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1 Bk2 Ch.09 Water Relation and Transport in Plants/1 Chapter 9 Water Relation and Transport in Plants Summary Table 1: Transpiration in plants (Section 9.1; Fig. 9.4) Definition of transpiration Transpiration Transpiration pull A process by which a plant loses water as water vapourizes from the surface of the plant into the atmosphere A thin water film, surrounding the mesophyll cells, evaporates to form water vapour The concentration of water vapour in the air spaces between mesophyll cells is usually higher than that of the atmosphere Water vapour in the air spaces constantly diffuses through the stomata into the atmosphere Water loss from mesophyll cells by transpiration is replaced by water Water is drawn into mesophyll cells from the neighbouring cells and eventually from xylem in the veins of the leaf by osmosis Force is developed to pull water up the xylem vessels from the roots is known as transpiration pull Table 2: General adaptation of leaves for reduction of water loss by evaporation (Section 9.1; Fig. 9.5) Leaf structure Outer surface covered with cuticle More stomata in the lower epidermis than in the upper epidermis Guard cells control the sizes of stomata opening Adaptation Provide a waterproof layer Reduce water loss in the upper epidermis direct subject to sunlight Tend to close up when light-dependent stage of photosynthesis is not occuring Table 3: Adaptive features of some plants (xerophytes) in a dry environment to reduce evaporation (Section 9.1) Leaf structure Numerous epidermal hairs (Fig. 9.6) Possess sunken stomata (Fig. 9.7) Possess rolling leaves (Fig. 9.8) Possess small, needle-like or spiny leaves (Fig. 9.9) Possess freshly stems or fleshly leaves (Figs and 9.11) Adaptation Trap moisture above leaf surface Trap moisture and greatly reduce evaporation Enclose stomata and reduce the contact of stomata with dry air Provide a small surface area to reduce evaporation Effectively store water when available 1

2 Table 4: Distribution of stomata in the leaf (Section 9.1) Bk2 Ch.09 Water Relation and Transport in Plants/2 In land plants In aquatic plants (e.g. Hydrilla) In floating plants (e.g. water lily) Site The lower epidermis contains more stomata than the upper epidermis Less stomata on leaf surface in some plants that live in dry land Stomata absent on leaf epidermis and without covered by cuticle Stomata are only present in upper epidermis Adaptation Reduce water loss by transpiration Freely permeable to dissolved gases in water and allow gas exchange which takes place over the whole leaf surface Allow gas exchange with air through the upper surface Table 5: Factors affecting the rate of transpiration (Extension Study) (Section 9.2; Fig. 9.18) Light (Fig. 9.19) Temperature (Fig. 9.20) Relative humidity (Fig. 9.21) Air movement (Fig. 9.22) Availability of water (Figs and 9.24) Effect Increase in light intensity leads to a higher transpiration rate Increase in air temperature leads to a higher transpiration rate Decrease in relative humidity in the surrounding air leads to a higher transpiration rate Increase in air movement leads to a higher transpiration rate Lack of water supply in soil or the rate of transpiration exceeds the rate of water absorption, leads to the decrease in transpiration rate Reason Stomata open widely Rise in air temperature The air can hold more water vapour Increase the rate of evaporation of water from mesophyll cells The air can hold more water vapour in a low relative humidity Water vapour evaporates from the leaf is blown away more rapidly and thus maintains a high concentration gradient of water vapour between air spaces in the leaf and the atmosphere The stomata may close to reduce water loss 2

