Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
3.8. Leaf Morphology and Anatomy. Definition and Functions of the Leaf. Leaf Types. Metamorphoses of Leaves

The leaf is a lateral plagiotropic organ characterized by limited intercalary growth. Only in a few plants, such as Welwitschia, is the leaf a permanent organ of unlimited growth, while in ferns, the leaf grows from its apex. Evolutionarily, leaves emerged in connection with the transition of plants to a terrestrial lifestyle. The analog of a leaf in some spore-bearing plants were phyllids—outgrowths of the SHOOT. True leaves of telomic origin first appeared in phylogenetic development among horsetails. The primary Functions of the leaf are Photosynthesis, Respiration, and Transpiration. A typical leaf consists of the following morphological parts: the blade (its largest part), the petiole, and the stipule. The petiole serves to attach the leaf to the shoot and position it optimally relative to light. Leaves with petioles are called petiolate, while those without are sessile. At the Base of the leaf in many plants, stipules form, which can fuse to create an ochrea. In grasses, the lower part of the leaf forms a sheath or tube. The junction where the sheath meets the leaf blade is called the ligule, flanked by auricles. The stipules of most plants appear as small paired leaflets, scales, bristles, and sometimes spines (in black locust) or large blades (in garden peas, wild pansy, or bedstraw).

Stipules perform a protective function; in some plants, they persist throughout the plant's life, while in others, they are shed soon after the leaf blade unfolds.

The most important part of the leaf is its blade. Leaf blades vary greatly in size, shape, and texture. Simple leaves are those that have a single blade on the petiole (such as apple, buckwheat, and nettle), whereas compound leaves feature multiple simple leaflets on a single petiole that detach independently during leaf fall (such as acacia and lupine). The common axis to which the simple leaflets attach is called the rachis.

The Diversity of blade shapes can be categorized by the following morphological indicators: overall shape, degree of dissection, margin type, apex shape, base shape, and venation.

When determining the shape of the blade, the ratio between its length and width is primarily taken into account. By shape, the following blade types are distinguished: round, when the length and width are nearly equal (as in aspen); oval, when the length is no more than twice the width (plantain); ovate, an oval leaf where the widest part is near the base of the blade (lilac); obovate, an oval leaf where the widest part is in the upper portion (elm); lanceolate, when the length is four times the width (willow); and linear, when the length greatly exceeds the width (grasses, sedges). Regarding the degree of dissection, blades can be lobed, if the incision reaches no more than a quarter of the blade width (oak); cleft, if the incision goes about halfway to the center (geranium); or divided, if the incision reaches nearly to the midrib (tomato).

By apex shape, leaf blades can be obtuse (marsh marigold), acute (willow), acuminate (linden), or emarginate (ginkgo). By base shape, they may be cordate (linden), reniform (wild ginger), cuneate (poplar), hastate (bindweed), or sagittate (arrowhead).

Leaf margins exhibit even greater diversity in shape: entire (wheat), dentate (stinging nettle), serrate (pear), sinuate (aspen), and crenate (ground ivy).

Leaf venation can be parallel, where the Veins run parallel to each other without a distinct midrib (grasses, sedges); arcuate, where the veins converge near the base and apex of the blade (lily of the valley); palmate-reticulate (maples); or pinnate-reticulate (oak). The most ancient type of venation is dichotomous, found in the leaves of ferns and maidenhair tree (Ginkgo biloba). As a rule, parallel and arcuate venation are characteristic of monocotyledonous plants, while reticulate venation is typical of dicots.

Compound leaves also vary: trifoliate leaves are typical of strawberries and clover; palmate leaves of lupine and horse chestnut; odd-pinnate leaves of black locust, rowan, and dog rose; and even-pinnate leaves of peanut and Siberian peashrub. Bipinnate or tripinnate leaves also occur, such as in honeylocust.