Vessel structure of Gnetum gnemon L. and G. montanum Mgf. was investigated by means of light microscopy (brightfield, phase contrast and Nomarski interference optics), and scanning and transmission electron microscopy. The gyres of the first order framework are compound, consisting of three elementary strands (subunits). Pits ranging from slit‐shaped to circular are formed either between the elementary strands of one gyre or between adjacent gyres. (Compound gyres are also figured for Alnus and lens‐shaped pits in annular‐helical elements are demonstrated for Juglans and Salix). In the two species of Gnetum studied, scalariform pits occur in addition to the well known circular pits. Scalariform perforation plates are present besides scalaroid, foraminate, and simple perforation plates as well as intermediates between the above types. These findings invalidate a common argument against the gnetalean origin of angiosperms, namely that pits and perforations of Gnetum are totally unlike those of primitive angiosperms and that therefore Gnetum must be ruled out as a potential ancestor of angiosperms. Variation in vessel structure of Gnetum is so great that it encompasses the typically circular pits of the coniferopsids as well as patterns of pits and perforation plates found in angiosperms. Some photographs of angiospermous taxa are interspersed with those of Gnetum to indicate the striking similarities. The determination of the patterns and shapes of pits and perforations is discussed in terms of four parameters: 1) the ratio of the width of the cell face to the distance between the gyres; 2) the types and distribution of the second order framework; 3) the stretching and distortion of gyres and/or second order framework; and in the case of perforations, 4) the pattern of wall and bar breakdown. Since the first parameter may change continuously, a continuum between circular pits or perforations and scalariform ones may occur. Such a continuum actually exists in Gnetum as well as in angiosperms. Patterns due to the other three of the above parameters are also formed in similar ways in both Gnetum and angiosperms. These similarities may, of course, be interpreted as the result of parallel evolution. However, when one considers the large number of angiospermous features of Gnetum, one is led to ask whether Gnetum may not have been close to the ancestral stock of all or at least some taxa of angiosperms.
JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 132.239. ABSTRACTVessel structure of Gnetum gnemon L. and G. montanum Mg. was investigated by means of light microscopy (brightfield, phase contrast and Nomarski interference optics), and scanning and transmission electron microscopy. The gyres of the first order framework are compound, consisting of three elementary strands (subunits). Pits ranging from slit-shaped to circular are formed either between the elementary strands of one gyre or between adjacent gyres. (Compound gyres are also figured forAlnus and lens-shaped pits in annular-helical elements are demonstrated for Juglans and Salix). In the two species of Gnetum studied, scalariform pits occur in addition to the well known circular pits. Scalariform perforation plates are present besides scalaroid, foraminate, and simple perforation plates as well as intermediates between the above types. These findings invalidate a common argument against the gnetalean origin of angiosperms, namely that pits and perforations of Gnetum are totally unlike those of primitive angiosperms and that therefore Gnetum must be ruled out as a potential ancestor of angiosperms. Variation in vessel structure of Gnetum is so great that it encompasses the typically circular pits of the coniferopsids as well as patterns of pits and perforation plates found in angiosperms. Some photographs of angiospermous taxa are interspersed with those of Gnetum to indicate the striking similarities. The determination of the patterns and shapes of pits and perforations is discussed in terms of four parameters: 1) the ratio of the width of the cell face to the distance between the gyres; 2) the types and distribution of the second order framework; 3) the stretching and distortion of gyres and/or second order framework; and in the case of perforations, 4) the pattern of wall and bar breakdown. Since the first parameter may change continuously, a continuum between circular pits or perforations and scalariform ones may occur. Such a continuum actually exists in Gnetum as well as in angiosperms. Patterns due to the other three of the above parameters are also formed in similar ways in both Gnetum and angiosperms. These similarities may, of course, be interpreted as the result of parallel evolution. However, when one considers the large number of angiospermous features of Gnetum, one is led to ask whether Gnetum may not have been close to the ancestral stock of all or at least some taxa of angiosperms.IT IS COMMONLY ACCEPTED that vessels of Gnetum lack scalariform pitting and are instead characterized by circular pitting and perforation plates derived from this type of pitting (e.g., Thompson). Furthermore, it is frequently emphasized that the ...
The concentrations of minerals were studied in decayed, discoloured and clear wood of Aspen using both light and electron microscopes. Kevex X-ray energy spectrometer system and atomic absorption spectrophotometer analyses revealed that the concentration of K, Na, Ca, Mn, Mg were highest in the decayed wood and lowest in clear wood. Discoloured wood showed an intermediate concentration. Only crystalline calcium was present in the decayed wood and phloem. Association of wood rotting fungi seems to be closely related to the accumulation of calcium crystals.
The sequential development of vessel elements in the primary and secondary xylem of Comptonia peregrina (L.) Coult. was studied. Scalariform, transitional, simple and scalaroid perforation plates were common in this species. The structural variation of these plates was interpreted on the basis of some developmental factors such as: I) width of the ceJl face and the distance between helical gyres; 2) type and distribution of secondary wall material in the form of strand, sheet or both; 3) localised and differential deposition of wall material and bar breakdown. These factors may work alone or in combination to determine the perforation plate structure.
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