|Leaf Lobing and Division|
Leaves, the main photosynthetic organs of plants, are usually green, flattened structures that are formed as lateral outgrowths at stem nodes. Simple leaves are composed of a single lamina, or blade, which may be attached to the stem via a cylindrical structure called a petiole.
Leaves lacking a petiole are called sessile. Laminae of simple leaves may exhibit various patterns and degrees of lobing, which are often characteristic of individual species of plants and, together with reproductive features, are used in plant identification.
Other species have compound leaves, in which the leaf laminae are subdivided into smaller leaflets. The pattern of arrangement, the number, and the shape of leaflets comprising compound leaves are often useful characteristics for identification of plants.
Some species of plants exhibit either gradual or abrupt changes in leaf lobing and division during development and are called heterophyllous. For example, some species exhibit a mixture of pinnatifid (pinnately lobed) and pinnatisect (pinnately compound) leaves on the same stem.
Heterophylly is often observed in water plants, with one form of leaf being produced where the plant stem is submerged and another being produced where the stem is above water.
Light periodicity, intensity, and quality, as well as oxygen and carbon dioxide concentrations, are known to influence leaf form in some species. In other species, different portions of a single leaf lamina can be pinnatifid or pinnatisect. The developmental mechanism for leaves of this type is not completely understood.
Lobes typically extend greater than one-eighth of the distance from the margin to the midrib of the leaf or leaflet. The margin is the edge of the leaf lamina lying between the apex and base.
The midrib is the prominent vein that subdivides the leaf or leaflet into two halves from base to apex. Palmately lobed margins are indented toward the base of the leaf lamina, creating a pattern like fingers extending from a hand, or a digitate pattern.
Pinnately lobed margins are indented one-quarter to one-half of the distance to the midrib, with the indentions oriented toward the midrib in a feather like pattern. Pinnately cleft margins are indented a little more than half of the distance to the midrib.
Pinnately incised, or pinnatifid, margins are deeply indented toward the midrib, extending well over half to almost completely to the midrib. The term “pinnately lobed” is sometimes used in reference to lobed, cleft, and incised leaves, collectively.
Simple and Compound Leaves
|common patterns of |
leaf lobing and division
A simple leaf has only one blade, or lamina, associated with it. There are no leaflets. In singly compound leaves, the leaf is sudivided into leaflets, which attach to a central rachis (axis). The rachis is continuous with the petiole, which attaches to the node region of the stem, where the axillary bud will be found.
In doubly, or bipinnately, compound leaves, the primary leaflet lamina is subdivided into smaller secondary leaflets, which attach to a secondary rachis, or rachilla. The secondary rachis attaches to the primary or central rachis. The primary rachis is continuous with the petiole, which attaches to the node region of the stem where the axillary bud will be found.
The number of leaflets in a compound leaf is often constant in many plant species. In even pinnately compound leaves, all of the leaflets are paired. There is no terminal leaflet; thus, the total number of leaflets per leaf is an even number.
In odd pinnately compound leaves, there is one terminal, unpaired leaflet at the end of the leaf, making the total number of leaflets an odd number. Trifoliolate leaves have three leaflets, which may have petiolules or be sessile. The term ternate denotes groups of threes.
Leaf blade length is measured from where the blade joins the petiole straight to the tip of the leaf and perpendicular to the width. Width is measured at the widest part of the leaf perpendicular to the length.
The petiole is the more or less round stalk that connects the leaf blade to the node region of the stem. Petiole length is measured from the point of attachment of the leaf blade to the node region of the stem. Some species lack petioles or have very short petioles.
The singly compound leaf blade is subdivided into leaflets which attach to a central rachis. The leaflet blade, or lamina, is the flat part of the leaflet. The petiolule is the stalk extending from the base of the leaflet lamina to the rachis.
The rachis is continuous with the petiole, which attaches to the node region of the stem, where the axillary bud will be found. Leaflet length is measured from where the leaflet blade joins the petiolule straight to the tip of the leaflet perpendicular to the width. Width is measured at the widest part of the leaflet perpendicular to the length.
In the doubly or bipinnately compound leaf, the primary leaflets are themselves subdivided into still smaller secondary leaflets. The leaflet lamina is attached to a secondary rachis, or rachilla. The petiolule extends from the base of the leaflet lamina to the rachilla. The rachilla attaches to the primary or central rachis.