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The form and arrangement of the parts of a typical foliage leafare intimately associated with the part played by the leaf in thelife of the plant. The flat surface is spread to allow the maximumamount of sunlight to fall upon it, as it is by the absorption ofenergy from the sun's rays by means of the chlorophyll contained inthe cells of the leaf that the building up of plant food isrendered possible; this process is known as photo-; the first stageis the combination of dioxide, absorbed from the air taken in through the stomata intothe living cells of the leaf, with water which is brought into theleaf by the wood-vessels. The wood-vessels form part of thefibro-vascular bundles or veins of the leaf and are continuousthroughout the leaf-stalk and stem with the root by which water isabsorbed from the soil. The palisade layers of the mesophyllcontain the larger number of chlorophyll grains (or corpuscles)while the absorption of carbon dioxide is carried on chieflythrough the lower epidermis which is generally much richer instomata. The water taken up by the root from the soil containsnitrogenous and mineral salts which combine with the first productof photo-synthesis - a - to form more complicated -containing foodsubstances of a proteid nature; these are then distributed by otherelements of the vascular bundles (the phloem) through theleaf to the stem and so throughout the plant to wherever growth ordevelopment is going on. A large proportion of the water whichascends to the leaf acts merely as a for the other raw food materials and isgot rid of from the leaf in the form of water vapour through thestomata - this process is known as transpiration. Hencethe extended surface of the leaf exposing a large area to light andair is eminently adapted for the carrying out of the process ofphoto-synthesis and transpiration. The arrangement of the leaves onthe stem and branches (see Phyllotaxy, below) is such asto prevent the upper leaves shading the lower, and the shape of theleaf serves towards the same end - the disposition of leaves on abranch or stem is often seen to form a "," each leaf fitting into the spacebetween neighbouring leaves and the branch on which they are bornewithout overlapping.

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In , a leaf is an above-ground specialized for . For this purpose, a leaf is typically flat (laminar) and thin. There is continued debate about whether the flatness of leaves to expose the to more or to increase the of . In either case, the adaption was made at the expense of water loss. In the , when carbon dioxide concentration was at several times its present value, plants did not have leaves or flat stems. Many have flat, photosynthetic organs, but these are not true leaves. Neither are the of . The leaves of , , and are variously referred to as , , or euphylls.

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