Testing the effect of characteristics of leaves on the transpiration rate of * Rubiaceae, Verbenaceae, Oleaceae, and Rutaceae

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Ji Min Choi

12 Biology

Practical Number 10:

Testing the effect of characteristics of leaves on the transpiration rate of  * Rubiaceae, Verbenaceae, Oleaceae, and Rutaceae


There is significant difference between plants and animals. It is generally a plant that produces its own food while an animal has to consume other organisms whether plants or other animals. Plants thus can be classified as autotrophs and primary producers in trophic levels.

Plants undergo special process called photosynthesis to produce their own food and nutrients. In other words, light energy is converted into chemical energy useful for plants. They require carbon dioxide from the air and water from the ground. Additionally, the light from the sun and pigments like chlorophyll in green plants are also essential for reaction to take place. Then products for photosynthesis are glucose and oxygen.

6CO2 + 6H2O → C6H12O6 + 6O2

The importance of sunlight is that it is composed of a wide range of visible wavelengths. It is thus white light. White light is the mixture of all colors from short-wavelength red to longer-wavelength violet (Clegg, 2007). The relationship between sunlight and chlorophyll is that the structure of chlorophyll allows it to absorb some colors of the light and reflect the rest. As chlorophyll is green pigment in green plants, it, hit by white light, reflects only green light into human eyes while absorbing the rest of red and blue light (Clegg, 2007). Therefore, green plants look green to humans. Different pigments for different plants work the same. They absorb certain colors of light and reflect specific color.

The reflected color of light determines the color of plants. Then what is the role of absorbed light? Some of light energy absorbed by chlorophyll is used to initiate photosynthesis. This is reason for the need of presence of sunlight during photosynthesis, especially the first step of the process. The first stage is called photolysis. Light energy is to split water into one oxygen and two hydrogen atoms. Oxygen as waste product leaves a plant and ATP energy is also produced by absorbed light (Clegg, 2007). The second and last step is carbon fixation. In this part, carbon dioxide (CO2), hydrogen atoms from photolysis, and energy from ATP together form organic molecules of sugar, or glucose (Clegg, 2007).

As mentioned before, oxygen leaves a plant. Not only oxygen but also water leaves the plant. More exactly, water evaporates from the plant mainly at leaves. This process is so-called transpiration. On the underside of leaves, there are a number of structures of stomata, which look similar to a pore and they have what is called guard cells. In fact, stomata is the passage for intake of carbon dioxide and way out of oxygen. During this, water evaporates as well. When there is lot of water in plant, guard cells are widely open to transpire. If the plant lacks H2O, it closes guard cells for no more loss of water. Transpired water should be replaced by more water from soil through the roots, stem and xylem (Transpiration, 2009).

The significance of transpiration is a plant continues to pulls up water from the soil whenever water leaves the leaf to operate photosynthesis and bring and spread minerals throughout the plant (Transpiration, 2009). What’s more, evaporation of water takes up heat and cools the leaves.

This transpiration rate is as influenced by a number of factors as is photosynthesis. The factors involve temperature, light intensity, humidity, soil water and so on. This laboratory would like to focus on different species of plants.

Research question

How does the different species of plants (Rubiaceae, Verbenaceae, Oleaceae, Rutaceae) with different characteristics of leaves affect the transpiration rate of the plants by calculating the volume of water uptake per unit area per unit time?


Independent: the species of plants (Rubiaceae, Verbenaceae, Oleaceae, Rutaceae) with different characteristics of leaves

Dependent: transpiration rate of the plants (the volume of water uptake)


  • The strength of wind:

The stronger and greater wind blows, the faster the transpiration happens by evaporating water from stomata quickly.

  • Temperature of air:

The higher temperature of air causes faster evaporation of water. Moreover, stomata is close when the temperature is too hot. Closed stomata can not transpire water and it is huge influence.

  • Light intensity:

Photosynthesis briskly takes place in sunny days using sun as the source of light energy. The high rate of photosynthesis can lead to the high rate of transpiration.

  • Time of measuring:

If the apparatus remains longer, it means the plant has more chance of transpiring.

  • Humidity:

Within the high humidity, plants would not seem to lose water to their surroundings as there is already much water in the air. This is associated with diffusion. The lesser the amount of water molecule in the air, the more water molecules from plants would move into it.

  • The amount of water provided:

Plants with more amount of water would transpire faster and much more than those in low provision of water.

  • Surface area

The bigger the surface area of a plant, the more there are stomata and thus the greater the rate of transpiration is. When keeping the surface area constant, the number of leaves does not matter if the size of leaves is different. For example, five small leaves and five big leaves have certainly different surface area.


All four different species of plant *Rubiaceae, Verbenaceae, Oleaceae, and Rutaceae would transpire at the different rates from each other. Even though having the same surface area, each has its own characteristics of leaves, and therefore the transpiration rate would be various. More specifically, the four species have leaves with the different number of stomata, size of stomata, thickness of waxy cuticles, hair, succulence, leaf type and leaf shape. Especially the certain species in Rubiaceae family has palmate leaf type, obovate leaf shape, and thick waxy cuticles; Verbenaceae has a soft leaf of elliptic shape and small pinnate type and thick waxy cuticles; the leaves of Oleaceae are hard, big elliptic-shaped and palmate type with much waxy cuticles; Rutaceae has soft alternate single leaves, ovate leaf shape and thin waxy cuticles. The importance is cuticles. They are waxy layers on the surface of a plant including leaves. Because of their waxiness and thus the hydrophobic nature, cuticles limits the water movement through leaves. The thicker the waxy cuticle layers on a leaf, the slower the transpiration rate would be.

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  • a potometer with a rubber tubing, a syringe, capillary tube, and three-way tap
  • secateurs
  • leafy shoot of plants (*Rubiaceae, Verbenaceae, Oleaceae, and Rutaceae)
  • a 50(w) x 30(d) x 25(h) cm tub (big enough for a potometer to be completely put)
  • water
  • a clamp stand
  • 2 x clamps
  • stopwatch
  • a 30cm ruler (± 0.1 cm)
  • sheets of squared paper (1cm grid)
  • a calculator


  1. Cut four different kinds of branch of plants using secateurs *Make sure the thickness of the branches fits well to the rubbing tube of a potometer
  2. Put ...

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