The cells of the ground tissue

Parenchyma cells are the least specialised of plant cells; they are characterised by having intercellular air spaces which vary in size. Parenchyma cells are regarded as the basic cells from which other cells have evolved. Parenchyma cells form the packing tissue of plants, and include the palisade cells and spongy mesophyll cells which make up the main photosynthesising tissue in the leaf.

Palisade cells are a dense green colour due to the numerous chloroplasts they contain. These cells are packed tightly together in a regular arrangement near the upper surface of the leaf so they obtain the maximum exposure to light. The chloroplasts can move round inside the cells according to the amount of light available. If it is a dull day, they are often clustered at the tops of the cells, in the best position to trap light; in very sunny conditions, they may be grouped towards the bottoms of the cells to avoid being overexposed to light.

The spongy mesophyll is the chief site of gaseous exchange in the leaf. It consists of rounded or sausage-shaped cells with fewer chloroplasts than palisade cells. The cells are closely arranged and between each of them are air spaces connecting the mesophyll with stomata.

Collenchyma and sclerenchyma make up tissues that have a supportive, structural role in plants. In leaves, these cells are common around the vascular bundles (especially in midrib) and at the leaf tips. Collenchyma cells are elongated and have unevenly thickened cell walls with extra cellulose in the corners of the cells. There are two main types of sclerenchyma: fibres are very elongated and have very thick cell walls impregnated with lignin; sclereids (or stone cells) are more spherical in shape. Both types of sclerenchyma cells are specialised for support. Fibres in particular have great tensile strength and do not break easily when stretched. Mature sclerenchyma cells are dead because they are enclosed in a complete layer of lignin which is impermeable to water.

■ Glossary of essential terms for you to know

N	English term	Russian equivalent
12.	lamina	местовая пластинка
13.	stalk	стебель, ножка, черенок
14.	petiole	черешок листа
15.	vein	жилка листа
16.	tissue	ткань
17.	vascular tissue	сосудистая ткань, проводящая ткань
18.	ground tissue	покровная ткань
19.	rectangular	прямоугольный
20.	to gain	получать, достигать
21.	to escape	улетучиваться, ускользать
22.	stomata	устьице
23.	to flank	защищать, прикрывать
24.	guard	сторожевой,
25.	to expose	выставлять, подвергать действию (солнца, света)
26.	to raise	поднимать
27.	to arrange	располагать
28.	vascular bundle	сосудистый пучок
29.	xylem	ксилема
30.	phloem	флоэма
31.	palisade	столбчатый, палисадный
32.	spongy	губчатый, пористый
33.	mesophyll	мезофилл
34.	bulk	основная масса
35.	to reinforce	укреплять, усиливать
36.	to evolve	развиваться
37.	cluster	пучок, гроздь
38.	to trap	ловить, задерживать
39.	midrib	средняя жилка листа
40.	tip	кончик
41.	uneven	неровный, шероховатый
42.	to impregnate	оплодотворять
43.	tensile	растяжимый
44.	lignin	лигнин
45.	collenchyma	колленхима
46.	sclerenchyma	склеренхима
47.	sclereid	склеренда
48.	impermeable	непроницаемый

 

■ Your Essential Assignments

I. Quick check

1. a) Which structure forms a waterproof layer on the surface of the leaf?

b) Why is this structure thicker than the upper surface of the leaf than on the lower surface?

2.What is the main function of palisade cells?

3. How does collenchyma differ from sclerenchyma?

 

II. Use monolingual English dictionary and write down what could the words given below mean:

waterproof, shade, pore, infection, waxy coating, guard cells, root.

 

III. Fill in the missing words:

Term	Noun	Adjective
connect	…….	…….
require	…….	…….
support	…….	…….
protect	…….	…….
adapt	…….	…….
expose	…….	…….
reproduce	…….	…….

IV. Find synonyms among the pool of words:

Pool of words	Synonyms
1)1.amount /2.flat /3.trap /4.quantity /5.even /6.catch
2)1.volume /2.fabric /3.reach /4.bulk /5.gain /6.tissue
3)1.escape /2.produce /3.disappear /4.manufacture
4)1.evolution /2.waterproof /3.impermiable /4.development

V. Give English equivalents to the following word combinations:

N	Russian term	English equivalent
1.	группа цветущих растений
2.	поглощение света и углекислого газа
3.	главная жилка листа
4.	плоская форма листа
5.	физические повреждения листа
6.	пара охранительных (сторожевых) клеток
7.	верхняя поверхность листа
8.	потеря воды
9.	наименее специализированные растительные клетки
10.	плотные зеленые клетки листа
11.	согласно количеству света
12.	наилучшая позиция улавливания света
13.	главный участок газообмена
14.	кончик листа
15.	неровно утолщенные клеточные стенки

VI. Match these words with definitions:

N	Word		Definition
1.	absorption	A	any of the very thin tubes that form the frame of a leaf
2.	stalk	B	the act of protecting s/b or s/th from attack, criticism etc.
3.	terrestrial	C	not allowing a liquid or gas to pass through
4.	cluster	D	the process of a liquid, gas or other substance being taken in
5.	defence	E	living on theland or on the ground rather than in water or in the air
6.	vein	F	a group of things of the same type that grow close together
7.	impermeable	G	thin stem that supports a leaf, flower or fruit and joins it to another part of the plant

VII. Give Russian equivalents to the following English terms:

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