Hint:To study any process and its effects, we should always start from a zero point. When a plant is to be studied for photosynthesis, we have to ensure that photosynthesis is not occurring just before the experiment. This is done by keeping the plant in the dark.
Complete answer: The picture shows a plant that is being studied for the light reaction. In order to study the effect of the light reaction of photosynthesis, we need to know what happens if the light reaction does not occur. So, the plant is kept in darkness. In the absence of light, there will be no light stimulation, hence, no light reaction would occur. So, at the beginning of the experiment, we will have a zero point for the light reaction. Now, when the plant is kept in sunlight, whatever changes occur on the exposed part of the plant will be sole because of the incident light during that given period of time.
The given experiment is used to determine the impact of light on the leaves of the plant.
Photosynthesis is divided into two types of reactions, namely, light reaction and dark reaction. The light reaction is necessarily a light-dependent reaction. It will occur only if the light is present. The presence of light is compulsory for the light reaction to occur. While in the case of a dark reaction, the presence of light is not a compulsory requirement. It is just named as dark reaction whereas it should be called a light-independent reaction as it does not depend directly on the presence of light for its completion.
Note: Light reaction can be studied properly when a plant or a part of a plant is not exposed to light for a certain duration of time. This ensures that there is no new formation of glucose just before the experiment. These experiments are generally carried out by keeping the plant covered by black paper and keeping it in a dark room.