Shape and Colour
marbled or patterned in other colours. This is also true of some flowers, although to a lesser extent. The causes of these types of variegation include chimeras, transposons, pattern genes and, in a few cases, virus infection.
Chimeras These are discussed in detail in Chapter 3 but a few points may be mentioned here. Chimeras are made up of genetically different cells. When the genes of these cells cause obvious differences such as, for example, a change in colour, then the different cell types become visible. Chimeras develop because flowering plants have a layered structure (see Chapters 1 and 3). Because of this arrangement, the outer layers of one partner in a graft can grow over the inner layers of the other, producing a plant made up from cell layers from two different genetic types (for example, + Laburnocytisus adamii). Chimeras can also occur when a mutation occurs in the growing point that causes part or all of a layer to be genetically different from other layers in the same plant. This type of chimera is usually stable and, when the mutations affect the production of chlorophyll, can result in good garden plants with attractively variegated leaves, such as Hosta and Euonymus (and see Chapter 3 for other examples). Although in evolutionary terms such a chimera does not confer any selective advantage and many may have arisen and died out, gardeners have selected them for their ornamental value, maintaining the variegation by taking shoot cuttings since they do not breed true from seed. However, they cannot be propagated by root or leaf cuttings, which disrupt the stable layered structure of the original growing point. In some variegated chimeras, reversion to the all-green form may take place in one or more shoots. Because the variegated shoots have no chlorophyll in the white or yellow part of the leaf, they are less vigorous than the all-green shoots which, if left alone, will eventually swamp out the variegation. Thus it is important to prune out any all-green shoots as soon as they are seen.
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Transposons Another cause of variegation in flowers and leaves is the occurrence of transposons (see Chapter 3). Transposons can move around within the chromosome and occasionally 'jump' into a gene and inactivate it. If they affect a gene for flower or leaf colour, a plant with random spots, stripes or sectors may result. One of the bestknown garden examples of transposons giving rise to an ornamental plant is that of Rosa gallica 'Versicolor' (Rosa mundi) which has flowers with red streaks on a white background (see Chapter 3 for details of how the transposons cause the red streaks). Transposons also affect foliage patterns if they disturb the production of chlorophyll. For example, it is not uncommon for pale sectors to occur at random in leaves (e.g. in garden peas, Pisum sativum).
Pattern genes The variegation patterns in some leaves are controlled by pattern genes and are a normal genetic characteristic of the species. In this case, the plants will breed true from seed. For example, in species of the tropical genus Maranta the green leaves are regularly patterned with dark blotches, while in Actinidia kolomikta the leaves are tipped with pink and white. Another form of seedtransmitted variegation is seen in Our Lady's milk thistle (Silybum marianum) where pockets of air along the veins reflect the light and give a silvery effect.
Viruses Although not now of any great importance with respect to the ornamental value of plants, infection with viruses can result in some types of variegation. Two viruses are mainly responsible and both have a limited host range and so are not a threat to other plants. Infection with vein-clearing virus is the reason for pale leaf veins in Vinca major 'Reticulata' and similar forms of ground elder (Aegopodium podagraria) and sweet violet (Viola odorata). Abutilon mosaic virus only affects species of Abutilon producing a mosaic pattern of yellow/green on the leaves. The forms infected with this virus are A. pictum 'Thompsonii',