Variegation is the appearance of differently coloured zones in the leaves, and sometimes the stems, of plants. This may be due to a number of causes. Some variegation is attractive and ornamental and gardeners tend to preserve these. The term is also sometimes used to refer to colour zonation in flowers, minerals, and the skin, fur, feathers or scales of animals.
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.
Because the variegation is due to the presence of two kinds of plant tissue, propagating the plant must be by a vegetative method of propagation that preserves both types of tissue in relation to each other. Typically, stem cuttings, bud and stem grafting, and other propagation methods that results in growth from leaf axil buds will preserve variegation. Cuttings with complete variegation may be difficult if not impossible to propagate. Root cuttings will not usually preserve variegation, since the new stem tissue is derived from a particular tissue type within the root.
As these plants have some of their tissue unable to carry out photosynthesis, the plant will be weaker than the plain green plant. They should generally be expected to die out in nature and their only real source of survival is through cultivation.
Some variegation is due to visual effects due to reflection of light from the leaf surface. This can happen when an air layer is located just under the epidermis resulting in a white or silvery reflection. It is sometimes called blister variegation.
Pilea (aluminum plant) is an example of a house plant that shows this effect. Leaves of most Cyclamen species show such patterned variegation, varying between plants, but consistent within each plant.
Another type of reflective variegation is caused by hairs on parts of the leaf, which may be coloured differently from the leaf. This is found in various Begonia species and garden hybrids.
Sometimes venal variegation occurs – the veins of the leaf are picked out in white or yellow. This is due to lack of green tissue above the veins. It can be seen in some aroids.
The blessed milk thistle, Silybum marianum, is a plant in which another type of venal variegation occurs, but in this case it is due to a blister variegation occurring along the veins.
A common cause of variegation is the masking of green pigment by other pigments, such as anthocyanins. This often extends to the whole leaf, causing it to be reddish or purplish. On some plants however, consistent zonal markings occur; such as on some clovers, bromeliads, certain Pelargonium and Oxalis species. On others, such as the commonly grown forms of Coleus, the variegation can vary widely within a population.
In Nymphaea lotus, the tiger lotus, leaf variegations appear under intense illumination.
Virus infections may cause patterning to appear on the leaf surface. The patterning is often characteristic of the infection. Examples are the mosaic viruses, which produce a mosaic-type effect on the leaf surface or the citrus variegation virus (CVV). Hosta Virus X (HVX) has caused such dramatic and attractive mosaic-type variegations that, when the disease was new to the United States, affected plants were mistakenly thought to be new mutations and were registered as new cultivars.
While these diseases are usually serious enough that the gardener would not grow affected plants, there are a few affected plants that can survive indefinitely, and are attractive enough to be grown for ornament; e.g. some variegated Abutilon varieties.
By convention, the italicised term variegata as the second part of the Latin binomial name, indicates a species found in the wild with variegation (Aloe variegata). The much more common, non-italicised, inclusion of 'Variegata' as the third element of a name indicates a variegated cultivar of an unvariegated parent (Aucuba japonica 'Variegata'). However, not all variegated plants have this Latin tag, for instance many cultivars of Pelargonium have some zonal variegation in their leaves. Other types of variegation may be indicated, e.g. Daphne odora 'Aureomarginata' has yellow edging on its leaves.
Garden plants 
Variegated plants have long been valued by gardeners, as the usually lighter-coloured variegation can 'lift' what would otherwise be blocks of solid green foliage. Many gardening societies have specialist variegated plants groups, such as the Hardy Plant Society's Variegated Plant Special Interest Group in the UK. Several gardening books which deal exclusively with variegated plants are available.