236 



MORPHOLOGY 



trating the overlying nucellar tissue ; while in Gnetum it enters the 

 embryo sac and encounters the free egg nuclei (fig. 531). 



Embryo. — The development of the embryo of Gnetales shows a 

 remarkable modification of the usual gymnosperm method, and varies 



in accordance with the structure 





of the gametophyte in each genus. 

 In aU cases the embryo has two 

 cotyledons. 



Ephedra. — ■ In Ephedra there 

 is free nuclear division within 

 the egg until eight free nuclei 

 appear, and then these nuclei 

 enter into the organization of 

 eight free cells (the proembryonal 

 cells, fig. 534). Two or more of 

 these independent proembryonal 

 cells may function as follows : 

 the nucleus divides ; the cell 

 develops a prolongation hke a 

 pollen tube (fig. 535), which 

 penetrates the surrounding endo- 

 sperm, and into its tip one of the 

 nuclei passes. Later the tip of 

 this tube, containing the nucleus, 

 is cut off by a wall (fig. 536), and 

 from this cell the embryo is de- 

 veloped (fig. 537). 



Tumboa. — In Tumboa the 



Fig. 534. -Embryo of £Me<fra.- three of ^^^.^^ ^^^j^^^ .^ ^^^ 



the eight free proembryonal cells. — After *■ 



Land. thallial tube) is used in the for- 



mation of a free and independent 

 cell, which then behaves as do the independent proembryonal cells of 

 Ephedra. It should be noted that in this case the stage of free nuclear 

 di\asion in embryo formation has disappeared, and the first division of 

 the fertihzed egg is accompanied by wall formation, which is an 

 angio.sperm condition. 



Gnetum. — In Gnetum the fertilized eggs in the micropylar chamber 

 of the embryo sac behave as in the case of Tumboa, and as do the proem- 

 bryonal cells of Ephedra, the tubular prolongation penetrating the endo- 





