382 



MORPHOLOGY OF GYMNOSPERMS 



The development of the sporogenous tissue in the ovule of Gnetum 

 has been described by Steasburger (3) for Gnetum Gnemon. The 

 archesporium consists of a group of hypodermal cells which cut off 

 wall cells by periclinal divisions, and the wall cells, together with the 

 overlying epidermis, give rise to the usual thick mass of sterile tissue 

 above the sporogenous region (fig. 419, F). Before pollination the 



.on ' 





r«>., 



428 



429 







Figs. 428, 429. — Gnetum Gnemon: fig. 428, diagram of longitudinal section of 

 ovule, showing position of embryo sac and the "pavement tissue" beneath; XS4; 

 fig. 429, the embryo sac and pavement issue more highly magnified; X300. — After 

 Coulter (25). 



cells at the tip of the nucellus become disorganized, resulting in an 

 indefinite and shallow pollen chamber. The several primary sporo- 

 genous cells are said to divide in the formation of mother cells, but 

 this would be a very unusual occurrence for a gymnosperm. 



A remarkable differentiation of nutritive tissue occurs in the 

 chalazal region of the ovule of Gnetum Gnemon (25) during the early 

 stages of the embryo sac. At the eight-nucleate stage of the sac it is 



