A great appreciation goes to the famous Swedish naturalist Carolus Linnaeus who launched the binomial nomenclature system (International Code for Botanical Nomenclature), which is still applicable nowadays. How to identify the species based on morphological traits? A plant is easily identified through its flower or fruits because either the color or the structure of the organs are distinguished. However, not all plants have flowers and some flowering plants are seasonal and may have very short flowering periods only in favorable conditions. Leaves are the organs that used in species identification. The main advantage of using leaves in identification is that they exist almost all year round and the morphology of the leaf can be well-preserved in the herbarium.
Leaves are the physical platform for photosynthesis that produce food for the plants in the form of sugar by collecting solar energy and carbon dioxide. Besides, they also provide a large surface area for transpiration, gaseous exchange, and water collection. A leaf is known as folium that locates at stem nodes or stems axial or at the apical shoot or as leaf axils. A leaf possesses a blade or lamina; an edge called the leaf’s margin, the veins (vascular bundles), a petiole, two appendages at the base of the petiole called stipules, as well as the sheath. There are pigmented leaves full of the chloroplast that consists of chlorophylls that can be green in color (presence of chlorophylls) and other colors that consist of carotenoids, anthocyanins, and β-carotenoids at different concentrations.
Leaves of some species fall at the same time in the dry season. The thickness of the leaves varies from species to species. Generally, a thin leaf that is a dorsiventral function to aid in assimilation (photosynthesis), gas absorption or breathing (transpiration), water evaporation (transpiration), the formation of water droplets via hydathode (guttation), as well as the absorption in the form of water and nutrient. Thick leaf, which can be existed in the form of a needle or cylindrical shape, usually found as an acicular, temperate leaf or xerophytic leaves. A leaf can be a complete or uncomplete leaf. A complete leaf contains a sheath, petiole, and leaf blade, whereas an uncomplete leaf usually is found as a petiolated leaf (petiole with lamina) and sessile leaf (lamina only). For instance, the leaf base is fused, and the leaf seems to pass through the stem. The sessile leaf can be seeming like the lobe or auriculate leaf partially or fully (amplexicaul leaf, like grass) encloses the stem.
Leaf appendages are used in identifications for some species. A stipule is a lateral appendage under or at the base of the axial of the petiole. A free stipule is observed on the groundnut, adnate or winged stipule can be observed on rose, and stipula intrapetiolaris can be seen on the plants under the families of Rubiaceae, Rhizophoraceae, Melastomacese, and Apocynaceae. Furthermore, stipula antidroma that is conspicuous and wide, which continues to surround the node, can be observed from the plants under the family of Euphorbiaceae. A shingle-like covering of a bud that is commonly known as bud scale is obtained from Mesua sp. Stipules also modified into hard and pointed structures called spines usually seen on the shy plant, Mimosa sp. Besides, ocrea or ochrea is a plant structure that formed a fused stipule into a sheath surrounding the stem, typically found in the Polygonaceae.
Some plants have leaf sheath, which is one of the characteristics of monocotyledon like Graminae, Zingiberaceae, Musaceae, Palmae, and Cannaceae. It is formed by the basal part of the certain leaves where they embrace the stem. It is used to protect the leaf primordium or shoot in sugar cane. It also acts as a false stem to support the plant. Then, the ligule is an appendage found between the leaf sheath and lamina. A leaf petiole and rachis are also involved in plant identification due to their shape and anatomy. The petiole or rachis is usually circular with a swollen base. Carica papaya has circular and hollow petiole. The petiole of the leaves in the Citrus family possesses flattened and winged shapes. Sometimes wing continues through rachis or pinnate leaf. Bauhinia has swollen parts at both apical and basal petiole. Garcinia sp. and Diospyros sp. have wrinkle petiole. Then, the rachis of Moringa is found swollen in the nodal section. Apart from these, angular that shows many angles, petioles with half circular and ridges are also playing a key in plant identification.
