There is a Leaves (Part One) which you could look at before this if you are in any doubt about your leaf knowledge.
The above is a cross-section through a typical leaf. There is an actual microscope photograph of a similar leaf below. It should be fairly easy to compare structures between the two pictures. It is probably easiest to think about the leaf as a series of layers, with each serving a specific purpose.
From the top…
| Waxy cuticle | a waterproof coating which makes sure that rain rolls off the leaf |
| Upper epidermis | the top skin layer of cells that give the leaf its shape |
| Palisade layer | a layer of cells that have lots of chloroplasts and are the main sites of photosynthesis |
| Spongy layer | a layer full of gaps so that gases can diffuse in and out of the leaf |
| Lower epidermis | the bottom layer of skin cells |
Some of the features might need a little further explanation…
The spongy layer, sometimes called the spongy mesophyll, also has cells that contain chloroplasts, but given their location in the centre of the leaf they do not receive such intense light as the palisade cells. Carbon dioxide diffuses in from the air and oxygen can diffuse out because of the spaces between the cells. The spongy layer also contains the vascular bundle (which just means bundle of tubes). The xylem carries water up from the roots and the phloem carries sugary sap back down to the roots. Phloem must supply any cells that cannot photosynthesise with the glucose necessary for respiration.
Water, drawn up from the roots and produced during respiration, can evaporate through the stomata (leaf pores). Each stoma is flanked by guard cells which can open or close the gap. When water is scarce they collapse the opening but when there is plenty of water in the soil they swell up, opening the stoma.
As water evaporates from the stomata, more water is sucked in from the roots. This process of transpiration pumps water through the plant. The following excerpt from the Life of Plants shows the journey that the water takes from roots to leaves.
Questions…
- Which leaf cells are chiefly responsible for photosynthesis?
- What are the raw materials for photosynthesis?
- What are the raw materials for respiration?
- In the carbon cycle, respiration is one of the ways that carbon is returned to atmosphere. Suggest another way in which carbon is added to the atmosphere.
- What structures allow gases to enter and leave the leaf?
- The photograph has two regions labelled with the word ‘parenchyma‘. What does the word mean in this context?
- What would be a possible diameter for a cell in the upper epidermis of an oak leaf?
- Animals that eat leaves often have specialised digestive systems. Why is this?
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