Development of the epiphyllous appendages of Begonia hispida var. cucullifera: implications for comparative morphology

Abstract

The epiphyllous appendages arise from the two outer cell layers of the young leaf by a cell division pattern like that of leaf inception at the shoot apex. The leaf-like appendages, especially, show a developmental pattern that resembles that of the main leaf. These data as well as anatomical evidence provide a factual basis for homologizing the leaf-like appendages with leaves (phyllomes). Only the position of the leaf-like epiphyllous appendages contradicts that of typical phyllomes. This contradiction disappears by applying the evolutionary process of spatial shifting (heterotopy) to leaves. This process leads to the formation of epiphyllous leaves which may be contrasted with cauline leaves. Since secondary appendages have been observed on some of the epiphyllous leaves, the concept of a 'leaf system' is used in contrast with the concept of ‘shoot system.’ In a leaf system leaves are interconnected without the intervention of a shoot apex. Supporting evidence from experimental research, especially organ culture, is pointed out. Other unusual modes of plant construction are briefly discussed.