ORIGIN AND EARLY MORPHOGENESIS OF LATERAL BUDS IN THE FERN DAVALLIA

The general organography, vascular organization, and leaf and bud development in Davallia solida and D. trichomanoides are described. These epiphytic species have creeping shoots with dorsally-borne leaves in a distichous phyllotaxis and the buds occur near each leaf base. Roots are borne on the ventral and flanking surfaces of the rhizome, but only at bud positions. The vascular pattern of these species is a perforated solenostele. Leaf and bud traces have distinctly different origins. While the proximity of buds to leaves has suggested that bud origin is axillary, observations show that the origin of buds is cauline and that their position is extra-axillary from inception. The stages of structural morphogenesis in Davallia buds differ significantly from the scheme proposed by Wardlaw. The principal difference is the absence of a resting period occurring between the origin and continued development of buds in Davallia. The elongated intemodes which separate leaf-bud pairs from one another, the topographically distinct and predictable positions of leaves and buds, the structural equivalence of unexpanded buds, and vascular differences in leaves and buds make Davallia an useful species for physiological studies of differential bud expansion. THE STRUCTURE AND FUNCTION of lateral buds in seed plants have been extensively investigated, and literature on the origin and development of buds and the physiology of bud expansion has been amply summarized by Esau (1965) and Phillips (1969). However, our knowledge of bud morphogenesis in lower plant groups is far more rudimentary. The ferns, in particular, exhibit a marked diversity in bud insertion and assumed origin. For instance, examples of dichotomous or pseudodichotomous (Dryopteris), extra-axillary (Polypodium), axillary (Trichomanes), and phyllogenous (Dennstaedtia) branching have been described. Because of this variation, and especially because the axillary type which predominates in seed plants also occurs in this group, an opportunity is available for morphological comparisons to be made between the modes of fern branch origin and their ontogenetic patterns of development with the inception and early growth of lateral buds in seed plants. Accounts of branching by Bower (1923), Goebel (1928), and Troll (1937) emphasize the high degree of variability found in the ferns. Although each author presents a scheme of bud origin and provides a generic example to illustrate each branching type, the modes of branch origin typi1 Received for publication 15 January 1975. The author wishes to thank Dr. Donald Kaplan for his advice throughout the course of this study and for his critical evaluation of the manuscript. I also thank Dr. Ernest Gifford, Dr. Russell Jones, and Dr. Rudolf Schmid for their reading of the manuscript and helpful suggestions and Ms. Charlotte Mentges for her assistance in preparation of the illustrations. cally were deduced from the mature disposition of buds rather than from detailed developmental investigation. While it is clear from macroscopic observation of shoots of different fern species that buds can occupy a variety of positions, the critical evidence for distinct and different modes of bud inception, as these authors suggest, is wanting. An especially interesting and problematic type of branch insertion is seen in the epiphytic fern Davallia. The dorsiventral creeping shoots bear highly divided deltoid leaves in two ranks on the dorsal surface of the main axis with buds appearing near, but slightly ahead of, each leaf base. Although this positioning suggests axillary origin, Bower (1923) suggested three alternative interpretations to explain their unique position of insertion: (1) the bud arises acrogenously (from the apical meristem) and the associated leaf is its first appendage, (2) the bud arises phyllogenously (from leaf tissue) and is secondarily displaced to an axillary position, or (3) both buds and leaves are cauline in origin and their ultimate disposition is due to the rhythmic inception of these primordia and the correlative longitudinal growth of the axis. The present investigation of the anatomical features of leaf and bud inception was initiated to discriminate the actual mode of origin from these possible explanations. The origin of buds in the ferns is of fundamental morphological importance, but of equal interest is the continued growth and expansion of these buds which ultimately will reflect the branching pattern and overall form of the plant. Early bud morphogenesis previously has been characterized in the