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:

■ 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:

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:

V. Give Russian equivalents to the following English terms:

VI. Find synonyms among the pool of words:

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:

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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