The Elements of Botany Part 6

_Cordate_ or _Heart-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 or _sinus_) where the stalk is attached.

_Reniform_, or _Kidney-shaped_ (Fig. 131), like the last, only rounder and broader than long.

_Auriculate_, or _Eared_, having a pair of small and blunt projections, or _ears_, at the base, as in one species of Magnolia (Fig. 126).

_Sagittate_, or _arrow-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_, or _Halberd-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).

[Ill.u.s.tration: Fig. 128-132. Various forms of radiate-veined leaves.]

_Peltate_, or _Shield-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 naturally likened 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 princ.i.p.al variations:--

_Ac.u.minate_, _Pointed_, or _Taper-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_, or _Notched_, 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_, and _Bristle-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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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_, or _Saw-toothed_, when the margin only is cut into sharp teeth, like those of a saw, and pointing forwards; as in Fig. 142.

_Dentate_, or _Toothed_, when such teeth point outwards, instead of forwards; as in Fig. 143.

_Crenate_, or _Scalloped_, when the teeth are broad and rounded; as in Fig. 144.

_Repand_, _Undulate_, or _Wavy_, 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_, or _Jagged_, 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 called LOBES. The number of the lobes is briefly expressed by the phrase _two-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 phrases _two-cleft_, or, in the Latin form, _bifid_, _three-cleft_ or _trifid_, _four-cleft_ or _quadrifid_, _five-cleft_ or _quinquefid_, etc., or _many-cleft_, in the Latin form, _multifid_,--express the number of the _Segments_, 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 terms _two-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 the leaf compound. Here, using the Latin form, the leaf is said to be _bisected_, _trisected_ (Fig. 155), etc., according to the number of the divisions.

[Ill.u.s.tration: Fig. 148, pinnately lobed; 149, pinnately cleft; 150, pinnately parted; 151, pinnately divided, leaves.]

[Ill.u.s.tration: 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 Division= corresponds to that of the veining, whether _pinnately veined_ or _palmately veined_. In the former the notches or incisions, or _sinuses_, coming between the princ.i.p.al 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 be _pinnately lobed_ (in the special sense), the second _pinnately cleft_ (or _pinnatifid_ in Latin form), the third _pinnately parted_, the fourth _pinnately divided_, or _pinnatisected_.

143. Correspondingly in the lower row, of palmately veined leaves, the first is _palmately lobed_, the second _palmately cleft_, the third _palmately parted_, the fourth _palmately divided_. Or, in other language of the same meaning (but now less commonly employed), they are said to be _digitately lobed_, _cleft_, _parted_, or _divided_.

144. The number of the divisions or lobes may come into the phrase. Thus in the four last named figures the leaves are respectively _palmately three-lobed_, _three-cleft_ (or _trifid_), _three-parted_, _three-divided_, or better (in Latin form), _trisected_. And so for higher numbers, as _five-lobed_, _five-cleft_, etc., up to _many-lobed_, _many-cleft_ or _multifid_, etc. The same mode of expression may be used for pinnately lobed leaves, as _pinnately 7-lobed_, _-cleft_, _-parted_, etc.

145. The divisions, lobes, etc., may themselves be _entire_ (without teeth or notches), or _serrate_, or otherwise toothed or incised; or lobed, cleft, parted, etc.: in the latter cases making _twice pinnatifid_, _twice palmately_ or _pinnately lobed_, _parted_, or _divided_ leaves, etc. From these ill.u.s.trations 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 often _jointed_ (or _articulated_) 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 circ.u.mstances. 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 called _Lobes_, or in deeply cut leaves, etc., _Segments_, or _Divisions_, the separate pieces or blades of a compound leaf are called LEAFLETS.

148. Compound leaves are of two princ.i.p.al kinds, namely, the _Pinnate_ and the _Palmate_; answering to the two modes of veining in reticulated leaves, and to the two sorts of lobed or divided leaves (141).

[Ill.u.s.tration: Fig. 156-158. Pinnate leaves, the first with an odd leaflet (_odd-pinnate_); the second with a tendril in place of uppermost leaflets; the third _abruptly pinnate_, or of even pairs.]

149. _Pinnate_ leaves are those in which the leaflets are arranged on the sides of a main leaf-stalk; as in Fig. 156-158. They answer to the _feather-veined_ (i. e. _pinnately-veined_) simple leaf; as will be seen at once on comparing the forms. The _leaflets_ of the former answer to the _lobes_ or _divisions_ of the latter; and the continuation of the petiole, along which the leaflets are arranged, answers to the midrib of the simple leaf.