ONTOGENY OF THE TENDRIL IN VITIS VINIFERA

The apical meristem of the grape tendril possesses several remarkable features: bilateral symmetry, a minimal number of appendages, and an exceedingly brief period of apical meristem activity. The cellular configuration of the apex changes from tunica-corpus to zonate, as rudimentary leaves and branch-tendril apices are initiated. Eventually the apical meristem of the tendril itself ceases meristematic activity and differentiates as a large hydathode. Typical spongy epithem tissue, copious xylary tissue, and water-pores in the epidermis characterize the hydathode. Numerous vascular strands traverse the length of the tendril and terminate in enlarged tracheary elements adjacent to the epithem. Cessation of meristematic activity follows lowered mitotic rate in the summit and accelerated differentiation below and within the meristem. THE DETERMINATE tendril-shoot in grape, Vitis vinifera, is a short-lived vegetative apical meristem. Studying the ontogeny of such determinate apical meristems is useful to show how differentiation proceeds in a region which in most vegetative shoots remains undifferentiated. Histogenetic studies concerning the fate of determinate vegetative apical meristems are rather few: excellent examples include studies on the thorn-shoots of Gleditsia (Blaser, 1956), Hymenanthera (Arnold, 1959), and Ulex (Bieniek and Millington, 1967), and studies of tendrilshoots (Moens, 1956; Shah, 1962; Millington, 1963, 1966). Highly modified determinate shoots also occur in cacti (Boke, 1959). The tendril of grape (Vitis vinifera) has a type of determinate vegetative growth in which the components of the apex differentiate completely. Its development has not been reported previously, despite the numerous studies of grape morphology. Most attention has been focussed on the peculiar arrangement of leaves and tendrils, which is considered either sympodial (Troll, 1954) or monopodial (Bugnon, 1953; Moens, 1956; Millington, 1966). Millington (1966) reviews the arguments and contributes new ontogenetic evidence to support the monopodial interpretation. Other genera in the Vitaceae possess the same bizarre architecture: Ampelopsis (Moens, 1956), Cayratia (Shah, 1960), Cissus (Shah, 1962), and Parthenocissus (Millington, 1963, 1966). The 1 Received for publication 6 May 1968. This research was completed in the Botany Department, University of California, Davis. The authors wish to thank Dr. T. E. Weier, who generously allowed use of the electron microscope (maintained under National Science Foundation grant GB-2546) for this project. They also are grateful to Dr. Kenneth Stewart for assistance with preparations for electron microscopy and to Dr. R. J. Weaver, who provided the plant material from University stocks. tendril of vitaceous plants is considered a shoot by either interpretation, however. Hence one can compare the tendril apex directly with other vegetative apical meristems. Initiation and early development of tendrils in Parthenocissus has been described by AMillington (1966). In Parthenocissus and Ampelopsis, the tendril tip becomes a glandular disk, quite unlike the hydathode in Vitis. Moens (1956) reported that the disk in Ampelopsis is secretory and is composed of radially elongate epidermal cells. The number of tendril branches per tendril-shoot varies: 2-4 in Vitis, 2-5 in Parthenocissus, and up to 10 in Ampelopsis. MATERIALS AND METHODS-Shoot tips and young tendrils were collected from healthy, greenhouse-grown plants of 'Zinfandel' and 'Petite Sirah' varieties of European grape (Vitis vinifera). These were fixed either in FAA or CrAF III; the material was imbedded in Paraplast and sectioned at 7-10 ,u, and stained in a combination of iron alum haematoxylin, safranin, and fast green. Whole mounts of representative tendrils were prepared by bleaching in NaOH, clearing in chloral hydrate, and staining in safranin to emphasize the vasculature. Some material of the hydathode was prepared for electron microscopy. This structure is a refractory subject for infiltration. The most successful treatment for killing and fixation was in cold 4 % glutaraldehyde (in 0.1 M phosphate buffer at pH 7.2) for 4 hr, followed by cold 3 % KMnO4 for 3 hr. The tendril tips then were dehydrated, imbedded in Maraglas, and sectioned. The sections were stained with lead citrate, then prepared and photographed by the customary procedures. Monitor sections approximately 0.37 ,A thick were stained in methyl violet, dehydrated, and mounted permanently for phase photography.