Abstract
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The future of the seed is partly predetermined by events (flower formation, flowering, nutrient flow from mother plant, etc.) preceding fertilization and the formation of the gametophyte.
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The environmental conditions under which the seed matures affect its final physiological constitution. This faet has mostly been neglected by seed physiologists.
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It is not known how far the triantic nature of the diaspore (seed coat, pulp, etc., 2n of mother plant, embryon of δ +n of Φ, endosperm 2n of Φ +n of δ) affects seed development and germination.
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The integuments of the ovules of some species have stomata. It is not known if they are functional in gas exchange or are constitutional non-functioning relics.
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The causes of the growth-degeneration pattern of the nucellus are unknown.
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During the development of the megaspore mother cell into the mature embryo sac dramatic cellular ultrastructural changes take place. This probably signifies a “change of guards” during which the gametophyte is freed from part of the controls by the ultrastructural units of the mother plant, preparing the ground after fertilization for a new, genetically independent sporophyte.
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Upon closer examination, the seemingly simple processes of fertilization and embryogenesis, as described in textbooks, turn out to be very complex and full of problems. Is the role each male nucleus plays preordained or is it left to chance which male nucleus goes where? What causes the degeneration of the synergids and of the vegetative nucleus, and what protects the other two male nuclei from a similar fate? Which ultrastructural organelles are carried by the generative nuclei into their respective receptor cells and what is their role in them? Why do zygotes in some species develop after fertilization immediately into an embryo whereas in other species the zygote remains dormant for some time? What causes the polarity of the egg cell which, after fertilization, divides into one developmentally most active apical cell (giving rise to the embryo) and into another “lazy” basal cell which develops into the suspensor of “unknown function?”
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In the source-sink relationship between photosynthesizing organs and the maturing seed there is one point at which the photosynthates pass from symplast to apoplast to symplast. The mechanism involved is largely unknown as well as the effect which environmental conditions have on this transport.
Zusammenfassung
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Das Schicksal des Samens wird zum Teil durch Ereignisse bestimmt (Blütenbildung, Blühen, Nährstoffzufuhr von der Mutterpflanze u. s. w.), die der Befruchtung und der Bildung des Gametophyten vorausgehen.
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Die Umweltsbedingungne, unter denen der Samen reift, beeinflussen seine physiologische Konstitution. Die meisten Samenphysiologen haben diese Tatsache nicht genügend beachtet.
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3)
Es ist nicht bekannt, wie weit die triantische Natur der Diaspore (Samenschale, Perisperm etc., 2n der Mutterpflanze,n δ + n Φ des Embryos, 2n Φ +n δ des Endosperms) Samenentwicklung und Keimung beeinflussen.
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4)
Die Integumente der Samenanlagen haben bei manchen Pflanzen Spaltöffnungen. Es ist unbekannt, ob diese eine Rolle im Gasaustausch spielen oder ob sie nur konstitutionelle nichtfunktionelle Relikte sind. 145
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5)
Die Ursachen des anfänglichen Wachstums und der folgenden Degeneration de Nuzellus sind nicht bekannt.
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6)
Während der Entwicklung der Embryosackmutterzelle zum Embryosack findet eine fundamentale Änderung der zellulären Ultrastruktur statt. Die Bedeutung dieser Änderung liegt möglicherweise darin, dass sich durch sie der Gametophyt von der Kontrolle durch die ultrastrukturellen Organellen der Mutterpflanze befreit und somit die Basis für einen nach der Befruchtung genetisch unabhängigen Sporophyt gelegt wird.
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7)
Bei näherer Betrachtung zeigt sich, dass Befruchtung und Embryoentwicklung, die in den Lehrbüchern meist als unkomplizierte Vorgänge dargestellt sind, äusserst komplexe und problematische Prozesse sind. Ist die Rolle der beiden generativen Kerne prädestiniert oder ist es dem Zufall überlassen, wohin jeder der beiden Kerne wandert? Was ist die Ursache der Degeneration der Synergiden und des vegetativen Kerns, und was schützt die beiden generativen Kerne vor einem ähnlichen Los? Welche ultrastrukturellen Organellen wandern mit den generativen Kernen in ihre respektiven Rezeptorzellen, und welche Rolle spielen sie dort? Warum entwickeln sich in einigen Arten die Zygoten direkt nach der Befruchtung zu Embryonen während sie in anderen Arten für oft lange Zeiträume im Ruhezustand bleiben? Was verursacht die Polarität der Eizelle, welche sich nach der Befruchtung in eine entwicklungsmässig äusserst aktive apikale Zelle, aus der sich der Embryo entwickelt, teilt, und in eine basale “träge” Zelle, die sich zum Suspensor “unbekannter Funktion” entwickelt?
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An einem bestimmten Punkt der Leitungsbahn, durch die Assimilate der photosynthetisierenden Organe in die reifenden Samen befördert werden, passieren die Assimilate vom Symplast zum Apoplast zum Symplast. Sowohl Transportmechanismus als auch der Einfluss von Aussenbedingungen auf ihn sind im grossen Ganzen unbekannt.
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Evenari, M. Seed physiology: From ovule to maturing seed. Bot. Rev 50, 143–170 (1984). https://doi.org/10.1007/BF02861091
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DOI: https://doi.org/10.1007/BF02861091