Part 6

_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.

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 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 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 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 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 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 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 principal 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).

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.

150. Three sorts of pinnate leaves are here given. Fig. 156 is _pinnate with an odd_ or _end leaflet_, as in the Common Locust and the Ash. Fig. 157 is _pinnate with a tendril at the end_, in place of the odd leaflet, as in the Vetches and the Pea. Fig. 158 is evenly or _abruptly pinnate_, as in the Honey-Locust.

151. _Palmate_ (also named _Digitate_) 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 the _radiate-veined_ or _palmately-veined_ simple 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 either _entire_ (as in Fig. 126-128), or _serrate_, 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 becomes _doubly_ or _twice compound_, either _pinnately_ or _palmately_, as the case may be. For example, while the clustered leaves of the Honey-Locust are _simply pinnate_, that is, _once pinnate_, those on new shoots are _bipinnate_, or _twice pinnate_, as in Fig. 160. When these leaflets are again divided in the same way, the leaf becomes _thrice pinnate_, or _tripinnate_, as in many Acacias. The first divisions are called _Pinnae_; the others, _Pinnules_; and the last, or little blades themselves, _Leaflets_.

155. So the palmate leaf, if again compounded in the same way, becomes _twice palmate_, or, as we say when the divisions are in threes, _twice ternate_ (in Latin form _biternate_); if a third time compounded, _thrice ternate_ or _triternate_. But if the division goes still further, or if the degree is variable, we simply say that the leaf is _decompound_; either palmately or pinnately decompound, as the case may be. Thus, Fig. 161 represents a four times ternately compound (in other words a _ternately decompound_) leaf of a common Meadow Rue.

156. When the botanist, in describing leaves, wishes to express the number of the leaflets, he may use terms like these:--

_Unifoliolate_, for a compound leaf of a single leaflet; from the Latin _unum_, one, and _foliolum_, leaflet.

_Bifoliolate_, of two leaflets, from the Latin _bis_, twice, and _foliolum_, leaflet.

_Trifoliolate_ (or _ternate_), of three leaflets, as the Clover; and so on.

_Palmately bifoliolate_, _trifoliolate_, _quadrifoliolate_, _plurifoliolate_ (of several leaflets), etc.: or else

_Pinnately bi-_, _tri-_, _quadri-_, or _plurifoliolate_ (that is, of two, three, four, five, or several leaflets), as the case may be: these are terse ways of denoting in single phrases both the number of leaflets and the kind of compounding.

157. Of foliage-leaves having certain peculiarities in structure, the following may be noted:--

158. =Perfoliate Leaves.= In these the stem that bears them seems to run through the blade of the leaf, more or less above its base. A common Bellwort (Uvularia perfoliata, Fig. 162) is a familiar illustration. The lower and earlier leaves show it distinctly. Later, the plant is apt to produce some leaves merely clasping the stem by the sessile and heart-shaped base, and the latest may be merely sessile. So the series explains the peculiarity: in the formation of the leaf the bases, meeting around the stem, grow together there.

159. =Connate-perfoliate.= Such are the upper leaves of true Honeysuckles. Here (Fig. 163) of the opposite and sessile leaves, some pairs, especially the uppermost, in the course of their formation unite around the stem, which thus seems to run through the disk formed by their union.

160. =Equitant Leaves.= While ordinary leaves spread horizontally, and present one face to the sky and the other to the earth, there are some that present their tip to the sky, and their faces right and left to the horizon. Among these are the _equitant_ leaves of the Iris or Flower-de-Luce. Inspection shows that each leaf was formed as if _folded together lengthwise_, so that what would be the upper surface is within, and all grown together, except next the bottom, where each leaf covers the next younger one. It was from their straddling over each other, like a man on horseback (as is seen in the cross-section, Fig. 165), that Linnaeus, with his lively fancy, called these _Equitant_ leaves.

161. =Leaves with no distinction of Petiole and Blade.= The leaves of Iris just mentioned show one form of this. The flat but narrow leaves of Jonquils, Daffodils, and the cylindrical leaf of Onions are other instances. _Needle-shaped_ leaves, like those of the Pine, Larch, and Spruce, and the _awl-shaped_ as well as the _scale-shaped_ leaves of Junipers, Red Cedar, and Arbor-Vitae (Fig. 166), are examples.