3 Table 6: Simple histology of young dicotyledonous plants (Section 9.3) Bk2 Ch.09 Water Relation and Transport in Plants/3 Type Sites Functions Ground tissues (Fig. 9.26) Vascular tissues Living, thin-walled cells with a large vacuole Living, thick-walled cells, relatively tough but flexible Mesophyll cells and epidermal cells Found near the surface of the stem and in the petioles of leaves Food production and food storage Source of new cells for repairing of damaged tissues Provide support in young plants Dead, thick-walled cells Mainly found in xylem Provide mechanical support of plants Xylem: made up of xylem vessels which are dead, hollow, thick-walled cells joined end-to end (Fig. 9.27) Phloem: made up of sieve tubes and companion cells. The sieve tubes are cells joined end to end with sieve plates in end walls (Figs and 9.29) Locate in the roots, through the stem to the leaves Transport of water and minerals Transport of organic food substances Table 7: Tissue distribution in the young roots and stems of dicotyledonous plants Young stem Young root (Fig. 9.31) (Section 9.3; Fig. 9.30) Tissue The outermost layer of cells is the stem epidermis The vascular bundles are usually arranged in a ring close to the epidermis with xylem in the inner region and the phloem in the outer region A layer of thin-walled cells called cambium is found between the xylem and the phloem The region between the epidermis and the ring of vascular bundle is the cortex The region within the vascular bundle ring is the pith The outermost layer of cells is the root epidermis; some epidermal cells near the root tip develop root hairs The tissue underneath the epidermis is the cortex The vascular bundle is located at the centre of the root The root cap present at the tip of the root protects the delicate growing region of the root 3

4 Bk2 Ch.09 Water Relation and Transport in Plants/4 Table 8: Adaptive features of roots for absorption of water and minerals (Section 9.4; Fig. 9.32) Root structure Extensively branching root system Cuticle absent in the root epidermis Numerous root hairs develop near the root tip Adaptation Allow the plant to absorb water and minerals from a wide area and a great depth of soil Allow water and minerals to enter into the roots reaily form the soil Provide a large surface area for absorption Table 9: The process of the absorption of water and minerals by roots (Section 9.4) Process Water continuously lost through the stomata of the leaves Absorption of water (Fig. 9.33) Absorption of minerals Water is continuously pulled up through xylem vessels from the root region by the transpiration pull A gradient of water potential is set up from the epidermal cells to the xylem vessels in the root Roots absorb water from the soil by osmosis The concentration of minerals in soil water is usually lower than that of the root epidermal cells Root absorbs minerals is mainly achieved by the active transport Table 10: Transport of organic substances in flowering plants (Section 9.5; Fig. 9.34) Site of transport Phloem Translocation Transpiration pull Process Carbohydrates produced are carried away from the leaf in form of sugars (e.g. sucrose) Food substances are transported from the leaves through the stems to other parts of the plants for growth or for storage An important force for the transport of water and minerals A continuous stream of water is maintained in the xylem vessels from the roots, through the stem to the leaves Water together with dissolved minerals are transported as continuous stream inside xylem vessels 4

5 Bk2 Ch.09 Water Relation and Transport in Plants/5 Table 11: The importance of support in plants (Extension Study) (Section 9.6) Plant feature Displaying the leaves in a high position Displaying the leaves in the best orientation Displaying the flowers in a high position Importance Avoid being shaded by other plants Receive maximum sunlight for photosynthesis Faciliate dispersal of pollen or seeds by wind or animals Table 12: Support in dicotyledonous plants (Extension Study) (Section 9.6) Young stem Woody stem Root Structure The turgid thin-walled cells in the cortex and the pith expand and exert a outward pressure on the epidermis The tough epidermis can resist pressure Xylem vessels have thick and rigid cell walls containing lignin Vascular bundles arrange in a ring near the epidermis A layer of cambium between xylem and phloem and divides repeatedly to form new xylem and phloem Vascular bundles concentrate in the centre of the root Function The stem becomes hard and upright, and strong enough to support the leaves and other structures above the ground Provide support and resistant to bending Responsible for increasing the thickness and supporting the stem Provide a great penetrating power for the root to grow through the soil Make the root more resistant to stretching 5

