Plants due to external ecological factors, and gradually evolved a variety of form and structure to accommodate the growth environment. Which affect plant growth is the biggest around the water supply. Therefore, in accordance with the relationship between plants and water, plants can be divided into xeric plants, in plants, wetland plants and aquatic plants. However, different plants, is grown in the same niche, they overcome the adverse environmental conditions, special methods may also very different. Xeric plants are usually specified in the appropriate water plants dry type, different from the drought-type plants. Through morphological or physiological adaptation, can maintain a body of water in arid areas in order to maintain the survival of plants. Broad xeric plants, including drought-tolerant type of plant. In recent years, according to desert plants (plants grown in desert areas) studies have shown that desert plants to be able to survive in the desert region, in addition to tolerance of drought, it also must be able to withstand malnutrition ("hunger"). On the one hand to develop a mechanism to reduce water loss, while the need to maintain efficient photosynthesis. Desert plants to maintain the physiological balance can be called "water - photosynthesis integrated relationship." And their morphology also with these physical requirements, the occurrence of certain changes to adapt. Generally strong in the light of a serious water shortage and the growth of plants, the plants tend to become stout dwarf. Ground aerial part of the development of a variety of structures to prevent excessive water loss, while the roots are deep underground soil or underground water storage organ formation. On the other hand, smaller leaves on the stem or loss after the shoots or leaves, young stems to replace the role of the cortex in their cells or other tissues can be rich in chloroplasts, photosynthesis. In addition, many woody desert areas, arid as the result of long-term adaptation, and more into the bushes, which grow in the desert there are many advantages. As many of the growth in saline soil of the so-called halophyte, or dry - salt plants, due to the physical lack of water, also showing the generally xerophytic structure.
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Appropriate drought deformation
旱生植物 适旱变形
Xerophytic plant morphology and structural changes, from the roots, stems and leaves of three shown. First, the root changes General for the underground portion of the root growth of plant knowledge, far and above-ground stems and leaves. This is because the root systems, buried in a lot of difficulty observed. Moreover, many xeric plants root system deep-rooted, much less study. Are known xerophytic plant roots. Roughly following some changes. Xerophytic plants have a higher root / stem ratio. Some of the main root growth can be deep, such as a Atriplex (atriplex sp.) Ground stems, although only 1-2 meters high, but the main root but can be up to 4-5 meters deep. It is said that mesquite (prosopis) and camel thorn (alhagi) of the main root could be as deep as 20 meters! Roots with different degrees of succulent, this succulent is mainly the increase of some thin-walled cells, but the cortex is not simply part of the increase. Root cortical layers, however, reduced (Table 1). Some people think that this could make closer to the soil column. Some xeric plants can also be found in the distribution of stone cells in the cortex, but their physiological function is unclear. Endodermis cell wall thickening, Casparian band widened. Casparian band wider, like xerophytic traits with a certain relationship. Extreme cases, Casparian band can be surrounded in the radial cortical cell wall and lateral wall, such as a white thorn (nitraria retusa). Desert plants often form a separate dimension column. This is due to the formation of cork layers, or between the cortical parenchyma cells of vascular necrosis, separated from the vascular tissues of the results. In contrast, more developed xylem, which may be more effective in conveying water. Second, the stems to adapt Stem is an important part of the ground, and withstood the effects of drought, the roots than significant, is relatively easy to observe, they form the anatomical changes are: Desert perennials grown leaves are often very degraded, for example, some plants with a section of the Polygonaceae, a variety of Calligonum (calligonum sp.) Is a notable example, the long dry season, or they fall off before the start. Some xeric plants such as wormwood (artemisia sp.), The red sand (reaumuria sp.) Atriplex (atriplex sp.) And a wooden pink (cymnocarpos fruticosus), during the dry season, when shedding of leaves, can be replaced with some form of smaller, more naturally dry leaves. Some plants, such as a King (zygophyllum dumosum), in the dry season after leaf fall, with the petiole still preserved chloroplasts, photosynthesis. Shoots instead of leaves of features, such as a variety of Haloxylon (haloxylon spp.) (Figure 1) and Calligonum (calligonum spp.), Has not developed a leaf stems (or some very degraded scale leaves, 1 ), but in the young green branches for photosynthesis, the formation of the so-called assimilation stems. After some of these branches may fall off. Some desert plants, branches, dead in the dry season in time to reduce the evaporation of water, while plants need water to minimize the extent, but one rainy season, they are also able to rapidly grow new branches. Desert plants, especially the desert shrub, often seen as a characteristic that is split to form the stem. Such as a wormwood (artemisia herba-alba), Peganum harmala (peganum harmala), and a King (zygophyllum dumosum) the stem can be split into several parts. Split to form several separate parts, because of the conditions encountered may be different niche, so some dry dead, while others may survive and continue to grow. Xerophyte the cortex and the ratio of the larger column, the stem of the cortex than in the plants of the width, while the bundle is more closely around the narrow