28. Text 8.2. The Stem

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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:

N		English term		Russian equivalent
1		foremost		самый главный
2		allow		предоставлять, позволять
3		pollination		опыление
4		dispersal		рассеяние, разброс
5		replace		заменить, замещать
6		attachment		прикрепление
7		node		узел, нарост, утолщение
8		internode		междоузлие
9		point		быть направленным, направляться
10		swollen		разбухать, пухнуть
11		herbaceous		травянистый
12		dicotyledonous		двудольный
13		monocotyledonous		однодольный
14		bark		кора
15		stomata		устьица
16		penetrate		пронизываться, проникать
17		lenticel		чечевичка
18		impermeable		непронизаемый
19		embed		вставлять, внедрять
20		inflate		наполнять, надувать
21		turgid		тургесцентный, набухший
22		rigid		жесткий
23		transverse		поперечный
24		tough		несгибаемый, жесткий
25		liken		уподоблять
26		girder		брус, перекладина
27		scatter		разбрасывать, рассеивать
28		phloem		флоэма, луб
29	parenchyma		паренхима
30	resist		сопротивляться
31	pitch		сердцевина
32	devoid		лишенный (чего-л.), свободный (от чего-л.)
33	reinforce		усиливать, подкреплять
34	slit		продольный разрез, щель

■ Your Essential Assignments