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Sorus abscission from laminae of Nereocystis luetkeana (Mert.) post. and rup. Walker, David Cabot


The release of mature sori from the laminae of Nereocystis was first reported about the turn of the century. Although that report was confirmed repeatedly and the process has been called abscission, it has never been examined at any more than the gross morphological level. This work describes the process morphologically at both the light and electron microscope levels. This was done with the intention of discovering if, indeed, work with the vascular plants or if it was even comparable. It was hoped that some new insights might be gained into the reproductive mechanisms of Nereocystis as well. Assuming that soral margins were comparable to abscission zones made it essential to know the morphology of surrounding vegetative and included soral tissues during the process of sorus release. A gradual gradient of the developmental stages which produced the sorus is found in sections across young soral margins. Using the events of meristoderm cell periclinal division, paraphysis initial elongation and zoosporangial initial inception as markers the sorus margin can be divided into three regions. Periclinal division of the meristoderm cells begins the sorus margin and Region I. Paraphysis initial elongation indicates the boundary between Regions I and II. The appearence cf zoosporangial initials and sorus lumen demarks the border between Regions II and III. Region III ends at the sorus proper. Sorus release consists of two phases. Release begins with necrosis and eventual erosion of the paraphysis initials, zocsporangial initials and subtending cells at the borderline between Regions II and III. Phase two commences with the schizogencus dissolution of middle lamellae between underlying cortical cells. Beginning at both surfaces sorus release is completed with dissolution of the matrix of the central medulla. This localized weakening of sorus margin tissues and separation is influenced mechanically by water motion and is interpreted as a Laminariales morphological equivalent to vascular plant abscission. Observations on transition and expansion zones of young thalli and the process of mucilage duct inception in these led to a better understanding of the abscission process. Mucilage duct system inception involves the burial of meristematic cells accompanied by the schizogenous appearance of a duct lumen filled by the secretory initials. Confinement of this inception process to the meristoderm indicates that it is homologous to sorus inception, only in a different context and with different end products. This is supported by the discovery that zoosporangial initials begin in a secretory capacity filling schizogenous sorus lumen and that, lacking only divisions, paraphysis initials are meristematic. It is apparent that abscission is then a modification of the events of zoosporangial inception in a meristematic setting which leads to abscission by schizogeny. Several insights into the reproductive mechanisms of Nereocystis have also resulted. The mucilage duct system, through the inception process, maintains a relationship with the surface at its potential venting points and may assist in flushing spores from the sorus surface. Paraphyses play a primary role in the maintenance of the outer cell wall and appear to be regulated by the zoosporangia. Finally, an unanticipated role of the abscission process was found to be the releasing of the outer cell wall of the sorus at its perimeters to facilitate exuviation and ultimately spore release.

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