In some Acacia species, such as the koa tree (Acacia koa), the petioles are expanded or broadened and function like leaf blades; these are called phyllodes. The leaf surface is also host to a large variety of microorganisms; in this context it is referred to as the phyllosphere. Most leaves have distinctive upper surface (adaxial) and lower surface (abaxial) that differ in colour, hairiness, the number of stomata (pores that intake and output gases), epicuticular wax amount and structure and other features. Leaf movement like this may also increase turbulence of the air close to the surface of the leaf, which thins the boundary layer of air immediately adjacent to the surface, increasing the capacity for gas and heat exchange, as well as photosynthesis. A petiole may be absent, or the blade may not be laminar (flattened). [20], Monocot leaves in temperate climates usually have narrow blades, and usually parallel venation converging at leaf tips or edges. Both are embedded in a dense parenchyma tissue, called the sheath, which usually includes some structural collenchyma tissue. Their surfaces are waterproofed by the plant cuticle and gas exchange between the mesophyll cells and the atmosphere is controlled by minute (length and width measured in tens of µm) openings called stomata which open or close to regulate the rate exchange of carbon dioxide, oxygen, and water vapor into and out of the internal intercellular space system. Veins (sometimes referred to as nerves) constitute one of the more visible leaf traits or characteristics. research project gave homeless people $7,500 each — the results were 'beautifully surprising', Vancouver mayor recommends $30M in COVID-19 homeless relief measures. In most leaves, the primary photosynthetic tissue, the palisade mesophyll, is located on the upper side of the blade or lamina of the leaf[1] but in some species, including the mature foliage of Eucalyptus,[5] palisade mesophyll is present on both sides and the leaves are said to be isobilateral. At the same time water is being transported in the opposite direction. A structurally complete leaf of an angiosperm consists of a petiole (leaf stalk), a lamina (leaf blade), and stipules (small structures located to either side of the base of the petiole). The concentration of photosynthetic structures in leaves requires that they be richer in protein, minerals, and sugars than, say, woody stem tissues. Furthermore, several kinds of leaf-like structures found in vascular plants are not totally homologous with them. [43], In the course of evolution, leaves have adapted to different environments in the following ways:[citation needed], May be coarsely dentate, having large teeth, or glandular dentate, having teeth which bear glands. Examples include flattened plant stems called phylloclades and cladodes, and flattened leaf stems called phyllodes which differ from leaves both in their structure and origin. There may or may not be normal pinnate leaves at the tip of the phyllode. "I might one day be that important person that has a powerful voice... a seed can grow into an oak tree.". This assimilation tissue is the primary location of photosynthesis in the plant. Leaves are mostly green in color due to the presence of a compound called chlorophyll that is essential for photosynthesis as it absorbs light energy from the sun. In any square centimeter of a plant leaf there may be from 1,000 to 100,000 stomata.[15]. Structures located there are called "axillary". External leaf characteristics, such as shape, margin, hairs, the petiole, and the presence of stipules and glands, are frequently important for identifying plants to family, genus or species levels, and botanists have developed a rich terminology for describing leaf characteristics. Tree leaves come in all sorts and sizes, and knowing the shape of particular leaves can help to quickly identify the tree species. [25] although criticized as being unduly burdened with jargon. Sessile (epetiolate) leaves have no petiole and the blade attaches directly to the stem.

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