. Morphology of gymnosperms. Gymnosperms; Plant morphology. 33° MORPHOLOGY OF GYMNOSPERMS The archegonium initials become recognizable at various stages in the development of the gametophyte. In Taxus and Torreya they appear as soon as the sac is full of tissue; in fact, any earlier differentiation would occur in the stage of free nuclei and would result in the selection of eggs and the elimination of archegonia. This tendency to differentiate the female sexual organ earlier and earlier in the history of the gametophyte has been referred to under Pinaceae (p. 263); and in this feature Taxus an
. Morphology of gymnosperms. Gymnosperms; Plant morphology. 33° MORPHOLOGY OF GYMNOSPERMS The archegonium initials become recognizable at various stages in the development of the gametophyte. In Taxus and Torreya they appear as soon as the sac is full of tissue; in fact, any earlier differentiation would occur in the stage of free nuclei and would result in the selection of eggs and the elimination of archegonia. This tendency to differentiate the female sexual organ earlier and earlier in the history of the gametophyte has been referred to under Pinaceae (p. 263); and in this feature Taxus and Torreya stand in an extreme position. In Cephalotaxus (124), on the other hand, the archegonium initials do not appear until the endosperm is well developed; and in Podocarpus the gameto- phyte is two-thirds to three-fourths its full size when fertilization occurs. In some cases, therefore, the principal growth of the endosperm takes place after fertilization, and in other cases before fertilization. The development of the archegonium is exactly as described for the Pinaceae (p. 263), and no archegohial complex is formed, each archegonium having its separate chamber and jacket (in case there is one). In Phyllodadus occasionally two archegonia occur in a common jacket (174). The arche- gonial jacket, investing the central cell and later the egg, is not developed so constantly or so conspicuously as among the Pinaceae. In Phyllodadus (144) it is conspicuous,, consisting of a layer of cells with large nuclei or several nuclei; in Cephalotaxus (124) it is less con- spicuous, frequently being interrupted by ordinary cells; while in Torreya (loi) it is not evident until after fertilization and is then weakly organized. The condition of this jacket among Tax- aceae is not known well enough to justify a general statement, but among the taxads, at least, it gives the impression of a vanish- ing structure, which finds its parallel in the disappearance of the. Fig. 381.—Taxus bac- ca
Size: 792px × 3156px
Photo credit: © Central Historic Books / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpublisherchicagouniversityo
