The Importance of Transpiration in Plants

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In botany, transpiration is the loss of water by plants. This loss of water occurs through stomata on the plants’ leaves.  The stomata contain two guard cells that form a small pore on the surface of the leaves. These guard cells control the opening and closing of the stomata in response to different environmental stimuli.

Transpiration is affected by different environmental stimuli. These factors include: presence of sunshine, temperature, internal water deficit and humidity.  Transpiration plays an important role in providing energy for transport of water in the plants and aiding in heat dissipation in direct sunlight through evaporation. The opening and closing of the stomata are necessary to allow entry of carbon dioxide into the leaves and to allow for the process of photosynthesis. By doing this the plant is supplied with energy for survival.

The presence of sunlight in the environment causes the stomata to open up. This in turn allows the water from the plants through leaves to evaporate and escape into the environment hence bringing a cooling effect and transportation of water from the soil throughout the whole plant.  During extremes of temperature, plants lose a lot of water. When temperatures rise, the stomatal cells are open hence allow more water to be lost. This disadvantages the plants as they may wither and eventually dry up especially, when the replenishment of water through roots from soil is not sufficient.  Additionally, when the internal water deficit is so much, plants will tend to preserve water through closing of the stomata.

Other factors such as the structure of the leaf also affect transpiration. If, for instance, a leaf has a hairy structure, then little or no loss of water will occur. The hairy parts trap the evaporation water thus creating a humid environment at the leaf site. This causes the deficit in water to reduce and hence reduces the loss. Under normal conditions, plant leaves exhibit all these characterizes simultaneously.

The spermatophytes are seed producing plants. They are the common green plans and produce seeds through flowers or cones. These plants usually have roots, stems, leaves and seed bearing structures. They are rich in chlorophyll and thus photosynthetic. They also have highly developed vascular tissue containing of xylem tissues and tracheid.

The importance of transpiration cannot be claimed without a proper back up of facts or evidence. This can only be done through statics. In this experiments the rate of transpiration was studied under different conditions. However, as a class, we chose to study this only in two conditions: the normal conditions regarded as control and extreme conditions regarded as windy.


The purpose of this experiment was to study the statistically compare rates of transpiration in plants using students’ T-Test under different environmental conditions.


They hypothesis were formulated based on the conditions chosen to perform the experiment as below:

Null hypothesis stated that no difference exist between the control and the experimental conditions in determining the rate of transpiration

Alternate hypothesis stated that more water would be lost in plants placed under windy conditions than those in the normal laboratory conditions.


Students were divided in to different groups and each group of students was given a plant and a photometer. The plant was placed into small flask of water and the opening sealed with parafilm wax. The photometer was then placed on a weighing scale and the plant let to transpire under ambient conditions. For fifteen minutes. The total weight loss was recorded with four replicates. The other plant was treated to the same procedure but it was exposed to a windy condition. The wind was allowed to blow over the plants for 15 minutes and the loss on weight recorded very five minutes. This was repeated for four times.



Xi (individual values)

X (control average)



Replicate 1





Replicate 2





Replicate 3





Replicate 4





Sum of squares



Xi (individual values)

X (experiment average)



Replicate 1





Replicate 2





Replicate 3





Replicate 4





Sum of squares



Degrees of freedom

In statistics, degrees of freedom refers to the number of independent variables that can vary or estimate in an analysis without breaking any constraints. It is given by the sample size minus the number of parameters needed in calculation of analysis. It is usually a positive whole number. In this student t-test, the degrees of freedom was found by the total number of observations minus two. Thus,

DF= 8-2= 6


In statistics, variance is the expectation of the squared deviation of a random variable from its mean. It is used to measure how far a set of numbers are spread out from their average value. In this experiment, it was given by subtracting the individual value from the mean of the set i.e. Xi-X, the finding the sum of the squares of all the individual and dividing by n-1, where n is the number of replicates under each treatment type.

From the tables above,

Control variance is given by; 0.0078/ (4-1)


Experimental variance is given by, 0.0855/ (4-1)


Calculation of t-value

Steps used in calculation of t-value;

i) Absolute value of control variance – absolute value of experimental variance

= I0.0026-0.0295I


This values is called the t-value numerator

ii) t-value denominator

This is given by, dividing the variance of each group by the number of replicates in each group.

Control= 0.0026/4


Experimental = 0.0295/4


Then adding the two values above, =0.0065+0.007375


Then the square root of the sum above, = √0.013875


This is the t-value denominator

iii) t-value = 0.0269/0.1178


Calculation of critical value

The critical value is chosen from the 95% confidence level table. The probability is P=0.05

Selecting a two tailed test, and locating the degrees of freedom, the critical value is found to be;

Critical value= 0.0603

In this experiment, our t-value was greater than the critical value, thus the null hypothesis is rejected and the alternate hypothesis is accepted. This means that there was a statistical difference between the means of the groups.


Form the statistical results, the rate of transpiration is cleared affected by the environmental conditions. This is in particular to the windy conditions. The results showed that the plant under the experiment placed under windy conditions had increased rate of transpiration than that placed under ambient conditions.

This can be explained by alluding to the breeze in the sea. When there is no breeze in the air, the air is excessively humid while when there is breeze, the air is dry. In this case, the presence of wind made the humid air around the leaves of the plant to be carried away, leaving the surface of the leaves free of any interference. This caused the water in the leaves to evaporate quickly than the plant placed in the ambient conditions or control group. The results explains why such factors are important to the plant.

Transpiration is not only affected by wind but array of many factors. These factors include: light, temperature, humidity, and soil water.

Soil water

A plant cannot transpire continuously without replacement of the lost water. Otherwise it will wither and eventually dry.  Absorption of water through the root of the plants should be able to keep up with the turgor pressure that occurs due to this loss. When the roots absorption fail to keep up with this pressure, the stomata close. Closing of stomata mean transpiration does not occur, and other processes such photosynthesis will also stop. If the turgor pressure is excess and extends to other parts of the plant such as stems and branches, the plants withers and die. Water lost through transpiration is very high. As a fact, an acre of maize or corn is estimated to lose about 1.5 million liters of water.


Light is a stimulant for transpiration. This is because it causes the stomata to open and allow water loss. During the day, stomata open up for many processes, not only transpiration. The exchange of oxygen and carbon dioxide happens concurrently with water loss. In normal cases, the stomata open when light strikes the leaf in the morning and closes during the night. This opening cause guard cells to change in turgor pressure; their outer walls bulge out while the inner walls develop a crescent shape. This process allows entry of carbon dioxide and exit of water and oxygen.


At higher temperatures, water evaporates from water bodies very fast. This is because water is heated up to its vaporization temperature. For plants, this does not need to rise up to boiling temperatures, but only to about 30 C. at this temperature a leaf may lose as much as three time the water than at 20 C.


The concentration of substances in between two regions is called concentration gradient. When this concentration difference is high, the rate of diffusion is also increased. Thus when relative humidity in the air is high, little or no water is lost, while when it is low, it means the rate of diffusion is high and transpiration rate is increased.


Statistical data is important in science. Facts have to be backed up by evidence. In this experiment, not only the factors affecting transpiration were studied but also the statistics behind it was determined. The student’s t-test was calculated from the results obtained and compared to the critical value. The t-value was found to be higher than the critical value. This clearly explained why the null hypothesis was not accepted while the alternate hypothesis was accepted. Transpiration is an important process to plants in terms of water regulation and production of food through photosynthesis.

August 04, 2023


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