Marta A. Morbelli

Megaspore wall in Lycophyta-ultrastructure and function

Abstract New contributions on megaspore walls in the genus Selaginella are presented. The study was carried out with electron microscopy (SEM and TEM), light microscopy including phase and differential interference contrast, fluorescence and confocal microscopy. The observations were based on mature material from herbarium specimens and on fresh material in different stages of development. The results refer to: 1. (1) Units: They are coiled rods. Two types have been recognized, one with a central axis and the other without an axis. 2. (2) Patterns of ultrastructure: Two patterns have been distinguished, ordered and slack, according to the type of unit with which they are built. 3. (3) Gaps: They are located within the wall between units, in places where some features change and in the middle and inner parts of the exospore. 4. (4) Exospore ultrastructure in young stages: Two layers have been recognized. Both of them are built by coiled rods. 5. (5) Connections between the tapetum and the plasma membrane of the growing megaspore: During megaspore development there are highly organized structures called wicks. They run across the exospore between units. 6. (6) Mineral deposits within the wall: Deposits of sulphur and potassium in addition to silicon have been detected by X-ray microanalysis.

Megaspore wall growth inSelaginella (Lycopodiatae)

Different stages of megaspore and megasporangial development inSelaginella argentea (Wallich)Spring,S. bigelowiiUnerw., andS. kraussiana (Kze.)A. Br. have been seen and studied. Megaspore wall units give positive reactions for polysaccharides and protein in young megaspores, and become the thick and resistant wall typical of the genus only later.—Units forming the exospore and the spaces between units enlarge from widths of 5–10nm early during development up to over 200 nm at pregermination stages. The spaces enlarge first. Initially they are circular and mostly about 70 nm in diameter. Later, spaces toward the inner part of the exospore enlarge more than those near the outer surface. During pregermination, wall spaces range in size from 4 to 50 times the width of units with the larger spaces located near the inner surface. As a result the exospore would be under tension to spring outward during germination when the laesurae are lysed.—A gap in the exospore, shaped like a half-moon in polar sections, forms in equatorial and distal portions of the spore. This gap becomes enormous, three times the volume of the central space plus the mesospore, and is filled with lipids and other nutrients. Late in development, during the period of tapetal cell degeneration, the gap contents are moved into the central space and the gap is closed.—Late in development the mesospore is degraded. Its products, along with gap contents, seem to be added to the contents of the central cavity and appear as reserve storage globules. A primary wall-like endospore is formed during this period, at the inner surface of the exospore. During germination this endospore develops further at its inner surface.—Changes in the size and shape of megasporangia occur independently of the size of megaspores.

MEGASPORE DEVELOPMENT IN SELAGINELLA THE GAP AND THE MESOSPORE

A gap in the exospore and the presence of a mesospore is found to be a normal part of development in the ten species ofSelaginella we have studied. The early spore wall consists of an exospore and a mesospore forming successively on the plasma membrane of the megaspore protoplast when it is 10–15 µm in diameter. Enlargement of the exospore and mesospore creates a central space, the lumen of the megaspore, around the megaspore protoplast. After that there is a vast enlargement of the exospore and a relatively small enlargement of the mesospore. The exospore splits close to its contact with the mesospore forming a gap over equatorial and distal regions. The gap becomes greatly expanded and becomes filled with lipids, PAS-positive carbohydrates, proteins and is crossed by wicks. Experiments with solutions of different osmolality on fresh megaspores show that the exospore and mesospore are not osmotic barriers. The mesospore appears not to be resistant to acetolysis at the many stages tested but exospore is resistant. Thus the mesospore size and shape is retained by the inner exospore that enveloped the mesospore. At maturation the mesospore undergoes lysis and absorption. At the beginning of germination stages an endospore forms at the inner part of the exospore. This inner part of the exospore, that adhered to and enveloped the mesospore, becomes pressed near to the bulk of the exospore. Until pregermination stages the megaspore protoplast is small (10–20 µm in diameter).

Morphology and ultrastructure of megaspores and microspores of Isoetes savatieri Franchet (Lycophyta)

Abstract The general morphology, organization, structure and ornamentation of the sporoderm of megaspores and microspores of Isoetes savatieri Franchet have been studied with a stereoscopic microscope, light microscopy, transmission and scanning electron microscopy. The megaspores are trilete, 420–580 μm in equatorial diameter, subtriangular to globose in polar view with an evident equatorial flange (zone). The ornamentation is rugulate. A siliceous cover with a three-dimensional network overlies an exospore sporopolleninous apparently laminar with an equatorial–distal separation of laminae. Each exospore lamina is fused to those of other planes delimiting lacunae. The lacunae located in the outer part of the exospore are filled with silica. The endospore is thick, its structure is fibrillar. In section it is apparently composed of two zones. The microspores are monolete, 35–40 μm long and 20–25 μm wide and elliptic in polar view. A supra-laesural expansion is present. The sculpture is microechinate. A contrasted perispore composed of thin joined threads that form a lacunose structure can be distinguished in the microspore sporoderm. Two parts of the underlying exospore structurally different are evident: a laminar outer part and an inner part, mostly compact. An equatorial–distal separation between both parts of the exospore is evident. The endospore is fibrillar. The ultrastructural similarity of the spores of I. savatieri with those previously studied in Isoetes can be regarded as indicative of the homogeneity of this group of living lycopsids.

The spores of Pteris cretica complex (Pteridaceae-Pteridophyta) in America.

Abstract Spore morphology in the Pteris cretica complex, including Pteris brasiliensis, P. ciliaris, P. cretica, P. denticulata var. denticulata, P. denticulata var. tristicula, P. ensiformis, P. multifida and P. mutilata was examined using light and scanning electron microscope. The spores are trilete, triangular to circular in shape, with an equatorial thickening (= cingulum). Equatorial diameter ranges from 29 to 69 μm, and polar diameter from 18 to 53 μm. Exospore is proximally verrucate and distally rugate, with the exception of Pteris ensiformis, which has cones on both polar faces. Perispore is generally less than 1 μm thick, apparently single-layered in section, and translucent under light microscope. Spheroids were frequently observed on the surfaces of both perispore and exospore. Hyaline spores lacking cingulum were found in all specimens. Pteris brasiliensis, P. cretica and P. denticulata exhibit pronounced polymorphism and, in addition to trilete spores these taxa also produce atypical spores such as tetralete, monolete and intermediate types.

Palynological analysis of Cheilanthes Species (Adiantaceae-Pteridophyta) of Northwestern Argentina

Abstract The spores of seven Cheilanthes Sw. species that grow in the Northwest Region of Argentina were studied with LM, SEM and TEM. They were: Cheilanthes marginala H. B. K., C. micropteris Sw., C. myriophylla Desv., C. notholaenoides (Desv.) Weath., C. pilosa Gold., C. poeppigiana Mett. ex Kuhn, and C. pruinata Kaulf. C. volcanensis de la Sota, mentioned for the region, was not included because all the collected material was sterile or immature. The equatorial diameter is not useful in distinguishing species because of the interspecific variability of such species as C. micropteris, C. poeppigiana and C. pruinata. In other cases interspecific variation overlaps. All the species analysed here have one basic pattern of structure: a homogeneous, smooth exospore and a sculptured perispore. The perispore has two layers: an innermost one, granular, compact and an outermost one, reticulate with or without an outer lamella. The outermost layer constitutes the greatest part of the perispore. Five sculpture gro...

Spore Morphology of the Polypodiaceae from Northwestern Argentina

Abstract The spores of the following genera of Polypodiaceae growing in northwest Argentina were analyzed: Campyloneurum, Microgramma, Pecluma, Phlebodium, Pleopeltis and Polypodium. The study involved analyses of herbarium material using light microscopy and scanning electron microscopy. The spores are monolete, 40–90 μm in major equatorial diameter, eliptic to oblong in polar view and plane to concave-convex in equatorial view. The exospore ranges from 2–5 μm thick, is apparently double-layered, with a verrucate or tuberculate surface that is usually perforated. The perispore ranges from 0.3–1 μm thick, is apparently single-layered, attached to the exospore, perforated, and generally smooth or in some cases micro-ornamented. Most of the taxa analyzed have globules on the surface. These are single or associated in masses and irregularly distributed. Characteristics such as size, shape and exospore and perispore sculpture allow us to differentiate among some of the genera as well as recognize species groups. Microgramma, Campyloneurum, Pecluma, Pleopeltis and Polypodium have verrucate spores whereas those of Phlebodium are tuberculate.

