CELL CYCLE DURATION IN THE MERISTEM OF NEPHROLEPIS BISERRATA STOLONS: THE ROLE OF THE APICAL CELL

The stolons of Nephrolepis biserrata (sw.) Schott are thin axes that grow rapidly (from 2 to 4 mm per day) in the controlled conditions applied. In the cylindro-conical meristem, three histological zones are defined. Cell cycle duration was determined for each zone by autoradiographic methods after incorporation of tritiated thymidine and confirmed by the colchicineinduced metaphase-accumulation technique. The apical cell and its derivatives (Zone 1) are mitotically more active (cell cycle duration: 80 hr) than the cells of the subapical zones (2 and 3), where cell cycle lengths are 142 hr and 95 hr respectively. These data, compared to previous results, give evidence for the main role played by the relative rate of division of the apical cell compared to that of lateral cells in the organization and the shape of the meristem of pteridophytes. Moreover, the apical cell appears to be unique in having a diffierentiated cytological aspect not usually associated with an intensely proliferating cell. THE ROLE of the apical cell at the shoot tip of most ferns is not yet clearly established. The estimation of mitotic indices, which in many cases have been shown to be correlated with cellular proliferation (Corson, 1969; Rembur and Nougarede, 1977), and the determination of cell cycle duration in each histological zone allow a determination of division rates of the apical cell and its immediate neighbors in comparison with those ofthe other cells ofthe apex. The data that have been obtained, far from suggesting a general concept about the active (Stevenson, 1976; Bierhorst, 1 977) or passive role (D'Amato and Avanzi, 1968; Sossountzov, 1969) of the adult apical cell, show differences according to the material studied. In adult Pteris and Polypodium, the very small mitotic index of the central zone of the meristem (apical + derivative cells) is always lower than that of the lateral zone where leaf initiation takes place (Michaux, 1971; MichauxFerriere, 1981), and the cell cycle duration of the apical cell and its neighbors is the longest of the apical region (Michaux-Ferriere, 1973, 1981). However, in Ceratopteris(Polito, 1979) and Azolla (Gifford and Polito, 1981) the mitotic index of the apical cell is higher than or equal to that of the other cells of the meristem, and cell cycle durations are shorter than or as long as that of lateral cells. In Azolla and Ceratopteris, the meri stem has a cylindro -conical t Received for publication 22 June 1984; revision accepted S February 1985. or very elongated shape, respectively, and it gives rise to a thin axis with many small leaves. On the other hand, the apices of Polypodium and Pteris are flat and give rise to thick rhizomes, which grow very slowly and produce relatively few large leaves. It appears as though there was a relationship between relative rate of division of the apical cell compared to other cells of the meristem and the shape of the apex, the rate of growth of the axis, and the size of foliarorgans(Gifford, 1983;Michaux-Ferriere and Hallet, 1985). In order to test part of this assumption, apical meristems of Nephrolepis biserrata stolons were investigated because of their aphyllous development, cylindro-conical shape, and fast growth (Espagnac, 1973). The mitotic index and the total length of cell cycle were determined for each of the three histological zones, first defined in the stolon meristems, and, in particular, for the central zone, which consists of the apical cell and nearby derivative cells. In order to support the data on the cycle duration determined by the continuous labeling technique, which requires a prolonged period of tritiated thymidine incorporation and thereby is often criticized (de la Torre and Clowes, 1974), the colchicine-induced metaphase-accumulation method was also applied. MATERIALS AND METHODS-Growth conditions and histological studies-Nephrolepis biserrata (sw.) Schott shoot apices were collected from first or second order branch stolons growing at the rate of 2-4 mm per day. The plants