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stoma - an open aperture (the stomatal pore) in the epidermis surrounded  by 2 guard
stomata - plural
a) Usually more frequent on epidermis of lower leaf surface.
b) Found on some herbaceous stems, fruits and petals.

Mechanism of Opening
a) open when guard cells are turgid (due to water uptake in response to potassium  influx) 
b) closed when guard cells are flaccid (due to water loss in response to potassium efflux) 

Daily Cycle
C-3 and C-4 Plants
a) open during day 
b) closed during night 
CAM Plants
a) open during night 
b) closed during day 

Designed for gas exchange
a) CO2 in and 02 out for photosynthesis 
b) CO2 out and 02 in for respiration 
c) H20 out during transpiration 


Palisade parenchyma
a) Contains 70-80% of the chloroplasts in the leaf. 
b) Specialized for photosynthesis - because it contains a large number of chloroplasts
    and it occurs towards the top side of leaf 
Spongy mesophyll
a) Contains large air spaces 
b) Specialized for gas exchange - because of the large air space and more stomata occur
      in the epidermis of lower leaf surface 

Sun Grown Leaf
a)  thicker, due to thicker palisade parenchyma layer
Shade Grown Leaf
a)  thinner, due to thinner palisade parenchyma layer, therefore, higher proportion of
     spongy mesophyll
b)  larger size
c) softer and more pliable