marrow. This structure may be an adaptive mechanism, especially in the cork layer is formed before the thickness of the cortex may be related to protection of vascular tissue from drought. Xerophytic plant stems and roots increased cortical thickness in cortical layers of the reduction in contrast. With some sections of the Chenopodiaceae, such as Anabasis (anabasis sp.) And Haloxylon (haloxylon sp.), Cortex, succulent, and to carry out photosynthesis. To the very dry summer, you can gradually falling off, and in the phloem parenchyma cells produce a cork layer that protects the internal vascular tissue. Some desert plants, the stems of green groups in addition to photosynthesis, it also developed a storage parenchyma. This usually presents succulent stem cells within the gel material and crystallization (Figure 1). Some young stems without leaves by photosynthesis of plants, stems of the stomatal opening may be blocked, or to defend the cell wall thickening does not seem to open look. No fleshy cortex of some xeric plants, such as a Atriplex (atriplex halimus) and a King (zygophyllum dumosum), the initial formation of the periderm, deep inside, is located more inside the stem phloem parenchyma of the development . It may also be a xerophytic adaptation mechanism. Some desert shrubs, such as wormwood (artemisia spp.), At the end of each year when nearly xylem proliferation (rapid growth rings is the end of each year), stems often occur out of a "cork between the xylem ring." Moss (1940) pointed out that this feature has a very important adaptive value, can reduce water loss, and increased water restrictions can have a role to a narrow zone of secondary xylem. Xeric plants cambium activity for years rhythms, the rhythm in plants than strict, and more generally with the local rainy season to begin its activities, entered the dry season, activity ceased. However, according to reports, some desert areas in the eastern Mediterranean plants, annual cambium activity may have two peaks. Most of the growth of plants in the desert, sapwood of wood fibers and fiber tracheids, may contain protoplast and storage material, remains the state of life. The role of these two kinds of cells, much like wood parenchyma cells and ray cells. According to reports, in a white thorn (nitraria retusa) and a Calligonum (calligonum comosum), these can be seen live wood fiber. Mesophyte wood fibers and fiber tracheids are no lives have been lost protoplast cells, but in the desert plant life was reported in the presence of wood fiber, so this has been a debate on plant anatomy problems. Third, the abnormal leaf Leaves is a flowering plant mainly for transpiration organs, so the plants dry leaves to reduce transpiration, the phase of adaptation is most obvious structural changes, which in the last century has caused a lot of botanists' attention, Maksimov (1925,1931) summarized the work of their predecessors, that the plant grows in arid regions, in the dry conditions, transpiration will be reduced to a minimum. As previously mentioned, many desert plants leaves degraded or retained only a few leaves, young stems often instead of leaves for photosynthesis. Now generally believed that the cause of dry leaves to show, roughly three points: 1) the lack of water; 2) a strong light; 3) the lack of nitrogen. The growth of desert plants, often lack these three, the leaves of the drought was most prominent structure. This leaves an important morphological and structural changes, about some of the following areas: Structure of the dry leaves with the most notable feature is the leaf surface area and its volume ratio decreases. Many workers also pointed out that the reduction of the outer surface of the leaves, often accompanied by some internal structural changes, such as smaller leaf cells, cell wall thickening, vascular system, the density increases, the increase in the development of palisade tissue, spongy tissue corresponding reduction in Therefore, the ability of photosynthesis also increases. Reduction in leaf size, corresponding to reduce transpiration, but in some plants, leaves smaller size, the number of leaves on the plants, but rather increased. Thus, the total surface area but larger. For example, the leaves of some conifers of the total area, than the larger number of dicotyledonous plants. Generally believed that the xeric plants to increase the density of stomata is also a feature. This increase may be due to reductions in the relative increase in leaf area results. Xeric plants increased stomatal density, but also waiting for the water supply is sufficient to increase the gas exchange and improve the efficiency of photosynthesis. Some xeric plants, stomata in the epidermis in depth, the subsidence of the pores can be formed nest, nest tables inside or trench covered with fur, such as oleander and a woody monocotyledons xanthorrhoea. Dry leaves of plants often form dense fur or white wax, such as a Elaeagnus (elaeagnus ploarcroftii). This may reduce transpiration and reflected light are related. However, Shields (1950) that the life table fur itself to lose a lot of moisture, therefore it can not protect the plants over transpiration, trichomes only to death, in the leaf surface to form a covering layer to be able to reduce leaf transpiration . Dry leaves of plants often contain resin or tannin, or some other colloidal substances. Very early on that the main role of these substances obstruct water flow. In addition, such a small sorrel (rumex acetosella), in dry conditions, leaf epidermis and around the veins of the cells, the formation of resin droplets or oil droplets can be used to impede water flow. Some Mediterranean oak tree leaves, with tannins and resins, may also have the same effect. Some leaves may have essential oils, face drought, can reduce the evaporation of water vapor transpiration rate. Leaves in the water conduit, not only on veins and bundle sheath extension area, but also through the mesophyll and epidermis. In recent years, there were found in the leaves from the quality and type of quality after the two