The Evolution of Morphological Novelty Exemplified in the Growth Patterns of Some Gesneriaceae

SUMMARY The growth pattern of certain South African species of Streptocarpus sub‐genus Streptocarpus has been investigated, using in particular the species S. fanniniae. It is shown that the vegetative plant body consists of repetitive units, the first being formed by the accrescent cotyledon. These units are here called phyllomorphs: each has a stalk (the petiolode) showing a mixture of axis‐like and petiole‐like features, and a lamina. The development of the phyllomorph is largely controlled by a triad of meristems situated near junction of petiolode and lamina. The basal meristem provides new growth at the base of the lamina; the petiolode‐meristem contributes to the growth of the midrib as the lamina enlarges and to elongation of the petiolode; the groove‐meristem initiates the growth of new phyllomorphs and of the inflorescences. Recognition of the phyllomorph as a repetitive structural unit permits satisfactory description and comparison of anomalous growth patterns of various species of Streptocarpus and of certain other genera of Old World Gesneriaceae. Its evolution and diversification is briefly discussed.