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 andsclerenchyma 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
■ 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:
IV. Find synonyms among the pool of words:
V. Give English equivalents to the following word combinations:
VI. Match these words with definitions:
VII. Give Russian equivalents to the following English terms:
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