. Morphology of gymnosperms. Gymnosperms; Plant morphology. GNETALES 383 invested by the loose and undifferentiated tissue of the nucellus. A Httle later, the cells at the chalazal end of the sac become strikingly differentiated, becoming more and more compactly arranged, gradu- ally obliterating the intercellular spaces, and taking on the appearance of glandular cells (figs. 428, 429). This compact "pavement tissue" gradually extends deeper into the chalaza, and spreads laterally below, becoming fan-shaped in longitudinal section, but with so definite a contour that it was mistaken
. Morphology of gymnosperms. Gymnosperms; Plant morphology. GNETALES 383 invested by the loose and undifferentiated tissue of the nucellus. A Httle later, the cells at the chalazal end of the sac become strikingly differentiated, becoming more and more compactly arranged, gradu- ally obliterating the intercellular spaces, and taking on the appearance of glandular cells (figs. 428, 429). This compact "pavement tissue" gradually extends deeper into the chalaza, and spreads laterally below, becoming fan-shaped in longitudinal section, but with so definite a contour that it was mistaken by Lotsy (15) for a compact antipodal tissue within the embryo sac (fig. 419, I). After the fertiliza- tion stage has been reached within the sac, this pavement tissue begins to lose, its glan- dular appearance, and later it is destroyed en- tirely by the growing en- dosperm. A year later, the en- dosperm has destroyed all of the nucellar tissue except a very small amount at the tip. In destroying the nucellar beak, the central region of the endosperm ad- vances into the beak and then spreads laterally (fig. 430). In the meantime the peripheral region advances more slowly toward the beak, and as a consequence a ring of nucellar tissue is pinched between two growing masses of endosperm. The division of the megaspore mother cell has been observed only in Ephedra (3, 14, 18), in which a linear tetrad is produced. Appar- ently the outer daughter cell of the mother cell frequently does not divide, for the earlier accounts describe a row of "three ; A close series shows (18) that in the same species this outer cell may divide completely, may divide incompletely, or may not divide at. Fig. 430.—Gnetum Cnemon: tip of seed; small amount of tissue of nucellar beak not destroyed by endosperm; differentiation of central and peripheral regions of endosperm, the former having advanced into the center of the nucellar beak and spread laterally, resulting in crushing the
Size: 1836px × 1361px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpublisherchicagouniversityo
