I’m curious—what makes certain plants have variegated leaves? Is this an adaptation for survival, the way cactus features are or the way flowers are built to appeal to pollinators?
Answer: Variation in leaf color arises because of a lack of the green pigment chlorophyll in some of the plant cells. It isn’t an adaptation to the environment, but instead it is usually the result of a cell mutation, and can be inherited (genetic) or occur randomly (chimeric). If genetic, the color change is stable, which means that if you propagate a green shoot from a plant with colored leaves or sow its seed, the coloring will reappear in the new plant. This applies both to green leaves with irregular markings (variegation), say in white and yellow, and to those of a single solid color such as gold or purple.
A random mutation usually shows up as variegation. If you propagate from a green shoot or sow seed of the plant, the color will not recur. This kind of variegation is the most common, but is often difficult to stabilize. Propagation must be from variegated or colored shoots. In nature these forms usually die out, being weaker growers because of the lack of chlorophyll, which plants use to make the food they need for growth.
Variegation can also be the result of a viral infection, showing as discolored veins or leaf areas. This form of variegation is relatively rare, but it is stable. Lonicera japonica ‘Aureoreticulata’ has this type of variegation, with golden yellow veins netting the leaves.
Pictured: Lungworts (Pulmonaria) are plants more admired for their variegated foliage than for their flowers. This is Pulmonaria 'Spilled Milk'.
Image courtesy Missouri Botanical Garden PlantFinder