All plants require sunlight to grow. Plants can be grown under artificial lights, but these lights are just mimicking the effects of sunlight. Dependent on the type of plant and its natural habitat, the amount of sunlight required for healthy growth will vary, but no plant will survive for long in the absence of light. For example, no green plants have been found in any of the world’s deep cave systems.
Plants are classed as autotrophs, which means that they produce their own food. To do this they require an input of energy and this energy is provided by sunlight. All plants that produce their own food, (there are a few exceptions) contain the green pigment chlorophyll. Chlorophyll absorbs energy int the form of sunlight and this energy is used in the process known as photosynthesis. A series of chemical reactions occur in which atmospheric carbon dioxide, absorbed through the leaves, and water, taken up by the roots, are combined to form glucose and other basic carbohydrates. These carbohydrates are then used, together with mineral elements drawn up by the roots, to form larger molecules such as starches, fats and proteins. These larger molecules are then used to build roots, stems, branches, leaves etc. As a byproduct to photosynthesis oxygen is released back into the atmosphere.
Thus the amount of sunlight a plant receives will effect its rate and amount of growth. Other factors such as temperature and available soil nutrients are important, but without sunlight the plant cannot survive. Beans planted and keep in the dark will germinate and begin to grow, but this is due to the store of energy in the bean seed. The plants will be straggly and pale and will soon die. In the tropics, with lots of sunlight and warmth, plants can grow much faster than in colder northern climates, where light levels are also much reduced. the other limiting factor in plant growth, of course, being the availability of water.
Plants have adapted to this need for light in different ways. Some have developed large leaves which can contain large amounts of chlorophyll. Others grow tall to tower over their fellows and absorb more sunlight. The importance of light to plants is shown by the process of phototropism. This means plants actively grow towards light. Seedlings in a tray on a windowsill will grow towards the window, reaching for the light. If the tray is turned around so that the seedlings are then facing away from the window, they will slowly change their direction of growth, and in a day or so will again be slanting towards the window. With the advent of time-lapse photography it has been shown that plants can actively ‘follow’ the sun through the course of a day. Leaves move to always present the maximum amount of leaf area to the suns rays.
It is this need for sunlight, through the presence of chlorophyll that gives the majority of plants their green colour. Even purple or silver leaved plants contain chlorophyll, its colour being masked by other pigments. There are some plants without chlorophyll, an example being the Ghost Orchid, Epigogium aphyllum, which gets its name from its deathly white appearance. It cannot manufacture its own food and relies on a symbiotic relationship with a root borne fungus for its nourishment. Compared to the variety and potential size of chlorophyll containing plants, those without are rare and diminutive. Another example of the value of sunlight and its affect on plant growth.