A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants. Typically a leaf is a thin, flattened organ borne above ground and specialized for photosynthesis, but many types of leaves are adapted in ways almost unrecognizable in those terms: some are not flat (for example many succulent leaves and conifers), some are not above ground (such as bulb scales), and some are without major photosynthetic function (consider for example cataphylls, spines, and cotyledons). Conversely, many structures of non-vascular plants, or even of some lichens, which are not plants at all (in the sense of being members of the kingdom Plantae), do look and function much like leaves. Furthermore, several structures found in vascular plants look like leaves but are not actually leaves; they differ from leaves in their structures and origins. Examples include phyllodes ,cladodes, and phylloclades.
Leaves are a plants main photosynthetic organs. Leaf structure is closely associated with its photosynthetic function. Leaves must permit carbon dioxide access to the photosynthetic cells but impede water from diffusing out. The oxygen that is a waste product of photosynthesis must be allowed to escape from the leaf.
Leaves have a large surface area so that they can trap enough light for photosynthesis. Plants which live in the shade eg:-woodlands,usually have very thin leaves with a very large surface area.such plants would be unable to live in full sunlight because they would loose water so rapidly. Furthermore, their very thin leaves would easily be damaged in the wind. thicker leaves would not be any better for shade plants because the light intensity is too low for light to penetrate into the lower spongy mesophyll. the chloroplasts would be wasted.
Plants which live in very sunny places tend to have much thicker leaves.because the light is so strong,it can get through the pallisade mesophyll down to the spongy mesophyll underneath.such plants donot do very well in shady places because the light intensity is too low for it to get to the lower spongy mesophyll. Photosynthesis produces glucose .this can be converted into starch and stored.
Waxy cuticle:-The wax cuticle forms a waterproof layer over the leaf.it prevents excessive water loss.it also prevents the entry of viruses,bacteria and fungi. epidermis:-The upper and the lower epidermis produce the waxy cuticle.except for their guard cells,they do not contain chloroplasts.the epidermis is clearly transparent.
Mesophyll:-The mesophyll consists of 2 layers of photosynthetic cells:the palisade mesophyll consists of columnar cells and the spongy mesophyll consists of spherical cells.Air spaces:-There are air spaces between the cells in both spongy and pallisade mesophyll.the air spaces in the mesophyll are bigger and nearer to the stomata.this means that the spongy mesophyll will get more CO2 than the pallisade mesophyll.
Stomata:-These are tiny holes in the lower epidermis.a few planrts also have stomata on the upper surfaces of their leaves.plants living in very wet places can afford to lose water so they tend to lose more stomata.Guard cells:-Unlike the other cells in the epidermis,guard cells contain chloroplasts.they are able to change their shape to open the close of stoma they surround.they can close their stomata to prevent excessive water loss in dry,hot,windy conditions.they have to open their stomata to allow gasoues exchange.
Vascular bundles:-The midrib and veins in a leaf contain vascular bundles.each bundle contains 2 kinds of tissue.xylem serves 2 functions.firstly,it brings water and mineral salts from the ground in its xylem vessels.secondly,it helps to support the leaf.phleom can bring organic nutrients eg:amino acids into the leaf:it also carries sucrose out of the leaf.