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Leaves are normally extensively vascularized and typically have networks of vascular bundles containing xylem, which supplies water for photosynthesis, and phloem, which transports the sugars produced by photosynthesis. However, horizontal alignment maximizes exposure to bending forces and failure from stresses such as wind, snow, hail, falling debris, animals, and abrasion from surrounding foliage and plant structures. [28][24][23], The number of vein endings is very variable, as is whether second order veins end at the margin, or link back to other veins. [14], Simple, vascularized leaves (microphylls), such as those of the early Devonian lycopsid Baragwanathia, first evolved as enations, extensions of the stem. They were once thought to be typical examples of pattern formation through ramification, but they may instead exemplify a pattern formed in a stress tensor field.[34][35][36]. A simple leaf has an undivided blade. [14] Compound leaves are closer to shoots than simple leaves. At about the same time, Melville (1976) described a system applicable to all Angiosperms and using Latin and English terminology.
[14] 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)[6]:408 and from which flowers are constructed in flowering plants. Veins appeared in the Permian period (299–252 mya), prior to the appearance of angiosperms in the Triassic (252–201 mya), during which vein hierarchy appeared enabling higher function, larger leaf size and adaption to a wider variety of climatic conditions. A stipule, present on the leaves of many dicotyledons, is an appendage on each side at the base of the petiole, resembling a small leaf. Once sugar has been synthesized, it needs to be transported to areas of active growth such as the plant shoots and roots.
The veins in a leaf represent the vascular structure of the organ, extending into the leaf via the petiole and providing transportation of water and nutrients between leaf and stem, and play a crucial role in the maintenance of leaf water status and photosynthetic capacity.They also play a role in the mechanical support of the leaf. Used of leaves, such as those of birches. Palmate, in which three or more main ribs rise together at the base of the leaf, and diverge upward. [26] Within these the major veins function as the support and distribution network for leaves and are correlated with leaf shape. Strong wind forces may result in diminished leaf number and surface area, which while reducing drag, involves a trade off of also reducing photosynthesis.
Compound leaves are a characteristic of some families of higher plants, such as the Fabaceae. But large leaf size favors efficiency in photosynthesis and water conservation, involving further trade offs. Leaves attached to stems by stalks (known as petioles) are called petiolate, and if attached directly to the stem with no petiole they are called sessile. All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only. © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins [16] Other factors include the need to balance water loss at high temperature and low humidity against the need to absorb atmospheric carbon dioxide. [19] The terminology associated with the description of leaf morphology is presented, in illustrated form, at Wikibooks. The leaflets may have petiolules and stipels, the equivalents of the petioles and stipules of leaves. Leaf margins: main part of the leaf, usually large and flat. Most leaves show dorsoventral anatomy: The upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions. Some of the smallest veins (veinlets) may have their endings in the areoles, a process known as areolation. Horn pyramidal, stout, as long as the shell, with three dentate edges. [8] Green plants are autotrophic, meaning that they do not obtain food from other living things but instead create their own food by photosynthesis. The stomatal pores perforate the epidermis and are surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts, forming a specialized cell group known as the stomatal complex.
Considerable changes in leaf type occur within species, too, for example as a plant matures; as a case in point 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.
There may or may not be normal pinnate leaves at the tip of the phyllode. Leaves complicate-bilobed, the upper lobe smaller, the lower succubous; margins entire or dentate or ciliate; underleaves none. Perennial plants whose leaves are shed annually are said to have deciduous leaves, while leaves that remain through winter are evergreens.
Palmate: like the fingers of a hand spread open. Both the mechanics and architecture of the leaf reflect the need for transportation and support. 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. Undulate: having a wavy margin. [33] The epidermis serves several functions: protection against water loss by way of transpiration, regulation of gas exchange and secretion of metabolic compounds.
A vein is made up of a vascular bundle. The phloem and xylem are parallel to each other, but the transport of materials is usually in opposite directions. The petiole mechanically links the leaf to the plant and provides the route for transfer of water and sugars to and from the leaf. The situation, arrangement, and structure of the stipules is called the "stipulation". Veins (sometimes referred to as nerves) constitute one of the more visible leaf traits or characteristics. A leaf (plural leaves) is the principal lateral appendage of the vascular plant stem,[1] usually borne above ground and specialized for photosynthesis. “Affect” vs. “Effect”: Use The Correct Word Every Time.
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