Two distinct growth phases were demonstrated to exist in apple trees. These phases were closely related to the ease of regeneration of roots on stems. Stem cuttings of wood in the mature phase were very difficult to root without special treatments, while those made from shoots in the juvenile phase rooted very readily.
The juvenile condition was recognized by the thinness of the leaves and small amount of pubescence. Anthocyanin production was abundant in juvenile shoots. In some species of apple the shape of the leaf changed with maturity from an entire to a lobed form.
Shoots having the juvenile characteristics of young apple seedlings were produced from roots of older trees. Stem cuttings made from these shoots usually formed roots readily. Juvenile shoots were produced from adventitious buds on roots and possibly from adventitious buds on limbs of mature trees.
Watersprouts were found to originate from latent buds rather than from adventitious buds. The only adventitious buds produced on stems were found on sphaeroplasts. Adventitious buds were produced readily on roots, especially if the latter were placed under favorable conditions of temperature and moisture. These buds arose from parenchyma in the secondary cortex and thus had no connection with the cambium of the root.
The change from juvenile to mature form was not related to the beginning of secondary growth in the plant, the loss of primary structures, or to the stage of organization of internal tissues of the stem. The expression of growth phases, together with the accompanying changes in ease of root formation, is believed to be dependent upon certain biochemical factors not clearly understood at present.
Stoutemyer, V. T.
"Regeneration in various types of apple wood,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 19
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol19/iss220/1