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Leaf development is a highly important process in the life cycle of a photosynthesizing plant. Leaves exhibit astounding variation in shape, margin, venation pattern, thickness, life history strategy, photosynthetic efficiency, defensive mechanisms, and chemical makeup. All of these diverse forms of leaf originate from the same basic developmental pathways of development.

Hormonal development
Most findings on hormonal leaf development have emerged from studies on the quick-growing Arabidopsis thaliana. Leaves develop from meristematic tissue. The process of cellular differentiation is complex and involves many hormonal pathways.

The role of KNOX genes
Meristematic tissue in plants is made of undifferentiated cells, similar to stem cells in mammals. A meristem known as the Shoot Apical Meristem (SAM) is found at the top of the plant, and a meristem known as the Root Apical Meristem (RAM) is found at the tips of the roots. Meristems also develop along the stem in a regular, repeated pattern known as phyllotaxy. These meristems become leaf primordia, which eventually develop into leaves. Knotted1-like homeobox (Evolutionary history of plantsKNOX) genes maintain the meristem. This prevents differentiation. KNOX genes have been shown to inhibit the production of the hormone [[Gibberellic acid (GA), a molecule that is greatly involved in many aspects of plant development, including germination, elongation, and leaf development. Leaf development therefore involves the inhibition of KNOX genes, as this inhibition leads to the expression of GA. GA may be involved in leaf symmetry.

Dorsiventral patterning
The top and bottom of a leaf – known as the adaxial and abaxial surfaces – are often different. The differentiation of top and bottom cells is known as dorsiventral patterning. In leaves, the SAM promotes adaxial development, and YABBY and KANADI genes promote abaxial development.

Venation pattern
Another important aspect of leaf development is the development of venation pattern. Veins arise from the procambium, which is a type of meristem that differentiates into vascular tissue. The ATHB-8 gene is likely involved in this process.

Environmental control
Many aspects of leaf development are under environmental control. See Phenotypic Plasticity.

Leaf development in deciduous plants
Deciduous plants grow and drop leaves every year. They photosynthesize in warm months and go into dormancy during colder months as a strategy for tolerating extreme temperature differences between summer and winter. They develop leaf buds in autumn. Resources like nitrogen are reallocated from old leaves in the process of senescence to embryonic leaves contained in the leaf bud. These embryonic leaves overwinter and then emerge the following spring. The buds open in response to various environmental cues, including photoperiod and temperature. Many plants have a chill requirement for bud burst, meaning that their buds do not open if they did not experience a sufficient stretch of cold while in dormancy. These leaves also exhibit phenotypic plasticity. Leaf morphology can change from year to year in response to environmental conditions. This process is under-studied. However, it is known that the phenotypic change in deciduous leaves is in response to the conditions of the previous year, as the deciduous tree develops leaf buds in the autumn. Deciduous plants must prioritize rapid leaf growth and maturity in order to maximize productivity, as they do not experience the benefit of year-round foliage and year-round photosynthesis. ,

Delayed greening
Delayed greening is a phenomenon found in some rainforest plants, in which the leaf does not photosynthesize at maximum capacity until it has reached maturity. The mechanism behind this is not known. However, delayed greening may be a strategy to avoid herbivory. Sugars produced by a young and soft leaf are easily consumed by herbivores, but older and tougher leaves have better defense mechanisms and therefore are less susceptible to herbivory.

Phenotypic plasticity
Phenotypic plasticity refers to the change a plant exhibits in response to its environment. In other words, environmental factors can contribute to the development of the leaf. A few aspects of leaf morphology that are under environmental control include the ratio of leaf dry mass per unit area, known as LMA; stomatal density and distribution; leaf margin shape; leaf thickness; photosynthetic rate; and N investment.