Leaves are generally flat organs an the major photosynthetic organs, thanks to the high number of chloroplasts that leaf cells contain. In addition, leaves are the main responsible for regulating transpiration, therefore preventing water lost.
Anatomically, leaves can be divided into two parts: the blade (or limb) and the petiole (Figure 1). The blade is where most photosynthesis and transpiration occur. Most of stomata and the photosynthetic parenchyma is found in the blade. There are two surfaces in the blade: the upper, or adaxial surface, and the lower, or abaxial surface. Adaxial surfaces are usually more directly exposed to sunlight, whereas abaxial surfaces are more hidden. The margin of the leaf, or leaf contour, may show a broad variety of forms. The petiole is more or less long and cylindrical. It connects the blade with the stem at the level of a stem node. Axillary buds, found at the sharp angle between the petiole and the stem, can develop into lateral branches. Some types of leaves, known as sessile leaves, lack petioles and the limb is directly attached to the stem.
The leaf size is variable. Regarding the blade complexity, leaves can be divided into simple and compound. Simple leaves have an undivided blade, whereas compound leaves have divided blades into leaflets, sometimes called folioles. Each leaflet looks like a leaf blade, but it is not (Figure 2).
The blade shows a large variety of shapes, depending on the species. A particular shape is the result of the adaptation of the species to the environment during evolution. For example, light intensity, rain, temperature, altitude, herbivores, and other factors influence the blade morphology. Different leaf morphologies get different names (Figure 3).
The vascular bundles of the leaf are known as veins or nerves, and venation (or innervation) is how they are organized. Venation can be used to distinguish two main groups of plants. Microphyll plants, such as ferns, show simple venation, whereas megaphylls have more complex vascular bundles organization, like flowering plants. These two types of venation seem to have emerged independently during evolution.
At the base of the petiole of dicot plants, there is a structure resembling a little leaf or scale called a stipule. In monocot plants, stipule-like structures at the leaf base are wider and usually embrace the stem.
In some species, leaves develop into structures not directly related to photosynthesis. Some leaves are associated with the flowers and form the bracts that are around the petals. Others become thorns, as in hawthorn (but not the bramble thorns which are stem derivatives), or are modified to catch insects as in carnivorous plants.
Tissues
Epidermis. On the adaxial surface (upper surface) of the leaf, there is an epidermis showing a thick cuticle made up of cutin and waxes. Stomata are usually scarce in the adaxial epidermis, or they are not present at all. On the abaxial surface (lower surface), there is a thinner epidermis with a high density of stomata.
Mesophyll. The mesophyll is the parenchyma found between the adaxial and abaxial epidermis. Most leaves show two types of parenchyma: palisade and spongy. Palisade parenchyma is found close to the adaxial surface, and it is made up of photosynthetic parenchyma cells containing many chloroplasts. They are tidily-arranged, elongated cells, oriented perpendicular to the leaf surface. That is why the name palisade parenchyma. Spongy parenchyma, or aerenchyma, is closer to the abaxial surface, and it is composed of more or less rounded cells with not too many chloroplasts. In some leaves, it is difficult to distinguish the two types of parencgyma.
Vascular system. The vascular bundles of leaves are called veins. They bring water and mineral salts to the leaves, and take photosynthesized substances out of the leaves. Veins show different patterns depending on the species, and the organization is called venation. Besides conduction, vascular bundles work as rows that provide mechanical support.
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Bibliography ↷
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Bar M, Ori N. 2014. Leaf development and morphogenesis. Development 141, 4219-4230.
Dkhar J, Pareek A. 2014. What determines a leaf's shape? Evodevo 5:47.
Pongrac P, Baltrenaite E, Vavpetič P, Kelemen M, Kladnik A, Budič B, Vogel‑Mikuš K, Regvar M, Baltrenas P, Pelicon P. 2019. Tissue-specific element profiles in Scots pine (Pinus sylvestris L.) needles. Trees. 33:91-101.
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