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 bybark consisting of many layers of dead cells. Bark is penetrated by small pores calledlenticels, 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
	foremost	самый главный
	allow	предоставлять, позволять
	pollination	опыление
	dispersal	рассеяние, разброс
	replace	заменить, замещать
	attachment	прикрепление
	node	узел, нарост, утолщение
	internode	междоузлие
	point	быть направленным, направляться
	swollen	разбухать, пухнуть
	herbaceous	травянистый
	dicotyledonous	двудольный
	monocotyledonous	однодольный
	bark	кора
	stomata	устьица
	penetrate	пронизываться, проникать
	lenticel	чечевичка
	impermeable	непронизаемый
	embed	вставлять, внедрять
	inflate	наполнять, надувать
	turgid	тургесцентный, набухший
	rigid	жесткий
	transverse	поперечный
	tough	несгибаемый, жесткий
	liken	уподоблять
	girder	брус, перекладина
	scatter	разбрасывать, рассеивать
	phloem	флоэма, луб
	parenchyma	паренхима
	resist	сопротивляться
	pitch	сердцевина
	devoid	лишенный (чего-л.), свободный (от чего-л.)
	reinforce	усиливать, подкреплять
	slit	продольный разрез, щель

■ Your Essential Assignments

I. Quick check

1. What distinguishes stems from other plant?

2. List the four functions structures?

3. How do parenchyma cells support herbaceous stems?

 

II. Fill in the missing words:

Term (verb)	Noun	Adjective
disperse	.......	…….
attach	.......	…….
….....	location	…….
identify	.......	…….
store	.......	…….
maintain	.......	…….
penetrate	…..	…….
.........	strength	…….
grow	........	…….
add	.......	…….

 

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:

N	Russian term	English equivalent
1. 1	наземные растения
2.	получать оптимальную экспозицию к свету
3.	удерживать цветы и плоды
4.	эффективное опыление
5.	рассеивание семян
6.	заменять умершие клетки
7.	хранить (запасать) воду, питательные вещества
8.	трудно опознать (идентифицировать)
9.	эпидермис заменяется корой
10.	внутренние части стебля
11.	принимать форму пучков (узлов)
12.	придавать стеблю прочность и гибкость

V. Give Russian equivalents to the following English terms:

N	English term	Russian equivalent
	foremost organs of support
	attachment site of a leaf
	most stems point upward
	easily distinguished from
	unusual shape of location
	swollen underground stems
	maintain the shape of the stem
	reduce water loss
	rigid woody tissue
	in softer turgid tissues

 

VI. Find synonyms among the pool of words:

Pool of words	Synonyms
1)1.keep/2.replace/3.gain/4.store/5.substitude/6.obtain/
2)1.ground / 2. appear / 3. soil / 4. emerge
3)1.reinforce/2.like/3.situation/4.similar/5.position/6.strengthen
4)1.support / 2. seem / 3. maintain/ 4. appear
5)1.tough/2. strength/3.rigid/4.force/5.locate/6.situate

 

VII. Answer the following questions. Use all information given before:

1. How is the stem centre called?

2. What kind of form does the vascular tissue take?

3. Where is the tough rigid vascular bundles embedded in?

4. How are vascular bundles arranged in the:

a) dicotyledonous plants

b) monocotyledonous plants?

5. Are the stems of trees supported by parenchyma?

6. What does the epidermis help?

7. What does the inner part of the stems consist of?

8. What is epidermis covered with?

9. Where do most stems point?

VIII. Match the sentence halves. Make complete sentences:

1.	The stems of most plants are foremost organs of support because	A.	upwards from the ground.
2.	Most stems point	B.	parenchyma cells for storage or it may be devoid of cells.
3.	Some stems have an unusual shape of location which	C.	vascular bundles embedded in undifferentiated cells.
4.	All stems of whatever size, shape or location are distinguishable as such by	D.	gaseous exchange takes place.
5.	The epidermis is covered with	E.	they lift terrestrial plants above the ground rising their leaves towards the sun.
6.	Bark is penetrated by small pores called lenticels through which	F.	waxy cuticle to reduce water loss.
7.	The inner parts of the stems of most non-woody plants consist of	G.	makes them more difficult to identify.
8.	Vascular tissue in the stem take the form of	H.	the presence of nods and internodes.
9.	The stem centre is called pitch. It may consist of	I.	bundles containing phloem and xylem and reinforces with strong fibres.

 

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.

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