Investigating Number of Stomata on a Leaf

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Lab Design
“Investigate the effect of a factor on the number of stomata of a leaf.”

Research Question: How do differing leaf heights affect the number/density of stomata of a leaf?

Hypothesis

Stomata are pores, typically found under the leaf (lower epidermis), that control the gas exchange of transpiration, where water vapor leaves the plants, and carbon dioxide enters.

I predict that the stomatal density on high leafs is higher than on low leafs. During photosynthesis the chloroplasts in the leaf cells synthesize ATP from ADP as a result of exposure to light, while oxygen is produced as a by-product of the photosynthetic reaction. Carbon dioxide, which enters the plant through diffusion via the stomata, is needed for this process (photosynthesis) to occur. When the chloroplasts in the leafs cell is exposed to higher light intensities, more ATP is synthesized from ADP, while production of the by-product oxygen also increases. This increase in the rate of photosynthesis calls for more “fuel”, i.e. Carbon dioxide. So for a higher concentration of carbon dioxide to diffuse into the plant, the plant must grow a greater stomatal density (higher number of stomata). This will create a larger surface area for carbon dioxide diffusion, the excretion of water vapor (transpiration) and the large amounts of oxygen being produced.

As the higher leafs are exposed to higher light intensities I predict the stomatal density to be high. Lower leafs are exposed to lower light intensities due to, for example, shading by top leafs, and will so have a lower stomatal density than high leafs.

Variables

Controlled:
Type of plant- The type of plant that is going to be used will stay the same, i.e. controlled. The type of plant that is used for this experiment is called Quercus Ilex. Amount of leafs (10 'high' leafs, 10 'low' leafs)- the ensure fair testing the number of leaves tested from each variable will be the same. Apparatus used- Same set up each time.

Microscope magnification (400x)- Magnification at which the number of stomata will be counted at is at a magnification of 400x.

Independent Variable:
Leaf Source- The leaf source regarding to the 'high' and 'low' leafs is the variable which will be changed to test the difference in number of stomata of the two variables. Distance between high/low leafs- The distance between the height at which 'low' and at which 'high' leaves were picked each time had to be of a minimum of 20cm to ensure plausible results. Lower epidermis of leaf used to count stomatal density- Because Quercus Ilex is a dicotyledonous plant, the number of stomata on the lower epidermis will be higher than on the upper epidermis. This is because dicotyledonous plants hold up their leaves horizontally, which directly illuminates the lower epidermis. So, to prevent water loss, fewer stomata will then be located on the upper epidermis.

Dependent Variable:
Stomatal Density of high leafs
Stomatal Density of low leafs

Apparatus/Material

10 high leafs
10 low leafs
Clear nail polish
Slides
Pincette
Microscope
Clear Tape
Calculator

Method

Find a leaf source that has a significant height from which you will be collecting your leafs from throughout the entire experiment.

Determine a low area, of little height from the ground, on the source from which you will pick 10 'low' leafs.

Repeat step 2, except that the area must be at an increased height distance of at least 20cm, to ensure a fair test and collection of 'high' leafs from a higher area than that of the 'low' leafs.

Choose a leaf of which the stomatal density is to be examined but don't pick it off the plant. This is so that the plants photosynthetic process will not be disturbed which could lead to change in the leafs natural state and affect your results.

Paint a layer of clear nail polish on the lower epidermis of the leaf and wait until it has dried.

Use your tweezers to gently peel off the dried layer of nail...
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