Leaves are generally flattened plant organs derived from shoot meristems. They are the major photosynthetic organs thanks to the high amount of chloroplasts that leaf cells contain. In addition, leaves are the main responsible for controlling transpiration, therefore preventing water lost. The design and morphology of leaves can be explained if we take into account these two functions: photosynthesis and transpiration. During evolution, vascular plants probably formed leaves from branches.
1. Morphology
Leaves can be divided in two parts: blade (or limb) and petiole (Figure 1). Blade is where most photosynthesis and transpiration occur. The majority of stomata and photosynthetic parenchyma are found in the blade. There are two surfaces in the blade: upper or adaxial, and lower or abaxial. Adaxial surface is usually more directly exposed to the sunlight, whereas abaxial surface is more hidden. Initially, the leave primordium has no adaxial-abaxial axis determination, which is established once the leaf emerges from the stem. This leaf polarity can be observed because the vascular bundles become organized with xylem toward the adaxial surface and phloem toward the abaxial one. The margin of the leaf or leaf contour may show a broad variety of forms. Petiole is more or less long and cylindrical. It connects the blade with the stem at the level of a node. Axillary buds, found in the angle between the petiole and the stem, will 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. Small leaves are often found in higher altitude places, with little rain, poor nutrient soils, as well as in hot and dry places. The sunlight radiation is another element that affects the leaf size and thickness, even in the same plant body. Leaves under more intense sunlight are smaller and thicker, mostly due to the growing of photosynthetic parenchyma. They also show more well-developed vascular system and epidermis than shadow leaves.
Regarding the blade complexity, leaves can be divided in simple and compound. Simple leaves have an undivided blade, whereas compound leaves show divided blades into leaflets, sometimes called folioles. Each leaflet looks like a leaf blade, but it is not (Figure 2). Simple leaves and leaflets can be easily distinguished because leaflets have not any axillary buds.
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 nerves or veins, and innervation (or venation) is how they are organized. Innervation can be used to distinguish two main groups of plants. Microphylls plants, such as ferns, show simple innervation, whereas megaphylls have more complex vascular bundles organization, like flowering plants. These two types seem to have independently emerged during evolution. It means that they are not homologous. In flowering plants, dicot species usually have a main central nerve in the leaf blade that branches many times, and the diameter of the vascular bundle decreases after each branching. These branching pattern is called reticulated. The main nerve, known as rib, extends from the petiole to the tip of the blade through the middle line of the blade. In monoct species, veins run parallel to long axis of the blade and usually show similar diameters. This organization is called parallel innervation. However, there are smaller vascular bundles transversely connecting the parallel larger ones.
The general organization of leaf venation depends on the species, but the fine pattern may be influenced by the environment. The position of large veins is more predictable, but the distribution is more diverse as they branch and reduce the diameter, even comparing leaves from the same plant. In addition, at least in dicot plants, fully developed leaves keep some capability to modify the vein organization, for example to get adapted to wounds.
At the base of the petiole of dicot plants, there is a structure resembling a little leaf or scale called 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, etcetera.
The presence of leaves in the whole main stem and lateral branches happens in plants with short lifetime, usually one year. However, those species living several years have leaves in the segment of the branch that grew that year or a few years before. Leaves die and fall in deciduous plants, but remain for several years in perennial ones. New leaves grow in new shoots. The position of leaves in the stem may be spiral when they are arranged as a helix, may be opposite when two leaves are at the same level and at opposite sites of the stem, may verticillated when three or more leaves are at the same level. Phyllotaxy is the name of the organization of leaves in stems.
2. Tissues
Epidermis
In the adaxial surface (upper surface) there is a layer of cutinized epidermic cells showing thick layers of cuticle and waxes. Stomata are usually scarce in adaxial epidermis, or they are not present at all. In the abaxial surface (lower surface), there is a thinner epidermis with a high density of stomata (Figure 4). However, there are leaves, referred as epistomatic, showing stomata only in the adaxial epidermis, such as floating aquatic plants. Hypostomatic plants are those having stomata only in the abaxial epidermis. Anphystomatic plants show stomata in both surfaces. Finally, underwater leaves of submerged plants do not usually have stomata. Stomata appear to be randomly distributed through epidermis of dicot plant leaves. However, in monocot plant leaves, stomata are arranged parallel to the veins. Xerophyte plants, adapted to dry environments, usually show a higher density of stomata in the leaves because they permit a large and quick exchange of gases during short periods of time when water is available.
