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.
Chimeral
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.
Reflective
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 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.
Pigmentary
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.
Pathological
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). 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 attactive enough to be 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.
Garden plants
Variegated plants have been long-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.
Read about Variegation in the Standard Cyclopedia of Horticulture
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Variegation. This term is usually applied to a class of variations, especially in leaf-coloration, in which the leaves become striped, banded, spotted, and blotched with yellow; white, red, and various other colors in connection with the normal green of other portions of the leaves. In the case of yellow-and-white variegation, the term albinism is sometimes used, especially when the plants are largely marked with white or yellow, as in Abutilon Sellovianum, Pelargonium zonale, and variegated forms of Evonymus japonica, Hydrangea hortensis, Hedera helix, Polyscias Guilfoylei var. Victoriae, and others. Among the dracenas, caladiums, and codieums, besides the white variegation, there are developed beautiful reds, pinks, yellows, and so on. As a rule, the term variegation is not used in cases of color-variation in which only the surface of the leaf is involved, as in many of the begonias, sansevierias (S. thyrsiflora and S. zeylanica), Alocasia cuprea, Cissus discolor, and others. In many such plants the markings are due in part to hairs, scales, or air in the cuticle or epidermal cells, as in sansevieria and begonia. In some begonias, many varieties of calathea (as C. ornata var. albo-lineata), the epidermal cells develop decided and definite color-variation, though the changes do not usually involve the mesophyl or inner cells of the leaf. In some genera, however, especially calathea, all gradations are found between purely epidermal variegation and changes involving the deeper layers of the leaf, as in C. Veitchii and C. Makoyana. The same is true of many other genera. Different kinds of variegation are shown in Figs. 3906, 3907. True variegations may be distinguished from ordinary colorations, bleaching, chlorosis, and the like, by the fact that the colored areas are usually quite sharply defined. They do not gradually blend into each other, but have definite boundaries. Cells in the variegated areas are found, as a rule, to contain the same chlorophyl bodies (chromatophores) as the ordinary green cells of the plant. However, in the variegated parts, the green color is not developed, and the chromatophores are often smaller or are somewhat swollen and vacuolate. In the case of chlorosis due to the lack of iron, or yellowing due to the lack of light, a leaf will quickly develop its normal color if given the proper conditions. This is not the case, however, in variegated leaves. While the intensity of whatever color the chromatophores may have can be varied by light and food, a variegated cell can never be changed by these means to a normal cell. The chlorophyl granules (chromatophores) appear to have lost entirely, in many cases, the power to make starch and sugar from the carbonic acid gas in the air, and in other cases this power is very greatly reduced. In practically all cases, however, when the chromatophores are not destroyed, they retain the power to convert sugar into starch and they thus store up starch in their tissues from the sugar manufactured by the healthy cells of the leaf. White or albino variegation is of course due to a lack of any coloring in the chromatophores, and sometimes to the entire absence of these bodies. The cells seem to have lost completely the power of making chlorophyl. These albicant variegations are to be looked upon as the more extreme forms of variegation, and usually arise through a feeble or atrophied condition of the plant. Seedlings raised from parents both of which are variegated in this way are usually very weak. High feeding and favorable conditions of growth, while they will not cause a variegated plant to return to its normal condition, will often stimulate the development of a normal green shoot that takes most of the nourishment and thus causes the starvation and disappearance of the albicant parts. In other cases, as in codieums, modified chlorophyl is made. Large yellowish oil-like drops occur in the substance of the chromatophores, and the various changes that these undergo, as the leaf becomes older, produce the remarkable and beautiful colorations of this group of plants. The coloration here, as in dracenas and caladiums, is intensified by strong light and nourishing food. The more of the modified chlorophyl there is produced and the more rapid the changes in the modified chlorophyl brought about through the action of light and the acids and oxidizing ferments of the leaves, the more highly developed will be the colors, though here again high feeding is likely to cause the plant to revert to its normal condition. Variegated plants or parts of plants are usually of slower growth and smaller than green plants of the same variety or the green parts of the same plant. Variegation occurs either by bud-variation or by variations in seedlings. In the former, a variegated branch is likely to appear on an otherwise perfectly normal plant. Such variegations are easily reproduced by budding, grafting, or cuttings, but generally do not develop again from seeds produced on such branches. On the other hand, when variegation develops in seedlings, the seeds of such plants usually give a number of variegated individuals, even the cotyledons being sometimes affected. In some cases the proportion of variegated plants from seeds is very large and can be increased by selection. As a rule, the form of spotting or marking is not constant in seedlings, often being very different from the parent. In certain groups of plants, which have for many years been selected on account of the horticultural value of these markings, the variegated condition has become almost a fixed feature of the plant, as in dracenas, caladiums, and codieums. While the plants of these genera are not usually propagated from seeds, still when they are so propagated, a large number of seedlings show more or less variegation. Darwin and many of the earlier investigators believed that these variations were started in the plant by unfavorable nutritive conditions, and much has been written on the subject as to whether or not variegations should be considered as diseased conditions. The question as to whether a variegated condition could be transmitted to normal plants by budding and grafting has also been much disputed, but the weight of evidence indicates that in many cases such transmission certainly takes place. This has been thought to indicate the presence of some micro-organism, living either parasitically or symbiotically in the plant, and causing the changes known as variegation. Investigations conducted by the writer on the so-called mosaic disease of tobacco, which is a form of variegation, and also on many other forms of ordinary variegation, show quite conclusively that the disease is not caused by micro-organisms, but is due to a deranged condition of the nutrition of the cells. Without going into the details of the matter, it may be said that the condition is characterized physiologically by a marked increase in the oxidation processes in the cells, caused by the presence of an abnormal amount, or an abnormal activity, of oxidizing ferment in the protoplasm. These changes must, in most cases, therefore, be considered as pathological in their nature, as the vitality and vigor of the plants are reduced as a result. It is further evident that the initial causes of variegation may be quite diverse, some of the most usual being seed of low vitality; unsuitable nourishment, especially a lack of elaborated nitrogen; rapid growth in very moist soil; severe injury to the roots during a period of rapid growth of the upper parts of the plant; severe cutting back, and the like. Though started at first through the influence of environment, variegation, when of value horticulturally, has in many cases been increased and fixed by selection till it has become almost a specific character in some groups of plants, and is considered in the botanical description of them. A word might be said in this connection regarding autumn coloration. The production of color in autumn foliage is, as is well known, due in part to the gradual destruction of the chlorophyl when the leaves have reached maturity and approach the period of death, and in part to the action of acids on anthocyanin. Many of the destructive changes which take place in the chlorophyl are oxidation processes, the same as occur in the cells of highly colored variegated plants, and physiologically they are not very different from the changes occurring in calathea, caladium, codieum, and the like. The approach of maturity in the leaf, and the coming on of cool weather in autumn, stimulates the production of oxidizing ferments, and the action of these and the acids of the cell-sap on the chromogen, or color contents of the leaves, especially the chlorophyl and anthocyanin, causes many of the brilliant colors of autumn foliage (p. 431).
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