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Transpiration is a force that pulls

water from the soil through the roots and to the rest of

the plant.

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Water evaporates through the leaves and creates a “suction” effect which enforces the water to be sucked through the stem.Water is sucked from the lower part ofthe plant to the leaves, from where it escapes into the surroundings, through the vascular bundles. This is called the transpiration stream.

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What is transpiration for?A tree draws up water from soil.Most of this water evaporates from the

leaves and the rest is used for photosynthesis and to maintain the turgor of the cells.

The water which evaporates from the leaves can give a cooling effect to a tree on a very hot day.

The transpiration encourages the salts to be absorbed from the roots together with the water.

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4 factors which control rate of transpiration are

light intensity,

humidity,

air movement,

and temperature.

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Transport of salts.The liquid which travels in the xylem is not pure water, but contains from 0.1 to 1.0% dissolved solids, mostly amino acids, other organic acids and mineral salts. The organic acids are made in the roots, while the mineral salts come from the soil.

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Transport of food.The xylem sap is always a very dilute solution, but the phloem sap may contain up to 25% of dissolved solids such as sucrose and amino acids.

Translocation

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Cross sections of dicot stem and root

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Cross section through a monocot stem

Cross section through a monocot root

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Arrangement of vascular bundles in a tree trunk

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Measuring the rate of transpiration using a potometer

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To demonstrate Root Pressure

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Weighing method.

Weigh the plant now, and again after a given time. Assuming that a decrease in mass is due to loss of water, calculate the mass of water lost per unit time.

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How are plants adapted for living in dry places?They have relatively few stomata.Their stomata are sunk down into pits in the

epidermis.They have folded or rolled up leaves.They have a hairy epidermis.They have small leaves.They drop their leaves in the winter or dry season.They have deep roots for absorbing water from deep

down in the soil.They store water in swollen stems and leaves. (Plants

which do this are called succulents).

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succulentscacti

Xerophytes