It is possible to control the growth pattern by introducing artificial light or blocking out natural light. Seedlings grown indoors can become etiolated and flop over, using artificial light will prevent this happening. A light box with the correct intensity and wavelength of light will produce more robust seedlings and later, flowers and fruit at the required time. Many of the plants grown for garden shows are controlled in special light conditions so that they perform on the day and can flower completely out of season. The Poinsettia (Euphorbia pulcherrima) requires heat and about 14 hours of darkness to produce its bright red bracts so has to be grown in special light conditions to be ready for Christmas.
There are special grow-light bulbs, but an equal number of cool white and warm white fluorescent tubes should give off a balanced spectrum sufficient for the job. They should be arranged 10 to 20 cm above the seedlings or cuttings in a box lined with reflective material. A larger scale arrangement is required for growing-on the plants. Most actively growing plants perform at their best with 18 hours of light per day, but those from tropical regions respond well to 12 hours as they would receive in the wild. It is possible to grow healthy plants without any natural light at all (as growers of illicit substances have discovered!).
Phototropism
Light also allows plants to interact with their surroundings. They are able to respond to shading by growing towards light sources. This is controlled by hormones produced at the shoot tip called auxins (indole-3-acetic acid or IAA) which influence cell division, so if the light is to one side the cells on the dark side receive more auxin and divide more rapidly forcing the plant to bend towards the light. This is phototropism. If the plants are grouped together and the light is from above they will all grow upwards at a faster rate, the winners will be those which are best adapted to local conditions. These are usually the weeds, but the phenomenon is used to advantage in forestry producing straight-stemmed trees. The IAA also suppresses the growth of side buds so the effort is concentrated at the main tip of the plant - this is apical bud dominance a phenomenon we try to influence when pruning.
The parts of a plant which are above the ground are said to show a positive phototropic reaction and the roots show a negative phototropic reaction as they grow away from light. Small seed do not have the energy to support a long initial shoot or plumule, so need to be near or on the surface. They are responsive to light as well as temperature and moisture, so germination occurs only when all three are correct.
