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| aalized organs serving other functions, such as tendrils of peas and other legumes, the protective spines of cacti, and the insect traps in carnivorous plants such as Nepenthes and Sarracenia. Leaves are the fundamental structural units from which cones are constructed in gymnosperms (each cone scale is a modified megaphyll leaf known as a sporophyll):?408? and from which flowers are constructed in flowering plants.:?445? Vein skeleton of a leaf. Veins contain lignin that make them harder to degrade for microorganisms. The internal organization of most kinds of leaves has evolved to maximize exposure of the photosynthetic organelles (chloroplasts) to light and to increase the absorption of CO2 while at the same time controlling water loss. 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) stomata which open or close to regulate the rate exchange of CO2, oxygen (O2), and water vapor into and out of the internal intercellular space system. Stomatal opening is controlled by the turgor pressure in a pair of guard cells that surround the stomatal aperture. In any square centimeter of a plant leaf, there may be from 1,000 to 100,000 stomata. This is a cross section showing the different layers of a leaf. 1 - Upper epidermis 2 - Palisade mesophyll 3 - Spongy mesophyll 4 - Vein 5 - Xylem 6 - Phloem 7 - Collenchyma 8 - Chloroplasts 9 - Nucleus 10 - Vacuole 11 - Stomata 12 - Cuticle Near the ground these Eucalyptus saplings have juvenile dorsiventral foliage from the previous year, but this season their newly sprouting foliage is isobilateral, like the mature foliage on the adult trees above. The shape and structure of leaves vary considerably from species to species of plant, depending largely on their adaptation to climate and available light, but also to other factors such as grazing animals, available nutrients, and ecological competition from other plants. Considerable changes in leaf type occur within species, too, for example as a plant matures (Eucalyptus species commonly have isobilateral, pendent leaves when mature and dominating their neighbors; however, such trees tend to have erect or horizontal dorsiventral leaves as seedlings, when their growth is limited by the available light.) Other factors include the need to balance water loss at high temperature and low humidity against the need to absorb CO2. In most plants, leaves also are the primary organs responsible for transpiration and guttation (beads of fluid forming at leaf mar |
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