Leaves are the parts of a plant that carry out photosynthesis, the process where light and carbon dioxide are absorbed and converted, with the addition of water, into glucose and oxygen. The plant uses the glucose and expels the oxygen as a waste product – a very useful balancing activity in a world full of animals who take in oxygen and give out carbon dioxide.
Each leaf consists of three basic parts; a stalk or ‘petiole’ at the base that becomes the central ‘midrib’ in the leaf itself, lateral ‘veins’ spreading from the midrib to the outer edges of the leaf and the ‘blade’ which is the flat area connecting the veins (like the fabric in an umbrella). The thin blade provides a large surface area to absorb plenty of light and the thinness means the carbon dioxide only has to travel a short distance before diffusing into the leaf cells. The network of veins help to support the leaf (like the struts of our umbrella) and also transport the water and carbhohydrates (glucose). The entire surface of the leaf is covered in stomata (pores) through which the gases are exchanged.
The internal structure of each leaf consists of several layers; externally, top and bottom, a wax cuticle on the outside of a thin, transparent epidermis layer protects the leaf whilst allowing light through. The cuticles will be thicker where the light intensity is greater. The stomata are distributed throughout the upper and lower epidemis layers, and each stoma is surrounded by ‘guard cells’ containing the photosynthetic, chlorophyll-filled chloroplasts that absorb the available light and begin the transformation process. The guard cells are kidney-shaped and will open and close the stomata as required.
Immediately below the upper epidermis is a layer of ‘palisade mesophyll’ cells, also full of chloroplasts to absorb more light, and between this layer and the lower epidermis is a layer of ‘spongy mesophyll’, a layer of loosely arranged parenchyma tissues, increasing surface area and allowing air to move freely between the layers of active cells exchanging carbon dioxide for oxygen. The vascular bundles that form the leaf veins pass through this layer, ready to transport sugars, water etc as necessary. Each vein consists of a sheath protecting layers of xylema and phloem.
On the plant, leaves may be arranged alternately along the stem (a layout called ‘alternative’), in pairs (‘opposite’) or in groups (‘whorled’); and can be either simple (a single continuous unit) or compound (where several ‘leaflets’ are grouped on the same petiole or stalk). Although the leaf or leaflet shape is generally an oval of one sort or another (dictated by the necessity to contain the midrib/vein network and parts of the blade), there are many differences noted by those who wish to be able to identify leaves and to which plant they belong. For instance, the apex (tip) can be pointed (‘acute’), heart-shaped (‘emarginate’), flat (‘truncate’) or one of five other types; the general shape has some fourteen types, there are ten or so margin (outline) descriptions and even the base of the leaf (where the blade meets the stalk) can be one of eight types. Further information on these definitions and much more can be found at the Colorado State University website.
Leaves are the main source of ‘food’ for trees and incidentally help to keep the oxygen/carbon dioxide balance on this world; for such small and apparently insignificant items (not to mention temporary, as many die off each autumn and grow back in the spring, leaving their plants dormant over winter) they have an enormous effect on everything around them.