Investigation 11: Plant Transpiration

On April 15th, the lab group consisting of Vikram, Vinay, Shreyan and myself were tasked with observing plant stomata and recording our data to derive some sort of meaning from it. In plants, stomata are the small openings on the underside of a leaf that open and close to allow gas exchange to occur with the plant's surrounding environment. This is an extremely important role played by the stoma because it helps leaves take in water from the roots through a process called transpiration. Transpiration is the way a plant uses the cohesion of water molecules to pull water up from the roots as more water vapor is evaporated out of the leaves. The stomata also open and close depending on environmental factors, but we are not investigating the opening and closing of stomata in our experiment.

Transpiration in action

Now how will we count the number of stomata on the bottom of the leaf? We could manually count each one using a microscope, but that would take forever and the stomata are green so they blend into the leaf and are difficult to spot. The lab group decided that we would use clear nail polish and apply it to the bottom of the leaf, to get impressions of the stomata. Then we would use tape to remove the nail polish from the bottom of the leaf with the impressions still intact, and apply the tape to a microscope slide in order to view the stomata. Then, because we have a specific field of vision using a microscope, we would calculate how big the area under the microscope was and then extrapolate that data to make an estimate on how many stomata cover the leaf. Then we could use this data to make hypotheses about the relationship between plants, the number of stomata they had and their environment.

The group performed the procedure outlined above using three different types of leaves: ivy, grass and ginkgo. Using the nail polish, we painted a small section of the leaf with it and waited for it to dry. After it was dried, we carefully applied tape to a section of nail polish and delicately pulled it off of the plant so as not to disturb the polish. Then we looked at the slides that we had prepared under a microscope at different magnifications.

Here is the ivy leaf we observed at 400X magnification:

Here is the blade of grass we observed at 400X magnification:

Here is the ginkgo leaf we observed at 100X magnification:

Next, we had to calculate the field of view that the microscope gave us for each leaf at each different magnification so that we could determine the stomata to surface area ratio for each leaf. To do this, we started by putting a plastic ruler under the microscope at the lowest magnification, 40X, and saw how large the field of vision was measure on the ruler. We discovered that at 40X magnification, the microscope's vision had a diameter of 5 millimeters. Using this data, and our knowledge that the diameter of the field of vision was inversely proportional to the magnification of the microscope. Therefore, to find the diameter of the field at 100X magnification, we had to multiply the original 40X diameter by 40/100 or 0.4, and to find 400X magnification, we had to multiply the original by 0.1.

After finding the diameters of the microscope's field of vision, we wanted to find the surface area of the field, so we used our knowledge of the formula for the surface area of a circle, which is A = pi*r^2, where r is the radius. Also, we know that the diameter is twice as long as the radius, so the formula we will use will look more like A = pi*(d/2)^2, where d is the diameter of the field of vision.

Finally, using the number of stomata per leaf and the area of the field of vision, we determined the stomata to surface area ratio for each plant. This calculation is in stomata per millimeter squared.

Here are our calculations:

Diameter at 40X: 5mm
Diameter at 100X: 5mm * 0.4 = 2mm
Diameter at 400X: 5mm * 0.1 = 0.5mm

Area at 100X: pi * (1mm)^2 = 3.14mm^2
Area at 400X: pi * (0.5mm)^2 = 0.785mm^2


And here is a graph of our findings:

Leaf	Number of stomata counted in FOV	Are of the FOV (mm^2)	Stomata to surface area ratio (stomata/mm^2)
Ivy	30	0.785	38.2
Grass	10	0.785	12.7
Ginkgo	26	3.14	8.3

We found that Ginkgo leaves have the lowest stomata to surface area ratio of the 3 plants that we tested with only 8.3 stomata per millimeter squared, and Ivy had the highest ratio, with a whopping 38.2 stomata per millimeter. Now the question that the lab group pondered was why there was such a large difference in the number of stomata that each plant had.

We hypothesized that the ginkgo has a low number of stomata because of its environment. Used to be planted in full sun and in warmer climates, the ginkgo would lose a lot of water due to transpiration if it had many stomata. Therefore it only has a few so that it can bring water up the trunk from the roots, but not too many as to create an inefficiency by losing a lot of water. We also believe that the ivy has so many stomata because it is a vine plant that grows upwards and in the shade. Ivies, when given the proper support, can grow up the side of a building and reach 30 meters above the ground. Therefore, to create enough of a pulling force on the water in the roots to get it to the leaves 30m upwards, there must be many stomata to transpire rapidly. Also, because ivies grow in the shade, they dont have to worry about losing a lot of water due to evaporation and are able to have so many stomata. Finally, the blade of grass we looked at had a somewhat low amount of stomata per millimeter squared, about 12.8 which is slightly higher than that of the ginkgo. Unlike the ivy, grass does not grow very tall so it doesn't have to have a lot of stomata to bring water to the leaf. Also, grass grows in the sun, so like the ginkgo, it is in danger of losing a lot of water to heat, so it can't have too many stomata otherwise it will die.

In conclusion, this lab has shown us that there is a strong correlation between the number of stomata that a plant has on its leaves and its environment. The three plants we tested had differing stomata counts due to heat and height, but other plants also have different stomata counts because of humidity or scarcity of water. A plant in an extremely humid climate would have many stomata so that it could transpire as much as possible even though the concentration gradient of water to air is radically different than plants in a desert climate. Also, plants like cacti must conserve water because it is so scarce in the desert, so it cannot have many stomata.