It's All About Light

Green plants cannot function without light. They need it for photosynthesis, the process by which glucose is created from water and carbon dioxide. This simple sugar glucose can then be eventually combined with nitrogen ions to create amino acids, the building blocks for proteins.

So photosynthesis is the dynamo which allows the plant to function and grow. Although artificial light (but not really tungsten, or household light) can be used, the principal source is SUNLIGHT. This is important to the plant in two ways:

• It's INTENSITY affects the rate a plant can grow (assuming no other essential factor is limited)

• It's LENGTH (the length of day, or photoperiod) can affect reproduction (time of seed germination, flowering, food storage, and senescence (death))
  

Since light is so important, why haven't I mentioned it before? Because I find that novices have more problems with watering and feeding.