Frond Articulation in Species of Polypodiaceae and Davalliaceae

Abstract

Abscission, the natural detachment of a plant organ, is being studied by physiologists (e.g., Addicott, 1964; Rubinstein and Leopold, 1964; Jacobs and Kirk, 1966) after many years of interest on the part of morphologists. As new facts are discovered about growth-regulating substances, the physiological mechanisms governing senescence are becoming increasingly apparent. All such mechanisms must be correlated with anatomical characteristics of the abscission zone, thus necessitating a thorough knowledge of the cell types and patterns in this region of the plant. Miihldorf (1925) summarized the work done in the lower groups and also referred to the comprehensive coverage given to the higher plants: Monocotyledons (Bretfeld, 1880; Fouilloy, 1899) and dicotyledons (Tison, 1900; Lee, 1911). There are, however, several gaps in the anatomical literature concerning abscission, and one noticeable omission is a thorough description of the disarticulation of the rachis from the rhizome in ferns. In one early work (Basecke, 1908) three categories were described: (1) The frond falls before clear abscission lines are formed; (2) Disarticulation occurs along a line of cells present from the youngest stages; and (3) The frond disarticulates along a separation layer formed during maturity. Pfeiffer (1928, p. 163) examined other groups in addition to the ferns and established twelve classes of abscission based on various combinations of three characteristics of the separation layer. These included cell morphology, time of differentiation, and physiological condition of the cells after disarticulation. Most ferns are placed in class I of this system, in which the separation layer is formed by obtuse parenchyma, is evident as