6 Glossary Bk2 Ch.09 Water Relation and Transport in Plants/6 Vocabulary 詞彙 Page Description 解釋 transpiration 蒸騰作用 29 the evaporation of water from living plant cells, caused by the diffusion through stomata or cuticles into the atmosphere transpiration pull 蒸騰牽引力 30 the force that lead to a continual loss of water through the leaves due to transpiration, that pulls water up the xylem vessels from the roots xerophytes 旱生植物 31 a plant that is adapted to living in dry habitats sunken stoma 內陷氣孔 32 an adaptive feature of plant to reduce water loss; the stoma is enclosed in crypts on the lower epidermis bubble potometer weight potometer 氣泡蒸騰計 36 an apparatus used for estimating the rate of water uptake by a plant, determined by the change in position of the air bubble inside a marked capillary tube 重量蒸騰計 38 a simple apparatus used to measure the loss in weight of a plant using a balance, and indirectly estimate the transpiration rate of that plant dehydrated 脫水 42 the state of a substance in which water is being removed from it wilting 凋萎 42 a condition of plants in which the cells lose their turgidity as a result of excessive water lost diurnal 晝行性的 43 to describe animals that are active during daytime ground tissue 基本組織 44 refers to all tissues apart from the epidermis, reproductive tissues and vascular tissues in plant; forming the mesophyll in leaf, and the pith and cortex in young stem and root parenchyma 薄壁組織 44 refers to living plant cells that have thin wall, large vacuole and are not well differentiated; for storing food or acts as loose connective tissues 水分經氣孔或角質層擴散進大氣, 令水分從活植物細胞中蒸發的過程因蒸騰作用而導致水分不斷經葉片散失的力量, 從而把水分從根部沿木質導管抽向上適應生長於乾旱生境的植物植物為減少水分散失的一個適應特徵, 氣孔位於下表皮凹入的位置用以估量植物吸取水分的速率的儀器, 水分吸收的速率是以刻有刻度的微細管內水泡位置的變化而決定利用天平, 量度植物重量跌幅的簡單儀器, 能間接推算出該植物進行蒸騰作用的速率物質被移除水分後的狀態植物的細胞因流失過多水分而失去硬脹度的狀態描述在日間活躍的動物的特徵指植物體內, 除表皮生殖組織及維管組織以外的所有組織 ; 葉片的葉肉, 與及幼莖和幼根的髓和皮層都是由基本組織形成指植物中, 細胞壁很薄液泡大及未完全分化的活細胞 ; 負責儲藏食物, 或作為鬆散的結締組織 6

7 Bk2 Ch.09 Water Relation and Transport in Plants/7 Vocabulary 詞彙 Page Description 解釋 lignin 木質素 45 a complex, hard carbohydrate which thickens and strengthens the cell walls of xylem vessels and sclerenchyma phloem 韌皮部 45, 46 part of the plant tissue in the vascular bundles, made up of companion cells and sieve tubes, responsible for lateral, upward and downward transports of food vascular bundle 維管束 45 strands of conducting tissue in plant, consisting of xylem and phloem, and also cambium in dicotyledonous plant vascular tissue 維管組織 45 composed of xylem and phloem in plants, extend from the roots through the stem to other parts of the plant body; in some plants, there is a thin layer of cambium separating the xylem and phloem xylem 木質部 45 part of the vascular tissue in plant, a woody supporting tissue, a hollow tube for transporting water and minerals from the root through the stems to the leaves and other plant organs xylem vessel 木質導管 45 vessels in plants that are composed of dead, thick-walled, tube-like cells, join end to end with no end walls to form very long, empty tubes; responsible for transport of water and minerals companion cell 伴細胞 46 elongated cell in the phloem of flowering plants, with a nucleus and dense cytoplasm, lying side by side with the sieve tube sieve plate 篩板 46 cell wall with holes found at the top and bottom of a sieve-tube in phloem sieve tube 篩管 46 a series of elongated living cells in phloem joined together with perforated end-walls to form a long tube through which food is transported 一種結構複雜且堅硬的碳水化合物, 使木質管道和厚壁組織的細胞壁加厚度和更堅硬維管束中的部分植物組織, 由伴細胞和篩管所組成, 負責食物的橫向與上下行運輸植物中的多條負責輸導的組織, 包含木質部和韌皮部, 而在雙子葉植物中, 同時包括形成層由植物的木質部和韌皮部組成, 從根部經莖伸延至植物的其他部分 ; 在一些植物中, 維管束內有一層薄薄的形成層, 分隔開木質部和韌皮部植物維管組織的一部分, 是木質的支持組織, 作為一條空心導管, 把水和礦物從根部經莖運輸到葉片等植物構造在植物中, 由厚壁呈管狀的已死細胞構成的管狀構造, 細胞的末端相連, 中間並無隔壁分隔, 形成長的空心管道, 負責運送水分和礦物質在有花植物的韌皮部內的長形細胞, 細胞位處篩管的旁邊, 有細胞核和密集的細胞質在韌皮部的篩管中, 上下兩端處有孔的細胞壁韌皮部由多個拉長的活細胞組成, 它們互相接合, 並以末端的有孔細胞壁作分隔, 形成長長的篩管 ; 植物製造的食物經篩管運送到植物的其他部分 7

