On Phenotypic Expression, Morphogenetic Pattern and Synangium Evolution in Psilotum

Abstract

Both the typical form and the appendageless variant of Psilotum nudum produce terminal synangia at the ultimate tips of the aerial axes. One clone in particular of the typical appendaged form produced synangia entirely at the tips of the aerial branches, as in the appendageless variant, and also developed occasional lateral transitional entities on the upper aerial axis displaying appendagelike and axislike morphological qualities. A developmental comparison of synangium development at the ultimate tips of aerial branches and of unusually elongating and normal sized fertile‐appendages showed that the morphogenetic pattern of synangium development was similar. Anatomical and morphological evidence showed the synangium to be derived from terminal subdivisions or bifurcations of the apical meristem of each structure studied. This supports the phyletic concept that the synangium of the Psilotaceae is basically terminal to an axis or an axis homologue, and that it probably evolved from terminal bifurcative branching. Occasional multiple sporangium lobes may be formed on a P. nudum synangium which may not be represented by vascular bundles. Two hypothetical phyletic models of synangium evolution are proposed that could be used to explain this phenomenon and which should be tested by further evidence. Typical and appendageless P. nudum were compared in their morphogenetic pattern developed at the upper axis vegetative apical meristem, and a reconciliation was made between the structuring of the apparently disparate forms, which involved the presence or absence of serial ordering in apical derivatives. It is suggested that this could serve as a model for appendage evolution in the family Psilotaceae.

The science of plant morphology: definition, history, and role in modern biology

“…Psilopsida-Because of the seemingly simple, inconspicuous nature of its leaves, the isotomous nature of its aerial shoot branching and lack of roots, and its isomorphic alternation of generations, Psilotum has been seen by many paleobotanists and plant morphologists as a living ''rhyniophyte'' (Rouffa, 1971(Rouffa, , 1978Stewart and Rothwell, 1993). Certainly, at a superficial level Psilotum would seem to simulate those ancient fossil plants (Fig.…”

Architecture of the sperm cell of Psilotum

“…Morphological evaluation of this hypothesis is limited to the inclusion of general structural features of Psilotaceae in more comprehensive phylogenetic analyses (Stevenson and Loconte, 1996;Kenrick and Crane, 1997;Wolf et al, 1998;Rothwell, 1999). Since the 1970s and the works of Bierhorst (1971Bierhorst ( , 1977, Rouffa (1978) and Siegert (1973), few new developmental, anatomical, or ultrastructural studies have been undertaken that shed light on the position of this phylogenet- DEDICATION: We dedicate this manuscript to the memory of two inspirational botanists of the twentieth century: Warren Herb Wagner and David W. Bierhorst. May the interest they excited in Psilotum and basal pteridophytes be kept alive for the millennia to come.…”

Étude au microscope électronique à balayage de la bulbille de Huperzia selago (L.) Bernh. (Lycopodium) : méristème apical, initiation foliaire et phyllotaxie

“…Cette technique a surtout ttC appliquee chez des Phantrogames pour mettre en tvidence la transformation de l'apex lors du passage de 1'Ctat vCgCtatif iI 1'Ctat floral (Einert et al 1970;Bartholomew 1977) et les effets de ces changements sur la phyllotaxie (Meicenheimer 1979); chez les PtCridophytes, seul Rouffa (1978), iI notre connaissance, l'a utiliste pour dCceler les synanges au sommet des axes atriens de Psilotum.…”