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Green-leaved plants need light to live. Light acting upon leaf and stem cells helps change carbon dioxide taken from the air and moisture from the soil into sugars and starches, necessary elements for plant growth. This manufacturing process is called photosynthesis. To carry on photosynthesis plants must have, in addition to light, proper temperatures and nutrients.

Measuring Light

Light is measured in foot-candles. One foot-candle is "the illumination at all points that are 1 foot from a uniform source of 1 candle-power of light. "The intensity of light varies with the seasons. In summer, it may reach 10,000 foot-candles or more during the brightest part of the day. This is too much light for African violets, begonias, and decorative green pot plants. During winter months, light intensity may be reduced to a meager 500 foot-candles. Clouds and smoke from factories also reduce light.

When light intensity becomes too low, plant growth slows almost to a halt, stems become weak, leaves thin, and flowers are paler than under normal conditions. Under too-high light intensity, plants tend to wilt, wither, and may become yellowed or show burned spots.

With optimum conditions such as good soil, correct watering, and proper temperatures, food production in plants is stepped up as they receive more light. To achieve maximum growth and flowering, you must regulate light intensity to make up for seasonal changes. Shade your greenhouse during summer to cut down the light; remove shade to admit more winter sun; brighten all dark corners with artificial light.

Artificial Light

Given proper temperature and humidity, many flowering plants, such as African violets, gloxinias, other gesneriads, and foliage plants, thrive under artificial light. The addition of fluorescent or incandescent light, or a combination of these, can convert dark, wasted space under benches and on window-less wall areas, as well as in basements and closets, into profitable plant-growing space.

Some greenhouse owners have found that fluorescent light speeds propagation and flowering of African violets, so they grow the plants in a combination of natural and artificial light.

Day Length

The growth and flowering of many plants depend on their daily hours of exposure to light'either natural or artificial. Without going into technical detail, this, roughly, is called photoperiodism, a phase of which is called day length. Plants are informally classified into three groups.

1. Dahlias, delphinium, pansies, tuberous begonias, and other plants which come into flower more rapidly during the long summer days of maximum light'or under the stimulus of artificial lighting'are called long-day plants.

2. Chrysanthemums, gardenias, poinsettias, etc., which start to flower when their light-exposure period goes down, are called short-day plants.

3. A third group contains plants'including African violets, carnations, and roses'which are unaffected by day length; these are called indifferent.

Science is now trying to determine whether plant growth and flowering are actually governed more by the daily period of uninterrupted darkness than by the length of exposure to light. Thus the plants that are now classed as long-day and short-day eventually may be called short-night and long-night plants.