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Deserts are among the most hostile habitats on Earth: they cover approximately one-third of the planet’s landmass, and are inhabited by highly specialized plants and animals that are capable of withstanding its harsh climate. Deserts are characterized by a few key features, the most significant being extreme aridity or lack of rain. Often, a desert has been defined as a region receiving an average of 10 or fewer inches of precipitation annually. As a result, plants and animals are very sparsely populated, and the soil is directly exposed to wind and sunlight without the protection of tree foliage1.This direct exposure to sunlight enables deserts to experience relatively higher temperatures in both air and soil, and is also responsible for the high rate of evaporation found at such an environment. Land is known to withhold and release heat energy at a higher rate than water. Hence, as arid areas, deserts undergo drastic fluctuations in temperature from day to night. The lack of moisture and decaying organic matter prevents significant formation of humus, which is why desert soil is nutrient-poor but mineral-rich. Lastly, without protection from vegetation, the soil in deserts is very easily eroded by water and wind.Over millions of years, desert organisms have evolved in various ways to survive under these conditions. Evolution is defined as the cumulative change in the heritable characteristics of a population. In simpler terms, it is the mechanism by which the most advantageous characteristics of a species are passed down to the next generation2. It is evolution that enables organisms to be better suited to their environments.This process is made possible due to genes, the unit of heritability3. Genes affect the physical and chemical 1Citation 1 (on page 6 of this document)2Citation 2 (on page 6 of this document)3Citation 3 (on page 6 of this document)2characteristics of an organism. Every individual in a sexually reproducing species has a unique set of genes, which translates to different physical and chemical features. This gives rise to variation between individuals of the same species. As organisms live and grow in a certain habitat, some of these traits become more advantageous in comparison to others4. Individuals possessing these traits become more likely to survive and reproduce, thus increasing the frequency of such organisms within the population.Over many years, a species will experience changes in morphology, physiology and phenology, ultimately enhancing its ability to survive in its specific habitat. This is called adaptation.In this paper, I will be exploring the ways in which xerophytes, or plants with specialized structures that enable water retention, are adapted to living in desert conditions.Leaves: General Structure and FunctionLeaves are one of the most important organs of plants: they are responsible for the production of glucose, which is conducted by a phenomenon called photosynthesis. Photosynthesis is an endothermic process that converts the inorganic simple molecules of carbon dioxide and water into the organic monosaccharide glucose, releasing oxygen as a byproduct5. The equation is given by:6CO2 + 6H2O ? C6H12O6 + 6O2Photosynthesis requires exposure to sunlight and open air in order to function. It occurs specifically in the spongy mesophyll and palisade mesophyll tissues in leaves6.The spongy mesophyll layer is designed to have a high volume, in order to build a concentration gradient with 4Citation 4 (on page 6 of this document)5Citation 5 (on page 6 of this document)6Citation 6 (on page 6 of this document)3air molecules. As a result, atmospheric gases diffuse into the leaves, hence enabling the plant with access to CO2, a primary input for photosynthesis. Molecules enter the spongy mesophyll through stomata, which are pores on the leaf’s lower surface. Often, however, these pores allow water to escape the plant in the form of vapour, an occurrence called transpiration. This creates a tradeoff between supply of carbon dioxide and retention of water.The palisade layer, on the other hand, contains cells having many chloroplasts. Chloroplasts facilitate high absorption of energy from sunlight. This energy, obtained through photons, is essential for photosynthesis. It is used to break water into its ionic substituents, H+ and OH-. After a series of subsequent chemical reactions, it leads to the formation of adenosine triphosphate (ATP), the energy currency for most living organisms.At night, in the absence of the sun, photosynthesis halts and another chain reaction known as the Kreb’s cycle begins. The ATP produced earlier is broken down into Adenosine diphosphate, (ADP), releasing high amounts of energy and an organic phosphate molecule. This energy drives the Kreb’s cycle, which produces glucose7. In this manner, light energy from the sun is converted to chemical energy in glucose.Despite the necessity of exposure to sunlight, there is one major drawback: it raises