Арсланова_Г_А_и_др_Essential_English_for_Biology_Students (1). Kazan federal university
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Text 8.2. The Stem■  Essential targets: By the end of this text you should be able to: ● describe the structure of dicotyledonous stem; ● state the major functions of stems; ● explain how different tissues contribute to the mechanical support of stems. Pre-reading ■ With a partner consider the following questions and try to answer them. 1. How do you think, what are the functions of stems? 2. What are distinguishable marks of all stems? ■ Read the given text and make your essential assignments: Functions of a stem. The stems of most plants are first and foremost organs of support. They lift terrestrial plants above the ground, raising their leaves towards the Sun and holding them in the best position to gain optimum exposure to light and carbon dioxide. They also hold flowers and fruit in positions that allow efficient pollination and seed dispersal. As well as support, stems have three other major functions: they transport materials from one part of the plant to another; they produce new living tissue to replace cells that die and to make new growth; they store food and water. The stem as plant organ The attachment site of a leaf or bud on a stem is called a node, and the po rtion between nodes is called an internode. Most stems point upwards from the ground and are easily distinguished from other plant organs. Some stems, however, have an unusual shape or location which makes them more difficult to identify. Potato tubers, for example, appear root-like, but they are actually swollen underground stems specialised for food storage. All stems, of whatever size, shape, or location, are distinguishable as such by the presence of nodes and internodes. The tissues and cells of a stem In a dicotyledonous, non-woody (herbaceous) stem the epidermis is like that of a leaf: a single layer of cells perforated by stomata. The epidermis helps maintain the shape of the stem. It is covered with a waxy cuticle to reduce water loss. In woody stems of trees and bushes, the epidermis is replaced by bark consisting of many layers of dead cells. Bark is penetrated by small pores called lenticels, through which gaseous exchange takes place. The lenticels usually appear as raised spots surrounded by a powdery and impermeable material. Just inside the epidermis, a layer of collenchyma gives both support and flexibikity to the stem. Some collenchyma cells contain chloroplasts which make the stem appear green. The inner parts of the stems of most non-woody plants consist of vascular bundles embedded in undifferentiated parenchyma cells. When fully inflated with water (turgid), the parenchyma cells press against the epidermis and collenchyma, strengthening the stem. The stems of trees and bushes are supported not by parenchyma but by rigid woody tissue which makes up the bulk of these stems. The woody tissue consists of xylem and associated cell such as fibres formed by a process called secondary growth. New wood is added outside the old wood each growing season to form annual growth rings, visible in transverse sections of the stems of trees and shrubs. Vascular tissue in the stem takes the form of bundles containing phloem and xylem and reinforced with strong fibres. The xylem is located towards the inside of the stem and the phloem towards the outside. The tough rigid vascular bundles embedded in softer turgid parenchyma tissue have been likened to reinforced concrete, in which rigid steel girders are imbedded in softer concrete. This arrangement gives the stem strength and flexibility, making it well suited to resisting sideways bending in strong winds. The vascular bundles of dicotyledonous plants are arranged in a ring pattern around the outside of the stem, while in monocotyledons such as cacti the vascular bundles are scattered throughout the stem. The stem centre is called the pith. It may consist of parenchyma cells for storage, or it may be devoid of cells, in which case it is called a pith cavity. ■ Glossary of essential terms for you to know:
■ Your Essential Assignments I. Quick check What distinguishes stems from other plant? List the four functions structures? How do parenchyma cells support herbaceous stems? Fill in the missing words:
III. Use monolingual English dictionary and write down what could the words given below mean: ground, growth, woody stem, non-woody stem, exchange, flexibility, growing season, resist, bundle. IV. Find English equivalents to the following word combinations:
V. Give Russian equivalents to the following English terms:
VI. Find synonyms among the pool of words:
VII. Answer the following questions. Use all information given before: How is the stem centre called? What kind of form does the vascular tissue take? Where is the tough rigid vascular bundles embedded in? How are vascular bundles arranged in the: a) dicotyledonous plants b) monocotyledonous plants? Are the stems of trees supported by parenchyma? What does the epidermis help? What does the inner part of the stems consist of? What is epidermis covered with? Where do most stems point? VIII. Match the sentence halves. Make complete sentences:
IX. Read and translate the short text without any dictionary: Fact of life: The stems of woody plants have a layer of protective tissue called cork, just below the epidermis. Cork is made of dead cells coated with a waxy substance (suberin) which makes them waterproof. The exceptionally thick cork layer of the oak (quercus suber) is removed for commercial use. If cork formed a complete layer, stem cells would die because they wouldn’t be able to exchange respiration gases with the environment. However, slit-like openings called lenticels develop in the cork. The lenticels contain loosely packed thin-walled dead cells which lack suberin, and they have large intercellular spaces to allow gaseous exchange. X. Food for thought: Cacti live in hot dry American deserts. To conserve water and deter herbivores, their leaves lose their photosynthesising function and are modified into spines. Nevertheless, cacti may lose as much as 20 per cent of their tissue fluids in a severe drought. Suggest how the stem is adapted to: a) carry out photosynthesis b) minimise water losses and minimise the effect on the plant of water-losses. XI. Translate into English using all the active possible. Стебель удерживает плоды и цветы в таком положении, чтобы происходило эффективное опыление и рассеяние семян. Клубни картофеля кажутся похожими на корень, но на самом деле они представляют собой разбухшие подземные стебли. Эпидермис помогает поддерживать форму стебля. Внутри эпидермиса находится слой колленхимных клеток, который придает стеблю как поддержку, так и гибкость. При полном наполнении водой (тургесцентный) парехимные клетки давят на эпидермис и колленхим, укрепляя (усиливая) стебель. Древесная ткань состоит из ксилемных и ассоциированных клеток, таких как волокна, сформированных посредством процесса, называемого вторичным ростом. Такое расположение придает стеблю силу и гибкость, делая его хорошо приспособленным к сопротивлению наклонам из стороны в сторону при сильном ветре. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||