Plants absorb light with the help of Chlorophyll that is built into the thylakoids' membranes. This chlorophyll is green in colour and absorbs red and blue light. They do not and are incapable of absorbing green light. It is this fact, the inability to absorb green light, that makes the leaves of plants and chlorophyll appear green to the human eye. Plants require light energy for the process of Photosynthesis by which they convert this light energy to chemical energy. This energy is stored by plants in bonds of sugar. The process of converting light energy into a food source is unique to plants and some algae. No other type of organisms on earth can manufacture its own food. The chemical reaction that takes place during this process is represented as 6CO2 + 6H2O (+ light energy) → C6H12O6 + 6O2.
Learning these facts about the responses of plants to length of daylight made something else apparent. Plants must have something within them that detects the change in length of daylight and that causes them to respond accordingly. Just recently this substance, called "phytochrome," has been isolated. Phytochrome is a bluish, light-sensitive pigment that absorbs red light. It has been shown that many plants, when exposed to the red wavelength of light, mature more rapidly. Somehow the light acts on the phytochrome to regulate a plant's growth changes, from seed stage to maturity. But it is not understood just how this is accomplished. Many gardeners now use to good advantage this knowledge about the responses of plants to light. By adjusting the length of exposure to light they can make a plant bloom when they want it to. Thus in winter they enjoy flowers that normally grow only in summer, and those that normally bloom in autumn they may have in other seasons. A chrysanthemum, for instance, is normally an autumn-flowering plant. But it can be made to bloom in summer. Just cover it with a cardboard carton in the late afternoons, and remove the carton in the morning. The extended period of darkness will cause the chrysanthemums to react as if it were autumn, and they will bloom with the summer flowers
The majority of all plants are photo-autotrophs, meaning that they can synthesise food straight from inorganic compounds by making use of the energy obtained from light, of which the sun is the greatest provider.
Plants absorb light by way of a process called photosynthesis, which is done with the help of the chlorophyll present in plants. Owing to the presence of chlorophyll in plants, they can absorb light directly from the sun.
The energy for photosynthesis is eventually received from photons that are already absorbed and engages a reducing agent, which in the case of plants is water, which releases oxygen as a by product of the process. The light energy obtained is then made into chemical energy, which in turn serves to perform synthetic reactions, that is, fixing carbon dioxide into carbohydrates, among other things.
