Variegation
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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.
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Chimeral variegation
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.
There are several types of such variegation, depending on the tissues that have been affected. 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.
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.
Variegation due to reflective effects
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. Cyclamen hederifolium leaves 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 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.
Variegation caused by other pigments
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 common Coleus, the variegation can vary widely within a population.
Pathological variegation
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. While these diseases are usually serious enough that the gardener would not grow affected plants, there are a few affected plants that are grown for ornament; e.g. some variegated Abutilon varieties.
Nutrient deficiency symptoms may cause a temporary or variable yellowing in specific zones on the leaf. Iron and magnesium deficiencies are common causes of this.