given. Fig.156ispinnate with an oddorend leaflet, as in the Common Locust and the Ash. Fig.157ispinnate with a tendril at the end, in place of the odd leaflet, as in the Vetches and the Pea. Fig.158is evenly orabruptly pinnate, as in the Honey-Locust.

Fig.159. Palmate (or digitate) leaf of five leaflets, of the Sweet Buckeye.

151.Palmate(also namedDigitate) leaves are those in which the leaflets are all borne on the tip of the leaf-stalk, as in the Lupine, the Common Clover, the Virginia Creeper (Fig.93), and the Horse-chestnut and Buckeye (Fig.159). They evidently answer to theradiate-veinedorpalmately-veinedsimple leaf. That is, the Clover-leaf of three leaflets is the same as a palmately three-ribbed leaf cut into three separate leaflets. And such a simple five-lobed leaf as that of the Sugar Maple, if more cut, so as to separate the parts, would produce a palmate leaf of five leaflets, like that of the Horse-chestnut or Buckeye.

152. Either sort of compound leaf may have any number of leaflets; yet palmate leaves cannot well have a great many, since they are all crowded together on the end of the main leaf-stalk. Some Lupines have nine or eleven; the Horse-chestnut has seven, the Sweet Buckeye more commonly five, the Clover three. A pinnate leaf often has only seven or five leaflets, or only three, as in Beans of the genus Phaseolus, etc.; in some rarer cases only two; in the Orange and Lemon and also in the common Barberry there is only one! The joint at the place where the leaflet is united with the petiole distinguishes this last case from a simple leaf. In other species of these genera the lateral leaflets also are present.

153. The leaflets of a compound leaf may be eitherentire(as in Fig.126-128), orserrate, or lobed, cleft, parted, etc.; in fact, may present all the variations of simple leaves, and the same terms equally apply to them.

154. When the division is carried so far as to separate what would be one leaflet into two, three, or several, the leaf becomesdoublyortwice compound, eitherpinnatelyorpalmately, as the case may be. For example, while the clustered leaves of the Honey-Locust aresimply pinnate, that is,once pinnate, those on new shoots arebipinnate, ortwice pinnate, as in Fig.160. When these leaflets are again divided in the same way, the leafbecomesthrice pinnate, ortripinnate, as in many Acacias. The first divisions are calledPinnæ; the others,Pinnules; and the last, or little blades themselves,Leaflets.

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