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Embryologie der Gymnospermen

01 Jan 1933-
About: The article was published on 1933-01-01 and is currently open access. It has received 21 citations till now.
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Journal ArticleDOI
TL;DR: As the embryo continues to develop, cells differentiate into the cotyledons and the apical meristems of root and stem, and in the free-nuclear stage the cells of the spongy tissue of the nucellus surrounding the megagametophyte may be multinucleate, with extremely large polyploid nuclei.
Abstract: 1. At the time of shedding, the pollen grain of Ginkgo usually contains four cells: the first prothallial, second prothallial, generative, and tube cells. After germination of the pollen grain the first prothallial cell aborts, while the generative cell divides, forming the body and stalk cells. During a late stage of development of the body cell, two blepharoplasts are formed, and the nucleus changes from a spherical to a lens-shaped structure. While the nucleus is changing shape, two vacuole-like structures appear close beneath the blepharoplasts, and a conspicuous granule becomes visible beside the nucleus. 2. Chromatin in the nucleus of the body cell is diffuse but condenses into chromosomes during division. Each spermatozoid cell includes one of the blepharoplasts, which changes from a rounded to a crescent-shaped structure and then forms the cilial band. The vacuole-like structure separates from the blepharoplast and moves to the side opposite the cilial tail. Later it becomes impregnated with chrom...

58 citations

Journal ArticleDOI
TL;DR: The suite of characters that define the seed habit is considered, and the probable selective pressures that produced each character are discussed, with a major conclusion that most characters are a direct consequence of the origin of heterospory and of natural selection for propagules with larger food reserves.
Abstract: The evolution of the seed is one of the major events in the history of land plants. In this paper, we consider the suite of characters that define the seed habit, and discuss the probable selective pressures that produced each character. Our major conclusion is that most characters are a direct consequence of the origin of heterospory and of natural selection for propagules with larger food reserves. Seeds are traditionally defined by the possession of integuments. However, some heterosporous pteridophytes possess integument-like structures. Therefore, integuments cannot explain the evolutionary success of seed plants. Rather, we believe that the decisive character in this success is related to pollination. Seed plants differ from other heterosporous lineages in the capture of microspores before dispersal of the 'megaspore'. Modern gymnosperms all possess mechanisms whereby the maternal sporophyte withholds resources from potential propagules that have not been pollinated and/or fertilized. This represents an increase in efficiency over Pteridophytic reproduction. Wind-pollination means the propagule is vulnerable to pathogens that mimic pollen, and pathogen pressures may have contributed to some seed characters.

52 citations

Journal ArticleDOI
TL;DR: Analysis of the morphological nature of the gymnospermous male gametophyte has been based on evidence from fossil gymnosperms, on a comparative survey of living vascular plants, and on ontogeny within the various families of the Gymnosperms.
Abstract: Summary Analysis of the morphological nature of the gymnospermous male gametophyte has been based on evidence from fossil gymnosperms, on a comparative survey of living vascular plants, and on ontogeny within the various families of the gymnosperms. Although fossil coniferous and taxad pollen is unknown, the fossil pollen of other gymnosperms is rather uniform in the presence of a layer of parietal cells (presumably an antheridial jacket) which surrounds a central, probably spermatogenous region. The same organization occurs in endosporal gametophytes of lower Tracheophyta of the present day. Reduction in the number of vegetative prothallial cells appears to have occurred early in gymnosperm evolution, so that the male gametophyte may be regarded essentially as a reduced antheridium. Between the fossil gymnosperms and the present forms there has occurred a reduction of the antheridial jacket and the corresponding production of a pollen tube. For various reasons, the balance of probability is in favour of a homology between the antheridial jacket of Palaeozoic gymnosperms and the tube cell of modern gymnosperms. In the lower tracheophytes and in Cordaimthus and the Recent gymnosperms, a polar organization of the endosporal male gametophyte is characteristic. On several grounds, it is shown that in this structure the presence of an antheridial stalk is precluded, and that therefore the use of the term ‘stalk cell’ is greatly in error. The cell which is often called ‘stalk cell’ is potentially–sometimes actually–spermatogenous and is sister to the virile spermatogenous cell. It is frequently sterile, however, and is therefore recognized here as the ‘sterile cell’. In the evolution of the Coniferales and the Taxales (as well as in the lower Tracheophyta) disappearance of the prothallial cells from the male gametophyte is a characteristic trend. Prothallial cells are lacking in the Taxodiaceae, Cupressaceae, Cephalotaxaceae and Taxaceae. (However, in two families, Araucariaceae and Podocarpaceae, there has developed a secondary proliferation of the prothallial cells). Recognition of the same trend in the Chlamydospermophyta and consideration of details of ontogeny indicate that prothallial cells must be absent from the gametophytes of Welevitschia and Gnetum. In these, the embryonal cell functions as an antheridial initial and produces a tube and a generative cell. The generative cell then divides into spermatogenous and sterile cells. Later, two male gametes result from division of the spermatogenous cell. Based on the morphological analyses indicated above, a terminology has been proposed which attempts to recognize morphological identities but does accept non-committal terms of common usage, as long as they do not violate the morphological concepts. A representative synonymy is given, and some examples of the use of the suggested terminology are shown. Details of the ontogeny of the male gametophyte in the conifer and taxad families have been reviewed. Although many genera are still completely unknown, it appears correct to state that on the whole the development of the male gametophyte within a family is quite uniform. The Pinaceae are characterized by the production of two senescent primary prothallial cells from the embryonal cell of the spore. The latter then serves an antheridial initial, forming a peripheral tube cell and an inner generative cell. All the preceding divisions are periclinal, and the generative cell also divides periclinally into an inner sterile cell and an outer spermatogenous cell. Eventually the latter divides to form two male gametes. In the Taxodiaceae, Cupressaceae, Cephalotaxaceae and Taxaceae no prothallial cells are formed, but the embryonal cell functions directly as an antheridial initial. Differences among these four families occur mainly in the relative sizes (and functional state) of the male gametes, the time of the occurrence of the various gametophytic divisions, and in the number of male gametes formed (which may exceed two in the Cupressaceae). In the Araucariaceae the two primary prothallial cells are not senescent but undergo an active period of cell division. Also the division of the generative cell is anticlinal rather than periclinal. Otherwise the sequence of ontogeny is similar to that of the Pinaceae. The Podocarpaceae resemble the Araucariaceae in the anticlinal division of the generative cell and usually the secondary proliferation of the primary prothallial cells. However, this proliferation is limited, and one or both cells may not divide. In one genus of the Podocarpaceae, no primary prothallial cells are produced. This article owes its inspiration to the period of the author's association with the late John T. Buchholz, Professor of Botany at the University of Illinois. It is a privilege to dedicate such a study to his memory. Professor O. H. Selling of the Swedish Museum of Natural History graciously supplied the print of PI. 1, fig. A, and Dr Walter Tulecke of the Boyce Thompson Institute kindly allowed his negative photograph to be reproduced as Pl. 1, fig. B. Dr Rudolf Florin has given both his personal permission and that of the Swedish Botanical Society to copy Text-fig. 1. Permission to copy figures from C. J. Chamberlain's Gymnosperms: structure and evolution and from the Botanical Gazette was given by the publisher, the University of Chicago Press. Many thanks are due to these individuals and organizations and to G. Fischer Verlag, Jena, J. Heslop-Harrison, editor, Annals of Botany, and P. Maheshwari, editor, Phytomorphobgy, who have allowed figures to be copied for the illustrations here.

49 citations

Journal ArticleDOI
01 Mar 1959-Planta
TL;DR: In this article, bestaubungstropfen vonTaxus baccata wird auch nach volliger Hemmung der Atmung abgeschieden; es handelt sich also dabei nicht um einziger Zucker Saccharose; auch hier ist die Konzentration der Aminosauren (8 frei und 12 in einem oder mehreren Peptiden) and des Phosphats hoch.
Abstract: 1. Der Bestaubungstropfen vonTaxus baccata enthalt an Zuckern Saccharose, Glucose und Fructose; die Konzentration der Aminosauren (10 frei und 6 in einem oder mehreren Peptiden gebunden) und des Phosphats ist relativ hoch (etwa in der Grosenordnung der bisher analysierten Siebrohrensafte). Apfelsaure und Citronensaure sind nachweisbar. 2. Im Bestaubungstropfen vonEphedra helvetica findet sich als einziger Zucker Saccharose; auch hier ist die Konzentration der Aminosauren (8 frei und 12 in einem oder mehreren Peptiden) und des Phosphats hoch. 3. Der Bestaubungstropfen vonTaxus baccata wird auch nach volliger Hemmung der Atmung abgeschieden; es handelt sich also dabei nicht um eine aktive Sekretion. Eine Beteiligung der Mikropyle an der Abscheidung konnte ausgeschlossen werden. 4. Im wasserdampfgesattigten Raum zeigen auch Pinaceen (Picea excelsa undLarix leptolepis) einen Tropfenaustritt aus der Mikropyle; ob dieser auch unter naturlichen Bedingungen eine Rolle spielt, bleibt zu klaren.

48 citations

Journal ArticleDOI

44 citations

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