A morphological and anatomical investigation of some of the thelypterid ferns

PhD ThesisFrom this survey of the available literature it can be seen that although the thelypterids have already been studied in varying degrees of detail, by many workers, the information presented has always been of a fragmentary nature. This has been a consequence either of the restricted geographical knowledge of the worker or of the restricted lines of research along which he has worked. Thus the present situation is that at one end of the scale the taxonomy and morphology of the thelypterids have been intensively studied (although in various strictly delimited geographical areas only) whilst at the other end of the scale anatomy and palynology have been studied in only a very small number of species which, moreover, have been chosen in a somewhat arbitrary manner. The present situation regarding the thelypterids is therefore that although there has been a large volume of work carried out, much of this has been of an incidental nature in works whose emphasis was not on the thelypterids alone. Because of this the available information is scattered throughout numerous works and an overall picture has not been constructed previously.Science Research Council

Gametophytes of homosporous ferns

This article does not have an abstract

Biogeographic origin, taxonomic status, and conservation biology of Asplenium monanthes L. in the southeastern United States

Asplenium monanthes L. is an apogamous fern with a wide tropical and subtropical distribution that extends into the temperate zone in the southeastern U.S. These populations are quite distant from the nearest neotropical populations, so the circumstances of their origin (i.e. as pre-Pleistocene relicts or by later long-distance dispersal) were investigated using spore and gametophyte morphology and starch gel electrophoresis. The southeastern U.S. populations of A. monanthes can be attributed to multiple long-distance dispersal events during the Pleistocene or Holocene. This recent origin and A. monanthes\u27 small population sizes account for southeastern U.S. A. monanthes\u27 lack of genetic diversity within and in many cases among populations. The source of the various founding spores cannot be conclusively determined, but evidence favors the Caribbean over Mexico. The southeastern U.S. populations of Asplenium monanthes do not warrant distinct taxonomic status. Analysis of genetic structure revealed great diversity in Mexico, so Mexico is suggested as a possible birthplace of allotriploid A. monanthes. The rarity and disjunct nature of the southeastern U.S. populations also prompted investigation into their ecology in the temperate zone and their prospects for continued survival. Asplenium monanthes survives in the southeastern U.S. by inhabiting shaded gorges, sinkholes, and cave entrances that moderate temperature extremes and maintain reliable moisture levels. The populations are very small, so they are subject to environmental and demographic stochasticity and are fixed genetically for relatively few multilocus genotypes. Several historical populations have disappeared in recent decades and several remaining populations appear to be declining as well. These declines may be a natural part of the species\u27 regional population dynamics or they may be cause for alarm. Regular long-term monitoring is required to determine whether the species is truly at risk of local extinction in the southeastern U.S. and if so, which factors are most responsible for this trend. No management intervention is recommended at this time

The desiccation tolerance through a unicellular and chlorophyllous fern spore

251 p.La presente tesis, por un lado, establece un protocolo universal para definir la tolerancia a la desecación en tejidos fotosintéticos y posteriormente se centra indagar las características ecofisiológicas que posibilitan esa estrategia empleando como objeto de estudio esporas clorofílicas de helechos. Estas esporas presentan un aparato fotosintético completamente funcional en el estado maduro. Mediante la experimentación basada en esporas clorofílicas se han podido definir algunos de los aspectos claves en la fisiología de la desecación. Así, se ha podido concluir que, en estas esporas, al igual que en diversos organismos tolerantes a la desecación, existe un gradiente de tolerancia desde aquellas incapaces de tolerar la perdida de agua (consideradas sensibles a la desecación) y aquellas que llegan a tolerar varios ciclos de perdida severa de agua. Esta mayor o menor tolerancia a la desecación se ha podido relacionar con procesos fisológicos específicos y así como con los requerimientos ecológicos de las especies. Uno de los grandes descubrimientos ha sido la observación del cambio en la composición lipídica dado en las membranas de los cloroplastos en paralelo a la pérdida de tolerancia durante la germinación. En el plazo de 72h se observó una regulación a la baja de aquellos galactolípidos encargados de la estabilidad de las membranas. Además, los galactolipidos encargados de una mayor fluidez de la membrana y de una mayor actividad fotoquímica sufrieron un considerable incremento cuando la tolerancia a la desecación se perdió. Por otra parte, la disipación térmica activada en esporas clorofílicas secas se observó por primera vez en esta tesis, relacionándose este mecanismo de fotoprotección positivamente con la tolerancia a la desecación. En tercer lugar, se constató que factores como la luz y la presencia de oxígeno son los mayores condicionantes para el mantenimiento de la viabilidad de esporas clorofílicas. Por último, se concluye que el uso de este tipo de esporas ofrece una oportunidad única para estudiar este raro fenómeno en tejidos fotosintéticos. Su carácter unicelular y simple facilita el estudio de los mecanismos fisiológicos y fisicoquímicos que permiten a una célula sobrevivir a la perdida de agua, sin que esta respuesta se vea interferida por otros tejidos más complejos