X. Read and translate the short text without any dictionary:

Fact of life:

Plants are not very efficient at harnessing energy from the sunlight they receive. Under the most carefully controlled laboratory conditions plants can reach 25% efficiency but on cloudy days the natural photosynthetic efficiency of most individual plants is about 0.1%. The annual winter evening primrose, Oenothera claviformis, has the highest natural photosynthetic efficiency at 8%, closely followed by sugar cane at 7%.

XI. Food for thought:

If C₄ plants have a greater photosynthetic efficiency than C₃ plants, suggest why all plants do not have C₄ metabolism.

Biology Jokes

Enjoy funny biology jokes while taking a break from serious science.

• What do you call the leader of a biology gang? - The nucleus

• Blood flows down one leg and up the other.

• The pistol of a flower is its last line of defense against insects.

• Mushrooms look like umbrellas because they grow in damp places.

• A couple of biologists had twins. One they called John and the other Control.

• Genetics explain why you look like your father and if you don`t why

you should.

• When you breathe, you inspire. When you do not breathe, you expire.

• Three kinds of blood vessels are: arteries, vanes, and caterpillars.

26. Unit VIII. Structure and transport in plants

27. Text 8.1 The Leaf

■

Essential targets:

By the end of this text you should be able to:

• describe the structure of a dicotyledonous leaf;

• distinguish between parenchyma, collenchyma, clerenchyma and sclerenchyma.

Pre-reading

■ Working in pairs, discuss the following questions with your partner:

1. What does leaf shape provide?

2. How are leafs arranged on many plants?

■ Read the given text and make your essential assignments:

The leaf is the main site of photosynthesis, the process by which green plants manufacture their own food. The lamina or blade of a leaf is flat and thin. Its shape provides a large surface area for absorption of light and carbon dioxide. The leaf is attached to a stem or branch by a leaf stalk or petiole. The stalk holds the leaf in a position such that its surface is exposed to the maximum amount of light. From the stalk, the main vein leads down the leaf with side veins branching out on either side. These veins connect the leaf to the rest of the plant, bringing the leaf some of the raw materials required for photosynthesis, and carrying products of photosynthesis away from it. This veins also provide mechanical support, maintaining the shape of the leaf. The stem and branches raise the leaves above the ground so they are exposed to the light. On many plants the leaves are arranged on branches in such a way that they do not shade one another.

The tissues of a leaf

In common with stems and roots, leaves are made up of three main types of tissue: epidermal tissue, vascular tissue, and ground tissue. Each tissue forms a continuous system throughout the plant.

The epidermis covers and protects the leaves. It is the first line of defence against physical damage, infection, and being eaten. The upper epidermis consists of one or more layers of rectangular cells. In terrestrial plants, these epidermal cells secrete a waxy coating called the cuticle. The waxy cuticle is waterproof, minimising water loss from the surface of the leaf. It is often thicker on the upper surface, making this surface appear more shiny than the lower surface.

The epidermis is perforated by microscopic pores called stomata. Stomata allow carbon dioxide and oxygen to gain easy access into the plant, but also allow water to escape. Each stomata is flanked by a pair of guard cells that regulate the size of the pore, closing it in times of water stress. Water is more likely to be lost from the upper surface of a leaf because it is more exposed to sunlight. The upper surface usually has fewer stomata than the lower surface; this minimises water loss.

The vascular tissue consists of veins adapted to transpsort liquid substances around the plant, and it is made up of vascular bundles, groups of vessels running from the root up the stem and to the leaves. Xylem forms the upper part of a vascular bundle in the leaf, bringing water and mineral salts to the leaf. Phloem forms the lower part of a bundle, transporting sucrose and other products of photosynthesis away from the leaf.

Ground tissue is all the tissue in a plant other than the epidermis, reproductive tissue, and vascular tissue. It makes the bulk of a leaf and consists mainly of parenchyma cells reinforced by collenchyma and sclerenchyma.

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.

T

he 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
	lamina	местовая пластинка
	stalk	стебель, ножка, черенок
	petiole	черешок листа
	vein	жилка листа
	tissue	ткань
	vascular tissue	сосудистая ткань, проводящая ткань
	ground tissue	покровная ткань
	rectangular	прямоугольный
	to gain	получать, достигать
	to escape	улетучиваться, ускользать
	stomata	устьице
	to flank	защищать, прикрывать
	guard	сторожевой,
	to expose	выставлять, подвергать действию (солнца, света)
	to raise	поднимать
	to arrange	располагать
	vascular bundle	сосудистый пучок
	xylem	ксилема
	phloem	флоэма
	palisade	столбчатый, палисадный
	spongy	губчатый, пористый
	mesophyll	мезофилл
	bulk	основная масса
	to reinforce	укреплять, усиливать
	to evolve	развиваться
	cluster	пучок, гроздь
	to trap	ловить, задерживать
	midrib	средняя жилка листа
	tip	кончик
	uneven	неровный, шероховатый
	to impregnate	оплодотворять
	tensile	растяжимый
	lignin	лигнин
	collenchyma	колленхима
	sclerenchyma	склеренхима
	sclereid	склеренда
	impermeable	непроницаемый

■ Your Essential Assignments