In every shoot tip and root tip there is a small area of cells that are not committed to develop into any particular structure. These undifferentiated cells of the meristematic area are the cells that make mericloning possible. The meristematic area is very small, a sphere about one millimetre in diameter. But if this region is dissected from the shoot tip and placed in a suitable nutritive medium, it is capable of producing countless reproductions of the plant from which it has been removed.
It’s unnecessary to take such a small piece of tissue in the case of cymbidiums. A cube with a side length of 3 – 4 mm will suffice, provided that it contains the meristematic area. If this is placed into a nutritive medium, either liquid or solid (such as Knudson’s medium), it will produce protocorm-like bodies that will eventually grow into new plants.
Aseptic procedures must be practiced at all stages, because the nutritive media are equally supportive of bacteria and fungi and these will quickly overgrow and probably poison the explant. If the excision is successful, the base of the explant will swell within 4-6 weeks and from this swollen tissue one or more green spherical bodies, commonly referred to as protocorms, will develop. They consist of a pith core covered by an epithelial layer.
As they grow, each protocorm will eventually produce a shoot from an area on its epithelium and this will in turn produce a root, thus yielding a new plant. However, this procedure would result in a very limited number of replications, so prior to the development of the shoot, the protocorm is cut or crushed so that it produces a number of second-generation protocorms. This procedure is repeated until sufficient protocorms have been produced to yield the required number of plants (one per protocorm).
The possibility of mutations increases with successive replications, so a limited number of plants, ideally between 200 and 500, are made. Each plant should be an exact replica of the parent. However mutations can occur during multiplication and variation will occur. Most mutations are deleterious so these are to be avoided, although occasionally a desirable variant results.
It is undesirable to mericlone a mericlone. The original parent plant should be the only source of tissue used. Where mericlones are mericloned and these are in turn mericloned again, the incidence of undesirable mutations increases markedly. It is always the innocent orchid, not the proliferator that receives the blame when the mericlone turns out to be inferior. Try photocopying a picture and then repeatedly copying each copy. Which gives the best result?
In colchicine conversion, where the diploid state is converted to the tetraploid, colchicine is applied to the protocorm in the hope that some epithelial cells will double their chromosome count. If this occurs and the protocorm survives the treatment, a plate of tetraploid-converted cells then develops and if a leaf shoot arises from this area, then a tetraploid plant results.
Mericloning was developed by Morel as a side effect from his attempts to rid cymbidiums of virus disease. It has mixed blessings. It has made replication in many genera so easy that plants can be produced at a very reasonable cost. However, if a particular clone is over-exploited it can remove its exclusivity, and that of cymbidiums in general. Always attempt to purchase your mericlones from the original grower or his agent, and try to ensure that they come from a limited run of tissue taken from the original parent plant.