05 January 2013

#46 Leaf structure

The leaf consist of a broad, flat part called the lamina, which is joined to the rest of the plant by a leaf stalk or petiole. Running through the petiole are vascular bundles, which then form the veins in the leaf. 

Although a leaf looks thin, its is made up of several layers of cells. You can see these if you look at a transverse section (cross-section) of a leaf under a microscope.

1. Cuticle:
  • made of wax – waterproofing the leaf
  • secreted by cells of the upper epidermis

2. Upper epidermis
  • thin and transparent – allows light to pass through
  • no chloroplasts are present
  • act as a barrier to disease organisms

3. Palisade mesophyll
  • main region for photosynthesis
  • cells are columnar (quite long) and packed with chloroplasts to trap light energy
  • receive CO2 by diffusion from air spaces in the spongy mesophyll

4. Spongy mesophyll
  • cells are more spherical and loosely packed
  • contain chloroplasts, but not as many as in palisade cells
  • air spaces between cells allow gaseous exchange – co2 to the cells, o2 from the cells during photosynthesis

5. Vascular bundle
  • this is a leaf vein, made up of xylem and phloem
  • xylem vessels bring water and minerals to the leaf
  • phloem vessels transport sugars and amino acids away (translocation)
Photo credit: Pass My Exams

6.Lower epidermis
  • acts as a protective layer
  • stomata are present to regulate the loss of water vapour (transpiration)
  • site of gaseous exchange into and out of the leaf
7. Stomata
  • each stomata is surrounded by a pair of guard cells
  • guard cells – control whether the stoma is open or closed
  • water vapour passes out during transpiration
  • COdiffuses in and O2 diffuses out during photosynthesis

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