In the leaf epidermis of many species there are cells that differentiated in hair-like structures or trichomes. Trichomes may be unicellular or multicellular, and may have several functions, such as protection, glandular, and preventing water loss. Leaves lacking trichomes are called glabrous and those with trichomes are known as pubescent.
Mesophyll
Mesophyll is the parenchyma found between the epidermic layers of the adaxial and abaxial surfaces. Most leaves show two types of parenchyma: palisade and spongy. Palisade parenchyma is found close to the adaxial surface, is made up of photosynthetic parenchyma cells containing many chloroplasts. They are elongated cells that are tidily arranged and perpendicular to the leaf surface (Figure 4). That is why the name palisade parenchyma. Spongy parenchyma is closer to the abaxial surface, and it is composed of more or less rounded cells with not too many chloroplasts. There are large intercellular spaces in this parenchyma, so it looks like a spongy texture. Intercelullular spaces are connected with substomatic chambers and are important for the gas exchange needed during photosynthesis. However, it is difficult to distinguish between the two parenchyma types in some species.
Dorsiventral leaves are those with a typical distribution of palisade parenchyma obove the spongy parenchyma. Isolateral leaves has two layers of palisade parenchyma (close to adaxial and abaxial layers, respectively) and a layer of spongy parenchyma in between. Some xerophyte species show isolateral leaves. Homogeneous leaves are those with almost an evenly cell morphology and distribution of parenchyma cells forming the mesophyll, as in grasses.
Veins
Vascular bundles of leaves are called veins (Figure 5). They bring water and mineral salts and take photosynthesized substances out of the leaves. Xylema and phloem, as well as some parenchyma cells, form the veins. Xylem is usually facing the adaxial surface, whereas phloem is toward the abaxial surface.
The vascular system of leaves is a network of interconnected veins. Veins show different organization patterns depending on the species. However, it is a common feature to follow a hierarchical pattern, where veins decrease the diameter after every branching point. Primary and secondary veins are regarded as the main veins because their diameter is higher and are wrapped by perivascular parenchyma. Lower order and minor veins are embedded in the mesophyll. There may be sixth order veins. The lowest order veins have closed ends. Each type of vein has its function. Smaller veins are responsible for gathering photosynthesizates from mesophyll, whereas the larger veins are mostly involved in the transport of those substances. Actually, larger veins are surrounded by parenchyma, the perivascular sheath, that function as the endodermis of the root: control of substances entering and exiting laterally from the vascular tissue. Veins also provide mechanical support to the leaf structure. In some dicot plants with large leaves, veins are surrounded by collenchyma and sclerenchyma cells, whereas in monocot may have fibers associated with veins.
In grasses, there are two types of photosynthetic pathways, C3 and C4. C4 pathway is more productive. Leaves of both species of grasses show structural differences. C4 leaves show two concentric layers of parenchyma around the vascular bundles, the external one is typical mesophyll and the internal one is made up of large parenchyma cells containing many chloroplasts (this disposition is known as Kranz anatomy). In addition, the distance between veins is short, no more than the thickness of four photosynthetic parenchyma cells. On the other hand, C3 does not show this concentric layers of cell around vascular bundles and the bundle sheath is a layer of small cells with few chloroplasts. Inner to the bundle sheath, there is another layer of smaller cells called mestome. The distance between veins in C3 leaves is larger than 4 cells of photosynthetic parenchyma (around 12, on average).
3. Formation
Leaves are developed from the apical shoot meristems, and from the tip meristems of branches. Leaves differentiate from lateral cells of the meristem known as founder cells. The initial number of founder cells may go from 10 to 100. The initial process can be divided in three steps: initiation, growing (leave protrusions and leaf primordia are formed), and expansion and maturation (it is mostly an expansion to form the leaf blade). The increase in size of the leaf is mainly by intercalar growing, that is, by proliferation of the cells found between the tip and the base of the leave. The increase in size of the cells also contributes to the final size of the leave. It must be pointed that a leaf is a complex organ because it has three axes: proximal-distal, adaxial-abaxial, and medial-lateral. Cells must know where they are during the differentiation process.
In leaf primordium, procambium cells are differentiated from the ground tissue located near the epidermis. They are induced by a high concentration of the auxin hormone and are then organized in cords to finally form the veins. Veins are not generated by growing in length of cords, but by lateral addition of new procambium cells. The molecular mechanism is known as auxin canalization. It consists in that differentiated vascular cells drive auxin laterally favoring the differentiation of new vascular cells.
Bibliography
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.