Plastochron Indices for Juvenile and Mature Forms of Hedera helix L. (Araliaceae)

English ivy (Hedera helix) plants were assessed for the applicability of the plastochron index (PI). Juvenile ivy satisfied all requirements for the use of the PI and showed a plastochron of 4.23 days. Mature ivy grown under long day conditions flowered after 11–12 leaves. Two distinct groups of leaves were produced with different plastochrons (0.83 and 3.2 days, respectively) and leaf morphologies. Long-day-grown ivy did not satisfy the requirements for the use of the PI. Short-day-grown mature ivy continued production of leaves beyond the 12th leaf. Vegetative growth was perpetuated for at least 25 plastochrons. By 19 plastochrons (ca. 41 days after budbreak) a linear PI vs. time relationship was established with a plastochron of 3.16 days. This newly acquired ability to maintain vegetative growth in mature ivy plants may allow a direct comparison with the vegetative indeterminant juvenile in order to assess possible anatomical factors responsible for phase stability and phase change using the PI as a basis for comparison.