HETEROPHYLLIC DEVELOPMENT IN MUEHLENBECKIA (POLYGONACEAE)

Flowering shoots of Muehlenbeckia platyclados Meisn. bear only reduced scale leaves which resemble the membranous sheath portion (ochrea) of leaves of other members of the Polygonaceae. Shoots propagated from cuttings bear enlarged foliage leaves with distinct lamina, petiole, and ochrea zones. The developmental basis for this heterophylly is explored in order to determine whether scale leaves resemble foliage leaves in their pattern of ontogeny or are developmentally unique. SEM and histological analyses have shown that scale leaves and foliage leaves are distinctive from inception. The scale leaf arises as a collarlike growth and extends over the shoot apex as a hooded sheath without evidence of blade initiation. By contrast, the first stage of foliage-leaf ontogeny is the differentiation of the distal lamina from the future leaf base. As the foliage-leaf ochrea encircles the stem axis, the lamina grows erect and projects from the abaxial surface of the sheath. Lamina reduction coupled with ochrea elaboration in intermediate leaf types indicate a homology between the entire scale leaf and foliage-leaf ochrea. Despite this homology, the production of the bladeless scale leaf does not involve a mere suppression of the foliage-leaf lamina. Erect growth of the saccate ochrea of the foliage leaf contrasts with the hooded expansion of the scale. Early histological differences, including contrasting rates of cell differentiation, also distinguish the two organs. This disparity in modes of growth and differentiation from inception results from separate, predetermined courses of ontogeny. Unlike other plants studied, leaf size and degree of leaf elaboration decrease with shoot meristem enlargement in Muehlenbeckia. Leaf packing does increase with shoot development and may contribute to variations in leaf morphology. It is concluded that the peculiarities of the heterophyllic leaf sequence in Muehlenbeckia are a property of the shoot system as a whole. IN RECENT YEARS, there has been a re-examination of the developmental and causal bases for the marked change in leaf form exhibited by the shoots of some plants (Cook, 1969; Critchfield, 1970; Feldman and Cutter, 1970a,b; Mauseth, 1977; Schmidt and Millington, 1968; for earlier references see Allsopp, 1965, 1967; and see references cited in remainder of text). In this regard the polygonaceous shrub, Muehlenbeckia platyclados Meisn. (vegetative tapeworm-centipede plant-ribbon bush), is of interest. While previous attention has been directed toward the morphogenesis of its unique flattened stems (Kaussmann, 1955; Troll, 1937), this species is also distinctive because of its heteroblastic leaf series. Shoots of seedlings and cuttings bear large, green, laminate-petiolate foliage leaves (Fig. 1), whereas I Received for publication 29 May 1978; revision accepted 31 August 1979. Research supported by National Science Foundation Grant DEB 74-02823 AOI. The authors wish to thank Dr. Dominick J. Paolillo, Jr. for his thorough reading of the manuscript as well as helpful comments on the plates. We are also grateful to Dr. M. V. Parthasarathy for the use of darkroom facilities. 2 Present address: Section of Botany, Genetics, and Development, Plant Science Bldg., Cornell University, Ithaca, N.Y. 14853. flowering shoots produce inconspicuous, nonphotosynthetic scale leaves (Fig. 4-6). The ontogenetic processes by which these and intermediate leaf types arise are compared and contrasted in this paper in order to determine the morphological relationship between them. Particular emphasis is placed on elucidating the developmental modifications which yield the scale leaf. The present investigation also serves as an evaluation of the relationship between shoot and leaf development in a species in which the more elaborate appendage type is generated early in the plant's development while the reduced type appears later. This condition is contrary to that described in previous investigations of heterophylly and is in need of detailed study. MATERIALS AND METHODs-Adult material of Muehlenbeckia was collected from a plant grown in the Botanical Garden of the University of California, Berkeley. The collector was Robert Dutton, but the original source of collection is unknown (Muehlenbeckia is endemic to the Solomon Islands). Juvenile material was obtained by propagation from stem cuttings of the adult plant. Adult and juvenile shoot tips were fixed in either CRAF III (Sass, 1958) or FAA (70%