Apical Organization and Leaf Ontogeny in Haplomitrium mnioides (Lindb.) Schust.

Abstract

In nature, the vegetative shoots of Haplomitrium mnioides (Lindb.) Schust. are prostrate and display a novel pattern of anisophylly in which the reduced row of leaves is dorsal rather than ventral, while the fertile shoots are erect and nearly isophyllous. In both cases, the leaves are typically entire and subrotund to broadly ovate. When grown on moistened vermiculite in environmental chambers, however, the vegetative shoots assume an erect, nearly isophyllous habit, and, along with the fertile shoots, produce bifid or incised leaves. Both normal prostrate and cultureinduced erect shoots were studied using serial sections and scanning electron microscopy (SEM) in order to determine if changes in apical organization are correlated with these changes in habit. A tetrahedral apical cell with nearly equal cutting faces was found to be responsible for growth in both types of shoots and the apical derivatives were found to divide as in the Metzgeriales so that but a single leaf initial is formed per merophyte. Thus, in both the normal entire and culture-induced divided leaves, initial growth is by a single depressed-ovoid apical cell, which in the bifid leaves eventually cuts itself in two. The lateral projections of the incised leaves each develop from a cell on the margin of an already expanded leaf. In anisophyllous shoots, cell divisions in the developing dorsal leaves are arrested at an earlier stage than in the lateral ones. In basic patterns of apical organization and leaf ontogeny, H. mnioides resembles H. hookeri (Smith) Nees and H. blumei (Nees) Schust., and all three appear in these characters to be more closely aligned with the Metzgeriales than with the Jungermanniales or Takakiophyta. Haplomitrium Nees has long been the subject of taxonomic and phylogenetic speculation because of its peculiar array of characteristics, both those unique to the genus and those which are superficially indicative of other taxa, particularly the Jungermanniales, the Metzgeriales, and Takakia Hatt. & Inoue. Although Haplomitrium superficially resembles the leafy hepatics, it has recently been postulated to be more closely related to the Metzgeriales because it is anacrogynous, forms similar preprophase spore exine precursors (Brown & Lemmon 1986), and has been reported to possess a pattern of apical derivative development and leaf ontogeny like members of the Metzgeriales (Leitgeb 1875; Goebel 1891; Campbell 1920; Schuster 1966a, b; Crandall-Stotler 1981). Apical organization and leaf ontogeny have been discussed by Gottsche (1843) and Leitgeb (1875) for H. hookeri (Smith) Nees, an isophyllous species, while Goebel (189 1) and D. H. Campbell (1920) did the same for the anisophyllous Calobryum blumei Nees (=H. blumei (Nees) Schust. [Schuster 1963]). Although E. O. Campbell (1959) did not outline the stages of leaf development, she did discuss the apical organization and alluded to leaf origin in the isophyllous Calobryum gibbsiae Steph. (=H. gibbsiae (Steph.) Schust. [Schuster 1963]). In each case, plants p ssessing only the entire leaves typical of Haplomitrium were studied. Culture-induced bifid and ncised leaves have been reported in H. mnioides (Lindb.) Schust. by Kitagawa (1984); however, their ontogeny has not been described. The discovery of these divided leaves, which are superficially indicative of the Jungermanniales, raises the question as to whether more than one developmental pattern exists for the leaves of Halomitrium. As pointed out by Renzaglia (1982), the merophyte division pattern which is responsible for leaf formation is fundamentally different in metzgerialian and jungermannialian taxa. If the dissected leaves of H. mnioides depart from the metzgerialian onitogenetic pattern, in which but a single leaf initial is formed, and instead, develop from two initials as do jungermannialian leaves, that Haplomitrium is intermediate between the Jungermanniales and the Metzgeriales would be suggested. Clarification of these ontogenetic patterns is obviously essential to determine the position that Haplomitrium occupies in liverwort phylogeny. To this end, this study was undertaken. MATERIALS AND METHODS Live H. mnioides plants were obtained from Dr. M. Mizutani, collected 10 December 1985, from Kyushu, Mi0007-2745/87/383-389$0.85/0 This content downloaded from 157.55.39.59 on Mon, 17 Oct 2016 04:42:53 UTC All use subject to http://about.jstor.org/terms 384 THE BRYOLOGIST [VOL. 90