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 megaspores." 

 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 nucellar tissue 

 against the encroaching peripheral region of endo- 

 sperm; sections of two suspensors shown; X40. — 

 After Coulter (25). 