162. =Phyllodia.= Sometimes an expanded _petiole_ takes the place of the blade; as in numerous New Holland Acacias, some of which are now common in greenhouses. Such counterfeit blades are called _phyllodia_,--meaning leaf-like bodies. They may be known from true blades by their standing edgewise, their margins being directed upwards and downwards; while in true blades the faces look upwards and downwards; excepting in equitant leaves, as already explained.

163. =Falsely Vertical Leaves.= These are apparent exceptions to the rule, the blade standing edgewise instead of flatwise to the stem; but this position comes by a twist of the stalk or the base of the blade. Such leaves present the two faces about equally to the light. The Compass-plant (Silphium laciniatum) is an example. So also the leaves of Boltonia, of Wild Lettuce, and of a vast number of Australian Myrtaceous shrubs and trees, which much resemble the phyllodia of the Acacias of the same country. They are familiar in Callistemon, the Bottle-brush Flower, and in Eucalyptus. But in the latter the leaves of the young tree have the normal structure and position.

164. =Cladophylla=, meaning _branch-leaves_. The foliage of Ruscus (the Butcher's Broom of Europe) and of Myrsiphyllum of South Africa (cultivated for decoration under the false name of Smilax) is peculiar and puzzling. If these blades (Fig. 167, 168) are really leaves, they are most anomalous in occupying the axil of another leaf, reduced to a little scale. Yet they have an upper and lower face, as leaves should, although they soon twist, so as to stand more or less edgewise. If they are branches which have assumed exactly the form and office of leaves, they are equally extraordinary in not making any further development. But in Ruscus, flowers are borne on one face, in the axil of a little scale: and this would seem to settle that they are branches. In Asparagus just the same things as to position are thread-shaped and branch-like.

Sec. 2. LEAVES OF SPECIAL CONFORMATION AND USE.

165. =Leaves for Storage.= A leaf may at the same time serve both ordinary and special uses. Thus in those leaves of Lilies, such as the common White Lily, which spring from the bulb, the upper and green part serves for foliage and elaborates nourishment, while the thickened portion or bud-scale beneath serves for the storage of this nourishment. The thread-shaped leaf of the Onion fulfils the same office, and the nourishing matter it prepares is deposited in its sheathing base, forming one of the concentric layers of the onion. When these layers, so thick and succulent, have given up their store to the growing parts within, they are left as thin and dry husks. In a Houseleek, an Aloe or an Agave, the green color of the surface of the fleshy leaf indicates that it is doing the work of foliage; the deeper-seated white portion within is the storehouse of the nourishment which the green surface has elaborated. So, also, the seed-leaves or cotyledons are commonly used for storage. Some, as in one of the Maples, the Pea, Horse-chestnut, Oak, etc., are for nothing else. Others, as in Beech and in our common Beans, give faint indications of service as foliage also, chiefly in vain. Still others, as in the Pumpkin and Flax, having served for storage, develop into the first efficient foliage. Compare 11, 22-30, and the accompanying figures.

166. =Leaves as Bud-Scales= serve to protect the forming parts within. Having fulfilled this purpose they commonly fall off when the shoot develops and foliage-leaves appear. Occasionally, as in Fig. 170, there is a transition of bud-scales to leaves, which reveals the nature of the former. The Lilac also shows a gradation from bud-scale to simple leaf. In Cornus florida (the Flowering Dogwood), the four bud-scales which through the winter protect the head of forming flowers remain until blossoming, and then the base of each grows out into a large and very showy petal-like leaf; the original dry scale is apparent in the notch at the apex.

167. =Leaves as Spines= occur in several plants. A familiar instance is that of the common Barberry (Fig. 171). In almost any summer shoot, most of the gradations may be seen between the ordinary leaves, with sharp bristly teeth, and leaves which are reduced to a branching spine or thorn. The fact that the spines of the Barberry produce a leaf-bud in their axil also proves them to be leaves.

168. =Leaves for Climbing= are various in adaptation. True foliage-leaves serve this purpose; as in Gloriosa, where the attenuated tip of a simple leaf (otherwise like that of a Lily) hooks around a supporting object; or in Solanum jasminoides of the gardens (Fig. 172), and in Maurandia, etc., where the leaf-stalk coils round and clings to a support; or in the compound leaves of Clematis and of Adlumia, in which both the leaflets and their stalks hook or coil around the support.

169. Or in a compound leaf, as in the Pea and most Vetches, and in Cobaea, while the lower leaflets serve for foliage, some of the uppermost are developed as tendrils for climbing (Fig. 167). In the common Pea this is so with all but one or two pairs of leaflets.

