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How Plants Breathe

Primula, Bristol, UK © Shirley Burchill

Plants do breathe - they give out carbon dioxide and absorb oxygen from the air that surrounds them. Their tissues respire just as animal tissues do. Plants, however, do not have lungs or a blood stream, so we cannot say that they breathe in the same way as animals.

We also have to be careful when studying green plants because in the light the green parts of these plants carry out photosynthesis as well as respiration.

Wild Strawberries, Sheffield, UK © Shirley Burchill

Photosynthesis does the opposite of respiration. Carbon dioxide is absorbed and oxygen is produced. In order to study respiration in green plants we must block out the light, because although green plants respire all the time they only photosynthesize in the light.

Remember that a green plant respires all the time, day and night.
A green plant photosynthesizes only in the presence of sunlight.

All parts of the plant respire, the leaves, the stem, the roots and even the flowers. The parts above the soil get their oxygen directly from the air through pores. The pores in the leaves are called stomata (singular: stoma). The pores in the branches of trees are called lenticels.

Drawing of Ficus leaf with stomata magnified © Shirley Burchill

The drawing shows a leaf of a ficus plant. A small part of the underside of the leaf has been magnified to show the stomata. The average number of stomata per mm2 of leaf is around 300. The smallest number is found on Tradescantia leaves which have 14 per mm2 . The highest number of stomata is found on the leaves of the Spanish oak tree. Here there are around 1200 per mm2 .

The roots of a plant also need oxygen which they obtain from the air spaces in the soil. If you give too much water to a plant in a pot you could kill the roots by drowning them! Plants, such as rice, which normally grow in wet soil often have air spaces in their roots. This is so that they can carry air from the atmosphere down to the root tips to be able to respire under water.

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