THE FERTILE SPIKE OF OPHIOGLOSSUM PETIOLATUM. II. CONTROL OF SPIKE ELONGATION AND A STUDY OF ABORTED SPIKES

The peduncle subtending the sporangial area of the fertile spike of Ophioglossum petiolatum elongates considerably bringing the sporangial area above the sterile segment of the frond. Surgical removal of the sterile segment of the frond has no effect on subsequent peduncle elongation, nor does removal of the sterile tip or any fraction of the sporangial area, providing a few basal sporangia are left intact. However, excision of the entire sporangial area results in an immediate cessation in peduncle elongation. Replacing the excised sporangial area with various growth substances indicates that, of those investigated, only auxins are able to induce peduncle elongation, but to a lesser degree than if the sporangia are left intact. Auxins stimulate peduncle cells to elongate without maintaining the mitotic activity of the intercalary meristem. An investigation of 19 rhizomes shows that three types of fronds are normally produced by this species: sterile fronds, fronds bearing a normal fertile spike, and fronds with an aborted spike. Aborted spikes lack sporangia and an intercalary meristem and fail to elongate. ALTHOUGH many of the anatomical features of the sporophyte of Ophioglossum have been investigated rather extensively in the last and this century (Bower, 1895, 1908; Campbell, 1911), few experimental studies have been designed to elucidate the physiological basis of various growth phenomena in this or other genera of the Ophioglossales. To date, most of the morphogenetic studies on ferns have been confined to members of the Leptosporangiatae (Wardlaw, 1952; Cutter, 1956, 1957; Steeves and Sussex, 1957). In a previous paper (Peterson and Cutter, 1969) the mechanism of elongation of the peduncle of the fertile spike of 0. petiolatum was described. Elongation occurs predominantly by the activity of an intercalary meristem located in the most distal portion of the peduncle subtending the sporangial area. This intercalary meristem is similar in position and function to that found in many angiosperm flower scapes and peduncles (Miyake, 1904; Soding, 1926; Schmalfuss, 1930; Kaldewey, 1957; Sachs, 1961, 1968) and to that of the gynophore of the peanut plant (Jacobs, 1947, 1951). It is the purpose of this paper to examine some of the factors involved in the control of peduncle elongation in 0. petiolatum and to consider briefly the occurrence and possible reasons for the development of aborted fertile spikes in this species. MATERIALS ANE METHODS-Plant materialMaterials used in the present study were obI Received for publication 10 July 1968. A portion of a thesis submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree at the University of California, Davis. 2 Present address: Department of Botany, University of Guelph, Guelph, Ontario, Canada. tained from plants that had been grown in the botany greenhouse at Davis. All plants were placed under bell-jars on a window ledge of the laboratory during the course of experiments. Surgical experiments-To determine the effect of removal of various portions of the frond on peduncle elongation, surgical experiments were conducted. These involved: (1) the excision of the sterile tip of parenchyma and vascular tissue from the sporangial area of fertile spikes, (2) the excision of varying amounts of the sporangial area, including the total sporangial area in some experiments, and (3) the removal of the sterile segment of the frond of those fronds with developing fertile spikes. All cut surfaces were smeared with lanolin paste to prevent desiccation of tissues. Intact fronds served as the controls in all experiments. Surgical and growth regulator experiments-In this series of experiments the entire sporangial area of young fertile spikes was excised and one of a number of growth substances added to the cut surface. The growth substances used were: kinetin, 6-benzylamino purine (BA), gibberellic acid (GA), 3-indoleacetic acid (IAA), naphthaleneacetic acid (NAA), and combinations of IAA + GA, NAA + GA, NAA + kinetin over a wide range of concentrations. In addition, sucrose (8 %) was added to the cut surface of some spikes and combinations of IAA + sucrose or IAA, GA and sucrose to others. All of these substances were incorporated into 1 % bacto-agar and the mixture placed in small plastic capsules (Beem) and kept in the refrigerator until needed. Plain agar was used as the control. During any