You don't have to be a master gardener to know that a plant with bright green leaves is healthy or that a plant with yellow leaves is not. The green coloration of plants is essential to their survival because it indicates that they possess the cellular structures needed to synthesize food from sunlight. Reliance on the sun for food defines the plant kingdom.
Each cell in a plant's leaves is packed with tiny green structures called chloroplasts. Chloroplasts appear green because they absorb every color of light except for green. The green color that you see when you look at a leaf is literally green light being reflected back at you. According to Florida State University's microscopy website, leaves contain as many as a half million chloroplasts packed into a space just under the size of a period on a page.
Green chloroplasts power the conversion of light into energy through a process called photosynthesis. During photosynthesis, molecules of water taken up by the roots and carbon dioxide absorbed by the leaves rearrange to form glucose, a process powered by energy from the sun. Glucose feeds the plant, promoting growth and development. Chloroplasts and the photosynthesis reaction define the plant kingdom.
Plants alone on Earth can convert sunlight to chemical energy. The glucose produced through photosynthesis is then made available in their fruits and foliage, consumed by other animals that pass that energy up the food chain. When you see the bright green leaves of a plant, you are observing the conversion of sunlight into food, a process that, through plants, sustains all life on Earth.
The connection between green coloration and plant health can be observed by germinating seedlings in the dark. Seedlings grown without adequate light etiolate: They become stretched and weakened structurally, and they fail to develop green coloration. On his website Kimball's Biology Pages, retired Harvard professor John W. Kimball shows a photograph of a bean seedling grown in the dark. The seedling is spindly and colorless. It is not producing its own food and, once the nutritive contents of its seed are spent, it will die.
In 1953, De Deken-Grenson found that providing etiolated plants with light resulted in the formation of chloroplasts in only a few hours. In many plant species, this process, called greening, is dependent on access to light. Exposing seedlings to light causes the formation of chloroplasts and results in the bright green color associated with healthy plants.