One of the trees growing in my garden has an offshoot that is quite different from the rest of the tree (see photo). Even stranger than this, emanating from the anomalous branch is another offshoot which does look like the original main tree. What could be the reason for this? Have similar phenomena been seen on other plants?
鈥� Similar phenomena occur in a range of plants. A variegated plant is often a of two cell lines of different colours, and occasionally, a new bud grows from just one type of tissue. Its leaves look conspicuously different and generally a stem from such a bud cannot revert to the chimera type.
Variegation or leaves of paler colour often result from a virus infection, and if a new bud happens to shake off the virus it produces a healthy-looking shoot. Occasionally, an aphid, or a virus particle conveyed by internal flow of sap, reinfects a healthy daughter shoot, producing a new shoot with the parental variegation.
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鈥淚n trees or other plants, variegation or leaves of paler colour often result from a virus infection鈥�
Scandinavian foresters might correct me, but the photo apparently shows a normal parent plant with an abnormal offshoot bearing a normal second-generation offshoot. Without closer inspection I cannot tell whether a splash of chemical or a pathogen injected by mites or aphids, for example, caused the abnormal branch. It seems likely to me that a virus or the toxic saliva of a parasite was the guilty party. If so, a daughter bud somehow shook off the infection or cellular damage, producing a new, normal-looking branch.
Jon Richfield, Somerset West, South Africa
鈥� This is an example of a plant chimera arising from the multicellular and layered growing points or that plants have. The tissue produced by each of the few cells that make these are pretty stable. Normally, there are three or four distinct layers of cells, like a hand in inner and outer gloves. The outer produces the epidermis or 鈥渙ver-skin鈥�, which is very stable. The next couple of layers produce the cells that show the fruit colour and such like, and the inner, less-organised layers produce the pith.
Meristematic cell tissue produces a column down the plant. Mutations 鈥� natural or induced 鈥� are single-cell events, so when one occurs high in the meristem a long strip of the changed cell type is produced following on from the first mutation in a line of cells. If a branch grows from this strip it can be a stable chimera with a complete concentric layer of the new type of mutated cell and can be successfully propagated.
This is probably the source of the new growth in the question. There are still two or more genotypes in the plant, but they are kept stable by the layered meristems, one type surrounding the other. However, it is always possible for a cell in one layer of the meristem to replace that from another, revealing the different genotype. In the conifer shown, the original, more vigorous type invaded the dominant ornamental layer (probably layer two). A shoot grew from this original patch but left a column of the decorative type that reappeared when a further branch grew from this area.
In the 1970s, I worked on breeding new fruit varieties that exploited mutations. We solved the chimera problem by using a chemical that causes mutations to kill all of the meristematic cells apart from the one we wanted. We also used horticultural techniques such as root cuttings to produce offspring plants from only one layer of the original. A self-fertile Cox鈥檚 orange pippin apple tree and a more compact Bramley鈥檚 seedling that came from this work are on the market.
Colin Lacey, Chichester, West Sussex, UK