How does plant growth occur?
Photosynthesis is the process by which plants turn sunlight into food. Food that is created by this photosynthesis is used by the plant to add more tissue to itself, whether rit be more leaves, flowers, or fruits. Without the process of photosynthesis, plant growth would not occur, and eventually the plant would not only stop growing, but stop living.
How do plants get the materials they need to grow?
The majority of photosynthesis occurs with the help of the chemical chlorophyll, which is found in special cells within the leaves known as chloroplasts. The visible light that hits the earth does so at different wavelengths. The different wavelengths correspond to different colors (there are seven, and can be seen within a rainbow).
Chlorophyll is also what is responsible for making the leaves green and they use mostly wavelengths of light that are red. Leaves also possess pigments other than chlorophyll that are present so that the plant can make use of a greater range of visible light wavelengths.
How darkness affects plant growth
The photosynthetic process occurs most readily when the sunlight available to the plant is the greatest. This is why plants in the tropics grow so large. They are near the equator where the sunlight is directly overhead much of the year. However, photosynthesis can occur in the dark. It does not occur as rapidly as during the daylight, but it is possible. If this were not the case, plants would shut down entirely every night when the sun went down.
Therefore, the periodic darkness of nighttime does not affect plant growth very much. In areas of the world, like Alaska, that experience seasonal increases in the length of daylight photosynthesis does make plants grow very large. Although Alaskan vegetable gardens are filled with plants that do well with cool temperatures and a short growing season (turnips and cabbage, for example), they tend to grow very large. This is because the sunlight hitting the gardens can do so for up to 20 hours a day.
Plants would not do well in total darkness. If a plant were to be placed in a closet with no light it would stop growing. If placed there too long, even with adequate water, it would still die because plants depend on receiving some sunlight.