Diverted development of reproductive organs: A source of morphological innovation in land plants

Recent discussions of animal development, particularly at the level of molecular genetics, have emphasized modularity, dissociation and co-option as basic principles of evolutionary developmental biology. These concepts are discussed in relation to two specific structural innovations in land plant evolution: the leaves (microphylls) of lycopsids, and the interseminal scales of Bennettitales. Both structures appear to have been derived evolutionarily by the diverted development of reproductive organs. In the case of lycopsids, recent analyses of phylogenetic relationships suggest that leaves are sterilized sporangia modified for photosynthetic assimilation. In the case of Bennettitales, structural data suggest that the interseminal scales are sterilized ''cupules'' modified for protection of the ovules. In both cases, multiplication of reproductive organs seems to have accentuated functional redundancy, and together with the developmental autonomy (dissociation) already inherent in the modular construction of plants, appears to have facilitated sterilization and co-option of some of these structures for new purposes. Numerous other examples in plants illustrate the same principles.