Epidermis DUMMY
Epidermal cells form the outer layer covering a leaf, separating internal organs from the
external atmosphere.
Epidermis tissue has several functions:
- protection against water loss by way of transpiration (contains guard cells which open
and close) - regulation of gaseous exchange
- secretion of metabolic compounds
- absorption of water
Mesophyll cells
The mesophyll is located between the upper and lower layers of the leaf epidermis, and is
mostly made up ofparenchyma(ground tissue) orchlorenchymatissue. The mesophyll is
the primary location for photosynthesis and is divided into two layers, the upper palisade
layer and the spongy mesophyll layer.
Theupper palisade layerlies beneath the upper epidermis and consists of vertically elon-
gated cells that are tightly packed together to maximise the number of cells exposed to
sunlight. In addition, these cells contain many chloroplasts, thus maximising their photo-
synthetic ability. The palisade layer thickness depends on the extent of exposure to the sun.
Leaves that are exposed to the sun have a thicker palisade layer. Those that are typically
found in the shade have a thinner palisade layer. Beneath the upper palisade layer is the
spongy mesophyll. The cells in the spongy mesophyll are slightly rounder and less densely
packed and have air spaces to allow for gaseous exchange.
Figures below show the leaf and tissue structure of a dicot plant.
Figure 5.41: Leaf structure.Chapter 5. Plant and animal tissues 151