Plants that are artificially induced to grow in a desired manner by propagating selected material are not to be included in this class and are considered cultivariants. Abies procera 'Glauca Prostrata' which is described as a flat-growing plant, but invariably produces an upright leader and eventually becomes a tree. The grafting of a side branch of Abies will generally produce a cultivariant exhibiting this kind of behavior. Upper picture is the original broom of Taxodium distichum 'Secrest' and the lower picture is Abies procera 'Glauca Prostrata'. The mechanisms that produce cultivars are not very well understood, but there are some good observations and interesting theories about the various processes at work.
Cultivars tend to remain stable, and propagations grow like the parent plant.
However, reversions back to species normal do sometimes occur and serve to confuse the issue. This type of activity is really quite common in many species. Mutations occur in nature and are often induced by the background radiation present all around us. When cell divisions are occurring in growing tissues, they are most susceptible to damage by this radiation. If such damage occurs at the right time and place, a mutation may result.