MITOTIC ACTIVITY IN THE ROOT APEX OF THE WATER FERN MARSILEA VESTITA HOOK. AND GREV.

Roots of Marsilea vestita ranging from 1-120 mm in length, as well as root primordia, were analyzed to determine mitotic activity and ploidy levels in the apical cell, five well-defined regions of the root proper, and two regions in the root cap. The mitotic index of the apical cell tended to be above the overall mean mitotic index for the entire apical meristem. No diurnal rhythm in mitotic index was apparent. The cell-cycle duration of the apical cell ranged from 12.1-25.2 hr, that of other regions of the root from 16.1-41.5 hr. There was no indication of polyploidy in any part of the apical meristem except in a few procambial cells. Thus, the results support the classical concept that the apical cell is the ultimate source of cells in the root. THE CLASSICAL CONCEPT of the role of the apical cell found in many bryophytes and lower vascular plants, namely that whenever a single apical cell is present, it divides regularly and is the source of all other cells in the shoot or root, was deduced from careful analysis of the arrangement of the cells surrounding the apical cell (Naigeli, 1845; Hofmeister, cited in Nageli and Leitgeb, 1868). This concept seems to have remained unquestioned for many years although, according to D'Amato (1975), "in 1867 already, Reess had stated that growing shoot apices of Equisetum may show a discontinuous meristematic activity of their apical [cell]" (D'Amato, 1975, p. 321). However, I have been unable to find any statement to such effect in Reess' paper. On the contrary, he wrote, "Bei Allen [Equisetum Arten] wird das Scheitelwachstum vermittelt durch ununterbrochen wiederholte Theilungen der dreischneidigen, einem tetraedischen Kugelausschnitt ungefahr entsprechenden Scheitelzelle mittelst gerader, den Seitenwanden der Scheitelzelle paralleler Hauptwande" (p. 14-"In all species of Equisetum, apical growth is caused by continuously repeated divisions of the apical cell, which has three cutting faces and approximately the shape of a tetrahedral segment of a sphere; the divisions result in the formation of straight walls parallel to the lateral faces of the apical cell."). ' Received for publication 5 June 1980; revision accepted 29 August 1980. This paper represents a portion of a dissertation submitted to the University of California at Davis in partial fulfillment of the requirements for the Ph.D. degree. I am deeply indebted to Dr. E. M. Gifford, Jr., for suggesting this topic for my research and for his invaluable advice and guidance. I am also grateful to Drs. T. L. Rost and R. H. Falk for their help with various aspects of this study and to Sigma Xi for a Grant-in-Aid of Research which supported part of this work. In 1953 Buvat and Liard reported that the shoot tip of Equisetum arvense has a mitotically inactive apical cell and that it presents a typical example of a me'risteme d'attente subtended by an anneau initial. In a subsequent paper, Buvat and Roger-Liard (1954) described mitotic activity in the apical cell of the roots of E. arvense somewhat ambiguously, stating that the apical cell "est loin de proliferer en manniere continue" (p. 1258"is far from proliferating in a continuous manner") but also, "La 'cellule apicale' et ses voisines immediates se divisent sensiblement au meme rythme; il n'y a pas de territoire inactif comme dans les racines d'Allium ou de Triticum" (p. 1257-"The apical cell and its immediate neighbors divide in the same rhythm; there is no inactive region as in the roots of Allium or Triticum."). D'Amato and Avanzi (1965, p. 392), in presenting evidence for quiescence of the apical cell, quoted only the former statement. It may be noted here that the classical theory does not demand continuous activity of the apical cell. Indeed, Nageli and Leitgeb (1868) observed that in some cases, the cells in the first, second, third, and even fourth packets of derivatives (merophytes, using the term adopted by Gunning, Hughes, and Hardham, 1978) proximal to the apical cell divide even more actively than the apical cell itself. D'Amato and Avanzi's much cited contention that the apical cell is quiescent was based mainly on determinations of the DNA content of the apical cell as compared to that of nearby cells in mitosis and on labelling indices in plants fed with 3H-thymidine for periods up to 3 days in roots of Marsilea strigosa (D'Amato and Avanzi, 1965; Avanzi and D'Amato, 1970), Ceratopteris thalictroides, Polypodium aureum, and two species of Blechnum (Avanzi and D'Amato, 1967) and shoots of Equisetum