MITOTIC ACTIVITY IN THE ROOT APICAL MERISTEM OF AZOLLA FILICULOIDES LAM., WITH SPECIAL REFERENCE TO THE APICAL CELL

The meristematic activity of the apical cell and its derivatives (merophytes) in the unbranched, determinate roots of Azolla filiculoides Lam. was investigated. The plane of division of the apical cell indicates that it is the initial of each merophyte. The division plane of each newly formed merophyte is strictly periclinal to the root surface and provides confirmation that the immediate derivatives of the apical cell cannot be the ultimate root initials. The frequency of cell division as determined by the mitotic index, and by the duration of the cell cycle as determined by the colchicine method, confirmed the meristematic activity of the apical cell. As roots increase in length, the duration of the cell cycle in the total meristem increases, with the apical cell possessing the longest cell cycle, whereas the immediate derivatives maintain approximately the same cycle duration as in shorter roots. In determinate Azolla roots, cell division appears to play a major role up to a certain root length, then increase in length is produced mainly by cell elongation. SINCE the time of the classical studies of Nageli and Hofmeister on the apical meristems in vascular cryptogams, the apical cell has been considered to be an initiating cell and the center of root organization (Hofmeister, 1862; DeBary, 1884; Haberlandt, 1914). Recently, studies have been initiated to determine the nature and role of the apical cell in the root growth of certain lower vascular plants. The results obtained have led to two different conclusions-one that supports the apical cell concept and the other which does not. Buvat and Liard (1953) reported that the apical cell of the shoot apex of Equisetum arvense does not divide with the frequency that is necessary to provide support for the apical cell theory. Based upon the lower number of mitotic figures observed in an apical cell as compared with its purported derivatives, Wetmore (cited by Sinnott, 1960) suggested that cells adjacent to the apical cell were the initials. For the roots of E. arvense, Buvat and Liard (1954) concluded that there were essentially no differences in mitotic activity between the apical cell and the surrounding cells. Clowes (1956, 1958) chose Azollafiliculoides as a representative of the pteridophyte root which possesses a single apical cell. Autoradiographs in1 Received for publication 10 December 1979; revision accepted 8 April 1980. The authors thank Dr. Vito S. Polito and Dr. Eva S. G. Kurth for critical reading of the manuscript. Support, in part, provided by NSF grant PCM 79-10683 to E.M.G. 2 Present address: Biology Department, Mahidol University, Rama 6 Rd., Bangkok 4, Thailand. dicated that the root apical cell of Azolla synthesized DNA, RNA, and protein at the same rate as the adjacent meristematic cells. Clowes concluded that there is no quiescent center in roots with a single apical cell. The localization of tritiated thymidine in the shoot and root apices of Ceratopteris thalictroides (Gifford, 1960) indicated that the root apical cell synthesized DNA, and perhaps to a greater extent than the adjacent cells. Gifford (1960) concluded that if the synthesis of DNA is not due to endoreduplication, then the results confirm the concept that the apical cell does divide, and perhaps at a rate higher than has been estimated heretofore. A study of cell lineages, planes of cell -division, and growth rates in the root apices of Ceratopteris thalictroides (Chiang and Gifford, 1971) supported the apical cell theory. However, as the roots became older, the apical cell divided less frequently and the duration of the cell cycle increased. D'Amato and Avanzi (1965) criticized the conclusion that the apical cell is an initiating cell based on DNA synthesis alone because, from this, one cannot distinguish the synthesis of DNA prior to mitosis from the synthesis of DNA prior to endoreduplication. By using a combination of microspectrophotometry and autoradiography, D'Amato and Avanzi (1965, 1968) and Avanzi and D'Amato (1967) reported that the apical cell is quiescent and that the chromatin of the quiescent cell is blocked at the post-DNA synthetic phase (G2). According to them, the root apical cell of species of Marsilea, Ceratopteris, Blechnum, and Polypodium divides only during the initial stage of or-