Coloured Leaved Plants and Photosynthesis
Some garden plants, such as coleus, Japanese maples and rhus, are grown for their reddish-tinted foliage, but these colours are caused by pigments which mask the presence of green chlorophyll without impairing its efficiency. On the other hand, plants with yellow or white variegation in their leaves, like variegated ivies, have little or no chlorophyll and their efficiency is impaired. Such plants need good culture and ample light to do well.
The greener the leaf, the more chlorophyll it contains. Evergreen plants whose foliage has to function the year round tend to have richly green leaves. Plants that have adapted themselves to grow in shade have usually developed big leaf areas and densely green leaves.
Green leaves, photosynthesis and light go together. Without light, leaves turn white, and growth ceases. Generally, the more intense the light, the greater the rate of photosynthesis.
All light-loving plants have mechanisms, usually in their leafstalks, whereby they can turn their leaves to present the greatest amount of leaf surface towards the source of light. Any plant growing in an uncomfortable, ill-lit position tends to extend or lean its growth to the light, as do pot plants when grown in a window.
Temperature affects plants and influences the rate at which photosynthesis takes place. It decreases in very cold and very hot weather, but varies with the kind of plant and the natural conditions under which it has evolved. For most of the plants grown out-of-doors, photosynthesis begins in earnest somewhere about 40° F. (4° C.) and quickens progressively as the temperature rises to about 90° F. (32° C.). After that it diminishes.
Photosynthesis results in the setting free of oxygen into the air. But this process should not be confused with the breathing or true respiration of the plant. All living cells in plants and in animals breathe all the time, and in much the same way. Oxygen is used to convert food materials into new tissue and energy, and in the process carbon dioxide is liberated. In plants, the two processes of photosynthesis and respiration go on at the same time, even in the same cells, during daylight.
The carbon dioxide freed in respiration is used in photosynthesis, together with more taken from the atmosphere. During darkness, photosynthesis stops, and traces of carbon dioxide may be released into the air, but in nothing like the quantities to justify the superstitious removal o£ flowers or plants from a sick room at night, lest their respiration should poison the atmosphere.