kinds of transport, such mesophyll cells containing some of these substances, showing there are more important. Moisture in the leaves within the conduit, through the palisade tissue to the epidermis than through the spongy tissue much. While the arrangement of palisade tissue and have a great relationship. Some round or nearly round the xerophytic leaf, palisade tissue radial arrangement around the central vascular bundle, and therefore suitable for water supply when water from the vascular conduit to the epidermis can be greatly enhanced. Xeric plants can often be seen in the veins to increase, and some stone short tracheid cells. In Salicornia (salicornia) degradation of leaves, palisade cells is easy to see the wide between short tracheid-like cells. For these cells, different observers have different interpretations: first that they are within full of air, and later, or that these cells can transport moisture into the surrounding layers, or that only one of these tracheid-like cells, the role of water. In addition, the mesophyll tissue may also spread a different tracheid-like cells, such as Euphorbiaceae pogonophora schomburgkiana. Volume is also a dry leaf in leaves of drought-resistant plants way, especially in grasses (such as Stipa) can be seen. Grass leaf has a special class of many bubble-like cells (called the exercise or cells), when faced with very dry, due to this bubble-like cells and (or) other epidermal cells with thin or thick-walled mesophyll cells , enables the leaves within the volume. On the other hand, ordinary dry leaves of plants usually have a lot of sclerenchyma, and have great mechanical strength, which is thought to reduce wilting time of injury, the growth of desert plants often have this feature. In short, normally grows in arid environments, plants can exhibit a variety of xerophytic characteristics. But for some plants will not necessarily be fully applicable, such as oleander, usually can be grown in moist, sufficient water area, but has many xerophytic morphological and structural features. Another example is the almond trees and leguminous anagyris usual growth in arid habitats, but the leaf structure is school students. Of course, the general said, xerophytic structure and arid environment is basically a relationship, however, even above the xerophytic structure, some features are different.
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Classification
旱生植物 分类
Many types of xeric plants, on the division and inconsistency of standards, can generally be divided into four types: A succulent xerophytic plants Parenchyma through the body of these plants store large amounts of water to form a succulent stems or leaves to reduce water loss to adapt to the number of severe drought. Succulent leaves are reflected in the agave, aloe, etc.; reflected in the main stem of the cactus plant. Form has reduced the relative surface area, thicker cuticle, sunken stomata and so on. But the most special adaptation is a special mechanism of photosynthesis, stomatal opening at night, when the bright day but closed. Epidermal water adept. Such plants in absolute anhydrous conditions for the survival time is very long, but semi-lethal water content (usually low value shows drought resistance) is extremely high. Such as large Sedum (sedum maximum) total dry weight of water that is 630-590% of deaths by half. Second, the hard dry leaf plants Such plants have a typical xerophytic structure, but not succulent. Its mechanical organization developed or Jiao Zhiceng thicker, more time in the water loss can prevent the occurrence of leaf shrinkage cracking. Another feature of their adaptation to drought is a large root system, water and more. In the same environment in which plants stomatal closure due to the drought, they have continued to open holes for photosynthesis, and to promote water absorption. The ability of such plants is to endure dehydration most xeric plants, but the overall ability is not suitable for very dry, can only live in seasonally dry areas, such as the Mediterranean climate zone. Representative plants Scots pine, oleander, Stipa and so on. Third, the soft leaves xerophytes malacophyllous xerophytes Although the leaves of these plants have varying degrees of xerophytic structure, but softer, and the leaves are similar in plants. Soil moisture more season, it's even more than transpiration in plants. Deciduous in the dry season to adapt. Such as bindweed are some of the species. Fourth, the lobular type and non-leaf plants Also known as "super-drought plants," the strongest drought, desert regional distribution of the more common. The former strong narrow leaves, leaf area is usually less than 1cm ^ 2. The latter leaves completely degraded, with green stems for photosynthesis. If Calligonum, ephedra.
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Other suitable dry plant
First, change the water plant. Drought dry state without body dehydration was killed (just sleep), rapid recovery when wet positive life events, it is also known drought-tolerant plants. Second, the semi-arid plants. Living in arid areas, but little or no morphological characteristics of plants suitable for dry, including two categories: short-short-class plants and plants; deep roots thin-leaved plants. The former must rely on the rainy season, which must rely on rich groundwater to survive, it can not be considered truly xeric plants. Third, the halophytes xeric plants. Living in diving slightly darker saline soil was classified as a class of halophytes, but also has obvious characteristics of xeric plants. Such as Anabasis. Fourth, the drought has a suitable capacity in plants. Many of the plants grown in relatively dry areas, but also some similar structural and physiological characteristics of xeric changes, known as the "proper dry abnormal", but this metamorphosis is reversible, unlike the dry plants. One of the most typical is the plants and xeric plants, the transitional type, also known as "dry the plants." The "semi-dry plants" is also sometimes classified as "drought in plants" in the.