Stages in development of Selaginella diffusa megaspores.

In mature megaspores of Selaginella diffusa (C. Presl) Spring the units of the exospore are ordered and become unordered toward the outer and inner surfaces. The exospore surface is coated with silica at maturity. The insertion of the future gap begins in early stages with formation of many minigaps within the inner part of the exospore distally. The mesospore, like the exospore, is resistant to the acetolysis reaction and can, thus, provisionally be considered to consist of sporopollenin. Unit structures within the outer part of the mesospore are unordered, but become ordered in the middle and inner parts. The inner surface of the mesospore appears verrucate. In maturing megaspores, the mesospore is mostly disintegrated and the inner exospore, which encapsulated the mesospore, remains as a somewhat isolated structure, and is again near the outer exospore. There are connecting strands across the gap between the inner surface of the outer exospore and the surface of the inner exospore. There are also spheres on the outer surface of the inner exospore.

Application of confocal microscopy to exospore ultrastructure in megaspores of Selaginella convoluta and S-marginata

Information pertaining to the arrangement of the rodshaped units that form the exospore ofSelaginella convoluta (Walk. Arn.) Spring andS. marginata (Humb. & Bonpl.) Spring megaspores was obtalned using both a confocal laser scanning microscope and a transmission electron microscope. Units are helically coiled, as we interpret them. The highest levels of fluorescence with confocal microscopy were in the places where the coiled exospore units that contain the fluorochrome dye overlap. These sites of overlap occurred in a close packed (hexagonal or pentagonal) arrangement. A more-or-less circular central nonfluorescent area was contiguous between overlapping exospore units. We conclude that the space between exospore units is a continuous channel (conduit). Each conduit is embraced by five or six helical-units that interdigitate.

Microspore Morphology of Isoetes Species (Lycophyta) from Southern South America

Microspores of the 24 species ofIsoetes that grow in southern South America were analyzed under a light microscope and scanning electron microscope. The microspores are monolete, elliptic in polar view, 30–40 μm long, and 20–25 μm wide. A background with various characteristics is seen on each surface. A supra-laesural expansion is present. The perispore is ornamented and has a perforated background. In section, it has a lacunose structure. The exospore is smooth, and it has a compact structure in section. The studied species could be divided into three groups by their perispore ornamentation: equinate, rugulate, and tuberculate. Microspore size was positively correlated with increasing ploidy level, and larger microspores were associated with terrestrial habitats. A convergence in ornamentation was found between spores produced by the studied species and those that grow in regions outside of the area under study.ResumenSe analizaron con microscopio óptico y electrónico de barrido las microsporas de veinticuatro especies deIsoetes que crecen en Sud América Austral. Las microsporas son monoletes, elípticas en vista polar, de 30–40 μm de longitud y 20–25 μm de ancho. En superficie se observa una base con diversas características. Una expansión sobre la lesura está presente. El perisporio está ornamentado y tiene una base perforada. En sección, tiene una estructura lacunosa. El exosporio es liso y en sección tiene estructura compacta. Las especies estudiadas se pueden reunir en tres grupos de acuerdo a la ornamentación del perisporio, estos son: equinado, rugulado y tuberculado. Se ha observado que existe una correlación positiva entre el tamaño de las microsporas y el incremento en el nivel de ploidía y el hábitat. Se observó una convergencia en la ornamentación entre las esporas producidas por las especies estudiadas y las producidas por otras especies que crecen en regiones fuera del área de estudio. Palabras clave: Sud América,Isoetes, microsporas, morfología, escultura