New Evidence on the Organization of the Root Apex in Leptosporangiate Ferns

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Abstract
DNA cytophotometry and DNA labelling with 3H-thymidine were used to study the apical cell in lateral root primordia of Marsilea strigosa and in root apices of Marsilea strigosa, Blechnum brasiliense, B. gibbum, Polypodium aureum and Ceratopteris thalictroides. The initial cell of the lateral root in Marsilea makes a series of divisions which lead to a primordium with a tetrahedral apical cell. At different stages of development of the primordium, the apical cell becomes « quiescent »; that is, it divides rarely, if at all, whilst mitotic activity is retained by the mass of cells surrounding the apical. This situation is kept in the apex of adult roots in Marsilea and the other species investigated. Quiescence of the apical cell is due to a permanent, or long-lasting, block in the G2 phase without any interference with further DNA synthesis up to 8–16C intermediate DNA contents. From time to time, the apical cell enters mitosis with either 4C (diploid) or — more rarely — 8C (tetraploid) DNA content. The result is the formation of a cytochimeric (diplotetraploid) root apex. Quiescence of the root apical cell of cryptogams is discussed in comparison with the quiescent centre of Angiosperm roots and the view is advanced that the apical cell represents an « unicellular quiescent centre ». It is also suggested that quiescence may occur in the apical cell of the shoot apex of cryptogams.