170. In one European Vetch, the leaflets are wanting and the whole petiole is a tendril, while the stipules become the only foliage (Fig. 173).

171. =Leaves as Pitchers=, or hollow tubes, are familiar in the common Pitcher-plant or Side-saddle Flower (Sarracenia, Fig. 174) of our bogs. These pitchers are generally half full of water, in which flies and other insects are drowned, often in such numbers as to make a rich manure for the plant. More curious are some of the southern species of Sarracenia, which seem to be specially adapted to the capture and destruction of flies and other insects.

172. The leaf of Nepenthes (Fig. 175) combines three structures and uses. The expanded part below is foliage: this tapers into a tendril for climbing; and this bears a pitcher with a lid. Insects are caught, and perhaps digested, in the pitcher.

173. =Leaves as Fly-traps.= Insects are caught in another way, and more expertly, by the most extraordinary of all the plants of this country, the Dionaea or Venus's Fly-trap, which grows in the sandy bogs around Wilmington, North Carolina. Here (Fig. 176) each leaf bears at its summit an appendage which opens and shuts, in shape something like a steel-trap, and operating much like one. For when open, no sooner does a fly alight on its surface, and brush against any one of the two or three bristles that grow there, than the trap suddenly closes, capturing the intruder. If the fly escapes, the trap soon slowly opens, and is ready for another capture. When retained, the insect is after a time moistened by a secretion from minute glands of the inner surface, and is digested. In the various species of Drosera or Sundew, insects are caught by sticking fast to very viscid glands at the tip of strong bristles, aided by adjacent gland-tipped bristles which bend slowly toward the captive. The use of such adaptations and operations may be explained in another place.

Sec. 3. STIPULES.

174. A leaf complete in its parts consists of blade, leaf-stalk or petiole, and a pair of stipules. But most leaves have either fugacious or minute stipules or none at all; many have no petiole (the blade being _sessile_ or stalkless); some have no clear distinction of blade and petiole; and many of these, such as those of the Onion and all phyllodia (166), consist of petiole only.

175. The base of the petiole is apt to be broadened and flattened, sometimes into thin margins, sometimes into a sheath which embraces the stem at the point of attachment.

176. =Stipules= are such appendages, either wholly or partly separated from the petiole. When quite separate they are said to be _free_, as in Fig. 112. When attached to the base of the petiole, as in the Rose and in Clover (Fig. 177), they are _adnate_. When the two stipules unite and sheathe the stem above the insertion, as in Polygonum (Fig. 178), this sheath is called an _Ocrea_ from its likeness to a greave or leggin.

177. In Grasses, when the sheathing base of the leaf may answer to petiole, the summit of the sheath commonly projects as a thin and short membrane, like an ocrea: this is called a LIGULA or LIGULE.

178. When stipules are green and leaf-like they act as so much foliage. In the Pea they make up no small part of the actual foliage. In a related plant (Lathyrus Aphaca, Fig. 173), they make the whole of it, the remainder of the leaf being tendril.

179. In many trees the stipules are the bud-scales, as in the Beech, and very conspicuously in the Fig-tree, Tulip-tree, and Magnolia (Fig. 179). These fall off as the leaves unfold.

180. The stipules are spines or prickles in Locust and several other Leguminous trees and shrubs; they are tendrils in Smilax or Greenbrier.

Sec. 4. THE ARRANGEMENT OF LEAVES.

181. =Phyllotaxy=, meaning leaf-arrangement, is the study of the position of leaves, or parts answering to leaves, upon the stem.

182. The technical name for the attachment of leaves to the stem is the _insertion_. Leaves (as already noticed, 54) are _inserted_ in three modes. They are

_Alternate_ (Fig. 181), that is, one after another, or in other words, with only a single leaf to each node;

_Opposite_ (Fig. 182), when there is a pair to each node, the two leaves in this case being always on opposite sides of the stem;

_Whorled_ or _Verticillate_ (Fig. 183) when there are more than two leaves on a node, in which case they divide the circle equally between them, forming a _Verticel_ or whorl. When there are three leaves in the whorl, the leaves are one third of the circumference apart; when four, one quarter, and so on. So the plan of opposite leaves, which is very common, is merely that of whorled leaves, with the fewest leaves to the whorl, namely, two.

183. In both modes and in all their modifications, the arrangement is such as to distribute the leaves systematically and in a way to give them a good exposure to the light.