8 Bk2 Ch.09 Water Relation and Transport in Plants/8 Vocabulary 詞彙 Page Description 解釋 cambium 形成層 48 a layer of unspecialized cells between xylem and phloem of vascular bundles; cell division in the cambium in stem or root gives rise to the inner secondary xylem and the outer secondary phloem cortex 皮層 48 plant tissue forming the outer part of roots and herbaceous stems; lies between the vascular tissue and epidermis, and is made up of parenchyma cells pith 髓 48 soft tissue made of thin-walled cell and located at the centre of the plant stem and root; used mostly for water storage root cap 根冠 48 a protective cap around the tip of the root, protect the root when growing in soil root hair 根毛 49 a tube-like extension of epidermal cells in the root-hair region of the root tip; increase surface area for the absorption of water and minerals from the soil aphid 蚜蟲 51 small, soft-bodies insects of the family Aphididae that have mouthparts specially adapted for piercing and feed by sucking sap from plants translocation 輸導作用 51 movement of organic food through phloem in a plant eosin solution 曙紅溶液 52 a red dye; used in experiment to trace the pathway of water movement in a plant capillary action 毛細管作用 54 the force that draws water upwards through narrow spaces root pressure 根壓 54 a force by which water is pushed up the plant stem through xylem vessel into the lower leaves; not present in all plants and not strong enough to push water up to leaves at top of tall trees dispersal 散佈 55 the process of distributing or scattering substances in different directions 木質部與韌皮部之間的非特化細胞層 ; 根部和莖部的形成層進行細胞分裂, 構成內部的次生木質部, 以及外部的次生韌皮部在根部及草質莖外層的植物組織, 由薄壁組織細胞構成, 位於維管組織與表皮之間由壁薄的細胞構成的柔軟組織, 位處植物的莖和根的中心, 主要用以儲藏水分包圍根最尖端的組織, 負責為在土壤中生長的根提供保護作用表皮細胞中呈管狀的延伸部分, 位處根尖的根毛區域 ; 根毛增加從泥土中吸收水分和礦物質的表面積細小的軟體昆蟲, 歸入蚜科, 有口部, 能刺穿植物並以吸啜當中的汁液作為食物植物內有機食物經韌皮部的移動紅色染料, 在實驗中用以找出植物水分流動的途徑經窄隙引水上升的力量把植物莖部的水分沿木質導管向上推至低層葉片的力量 ; 根壓並非在所有的植物中出現, 且其力量不足以把水分向上推至高樹頂層的葉片把物質向不同方向散開分佈的過程 8

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