More often, the lamina of leaves tells the distinct feature that represented the family. Identification and classification of plants often use leaves with different shapes, sizes, apical, basal, margin, venations, hairs (trichomes), and colors to differentiate one from another species. There are two types of lamina, namely heterophylly and anisophylly. Heterophylly is the plants having different shapes of leaf in the same plant while the young leaf’s morphology is different from that of the older leaf. For example, carrot. Anisophylly is observed when the plant had a different leaf shape at the same petiole or branch-like Begonia sp. The shape and size of each leaf can differ like a circular (orbicularis), shield (peltate), oval (ovate), leaf blade wide and long (oblong), lance-shape (lanceolate), and mixed of two shapes. Furthermore, they have egg-shaped (ovate), three angels (triangularis), delta (deltoid), and diamond-shaped (rhomboid) leaves that have no notch at the base. The leaves with a notch at the base have heart-shaped (cordate), kidney-shaped (reniform), arrow-shaped (sagittate), sagittate leaf with its two lobes directed outside (hastate), and lobed at its base (auriculate).
Another distinct feature is the shape of the apex. Broad apex can be observed in various shapes like a breach of egg shape (obovate), breach hart shaped (obcordate), breach triangularly shaped (cuneate), and spatula-like (spatulate). Specifically, there is an apical end in different styles. The apical end with a long tail is known as caudate, short tail is referred to as cuspidate. Apical end up with spine (mucronate), and blunt end with a shallow notch (retuse), and deep notch (emarginate) are also acting as distinct characteristics in plant identification. On the other hand, the basal part of the leaves also have a clear shape like a sharp-pointed base (attenuate), sharp base (cuneate), lobed at the base (auriculate), heart shape (cordate), arrow shape (sagittate), spear shape (hastate), asymmetrical, and rounded (rotundate).
Leaf venation plays a pivotal role in plant identification as well when the leaf lamina is similar within the species of the same genus. Leaf venation is used to provide mechanical support and a conducting system. Leaf venation is comprised of the midrib, which is a continuity from the petiole that divides the lamina into leaf and right parts. The equal division produces an asymmetric leaf, whereas unequal division resulted in an asymmetric leaf like that of Begonia sp. Then, some leaves with more than one main vein arise from the same point observed in Nelumbium (water lily), Ricinus sp. and Manihot utilisima that have the petiole located beneath the center of the leaf. Subsequently, the lateral vein that branched from the midrib further divide into the primary and secondary lateral vein. The small vein is called veinlets. There are several types of venation such as reticulate and parallel venation. From there, both venations can be further categorized into pinnate (unicostate) and palmate (multicostate) types.
Moreover, the leaf margin, which is the leaves’ shape, tells the identifications of the species. A leaf can have either an entire or incision margin. For the incision, the angel incision can be proceeded inwards (sinus) and outwards (angulus) the leaf-blades. They are classified as teethed, serrate, double serrate, crenate, sawed, repanded, or waxy. Besides, the adaxial side of the leaf surface is used as another key to identifying a species. A leaf can be smooth with the shinning, dull, or waxy surface, glabrous (smooth without trichomes), scaber (rough and harsh to touch), wrinkle, wavy, pilose (thinly but covered by trichomes), villous (thickly covered with trichomes), hispidus (covered with long and stiff trichomes), floccose (covered with cottony hairs that are detachable), or tomentose (densely covered with long and cottony hairs). Generally, leaflets that are the same age, formed, and fall simultaneously are known as a compound leaf with no lateral bud on the leaf axil. The compound leaf types consist of pinnate, palmate, and pedate.
Apart from the morphological traits, leaf arrangement is also critical in plant identification. Referred to as phyllotaxy, there are three principal types mainly alternate, opposite, and whorled. Apart from playing an essential role in photosynthesis, the leaves are modified in an extreme environment. It can be used as a food and water reserve for xerophytes. Begonia and Bryophyllum used the leaves as the organ for reproduction. Some leaves are modified into tendrils that are observed in climbers. Furthermore, carnivorous plants have modified leaves that function as insect traps for getting nutrients from the prey. Plants also protect themselves from herbivores by having spines, bristles, hooks, glandular trichomes, and stinging hair that are used in the primary defense mechanism. The leaf that contains toxic compounds like latex, alkaloid, and calcium oxalate is used to defend the plants as chemical weapons.
Plants are sessile (non-moving), and they co-evolve with the predator and environment. Plant imitates the general appearance, color, shape, or particular feature into a deadly animal known as mimicry. For instance, Caladium, Amorphophalus, Ariesaema, and Sansevieria resemble multicolor and variously spotted snakes or the hood of a cobra snake. One of the carnivorous plants, the cobra lily also one of the examples of mimicry.
Further reading:
Simpson, M. G. (2019). Plant systematics. Academic press