the internal temperature of the plant and its immediate environment. This causes water molecules to get kinetically energized and evaporate faster. Once again, water is at risk of escaping through the stomata. Thus, there is another tradeoff, between availability of sunlight and rate at which water is lost.These tradeoffs exist in almost all photosynthesizing plants. However, most plants have ready access to water, which they absorb with roots, as a replacement for what is lost to the atmosphere as vapour. Since desert climates and habitats are extremely arid, absorbing water from the soil is practically impossible. Hence, the leaves of plants 7Citation 7 (on page 6 of this document)4here have special features that decrease the impact of these tradeoffs under heightened conditions.Adaptations: TissuesThe palisade mesophyll and spongy mesophyll layers of plants are generally protected between the upper epidermis and lower epidermis 8(where stomata are found). These protective tissues are usually thin, so as to promote maximum absorption of sunlight and easy diffusion of gases for photosynthesis. The thinness and permeability of leaves also, however, increases the rate of transpiration.Xerophytes have adapted to overcome this problem, by developing a thick layer of lipid polymer known as the cuticle on the outer edge of both epidermises. The thick cuticle prevents water from escaping easily. Cuticles of xerophytes often secrete wax, which provides an additional layer that helps conserve water further. The wax also makes the leaves more lustrous, which enables reflection of heat energy from the sun, thereby preventing an increase in rate of evaporation. This is seen in the mountain-dwelling Azorella compacta that is native to South America.The lower epidermis of some species, such as Gahnia microstachya of southern Australia, is covered by very tiny hair-like projections that are usually concentrated around the stomata. These unique structures reabsorb the moisture that escapes via transpiration. The hairs also provide partial shade for the plant.Adaptations: Leaf Shape and OrientationThe leaves found in tropical or temperate habitats are usually strategically arranged, and shaped flat and wide for absorbing sufficient sunlight in the presence of surrounding foliage. Having wide and flat leaves in deserts, however, proves to be disadvantageous because it is easier for water to escape into the atmosphere. 8Citation 8 (on page 6 of this document)5Instead, xeric species are adapted to minimize leaf surface area and decrease transpiration.Many xerophytes have leaves that are reduced to spines: this not only provides protection from herbivorous animals, but also drastically decreases surface area. In some species, such as the Ammophila arenaria, leaves may also be rolled up9, containing stomata inside as a way of decreasing water loss further. In such cases, the fleshy stems conduct photosynthesis: the palisade cells are found on the periphery of the stem10 rather than in leaf tissues.Other species, called succulents, have leaves with the ability to store water within. The leaves are usually small and cylindrically shaped, with a majority of the chloroplasts concentrated around the periphery. The center is packed with parenchymatous cells that contain relatively large intracellular vesicles called vacuoles, whose function is to store water and soluble materials11. These reserves are consumed when environmental water supplies dwindle. The extracellular environment of these plants is mucilaginous and contains soluble fibers, which ensure prolonged moistness even when leaves are partly broken off.Some xerophytes have leaves that are oriented in such a way that they funnel rainwater directly to the roots, where it is absorbed immediately.Adaptations: StomataAs the foremost causes of transpiration, the stomata in xerophytes are specialized to retain as much water as possible.9Citation 9 (on page 6 of this document)10Citation 10 (on page 6 of this document)11Citation 11 (on page 6 of this document)6They are positioned in sunken niches and ridges where it is improbable for sunlight to reach. This attempts at decreasing transpiration that occurs easily from cells in close proximity to the stomata. Some xerophytes open their stomata only during the nighttime, when wind speed and temperatures are drastically lower. They absorb and store CO2 at night, and begin photosynthesizing when sunlight is available the next day. This alternative form of photosynthesis is prevalent in the Agave genus. Thus, transpiration is decreased.ConclusionTherefore, in this manner, the leaves of xeric vegetation are able to decrease water loss, with the help of specialized structures that came about as a product of adaptive evolution over millions of years. Desert regions prove to be the hardiest of terrestrial habitats: however, the tissues, stomata and shape of leaves successfully allow vegetation to persist in this environment

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