"Plants bearing such
variegation are
chimeras, with more than one type of genetic makeup in their tissues. A lack of
chlorophyll producing tissue in some tissues causes variegation with white or yellow coloured zones on the leaf, contrasting with the usual green tissue. It is due to some of the plant’s
meristematic tissue losing the ability to produce
chloroplasts, so that the tissue it produces is no longer green.
In a common type of such variegation, the part of the meristem that produces epidermal tissue loses the ability to produce chloroplasts. The margins of leaves may be composed only of cells derived from this merismatic tissue. Hence, this marginal tissue is white or yellow, rather than green. There are several other types of such variegation, depending on the tissues that have been affected, and their relationship to each other. Variegation may be consistent and symmetric in appearance throughout the whole plant, or it may be quite random in location. In some plants, entire branches or stems including the leaves may be variegated. The variegation in some forms is unstable. The extent and nature of the variegation can vary, and sometimes the plant will return to the green form. In others it is stable and does not change under normal conditions and temperature."
http://en.wikipedia.org/wiki/Variegation