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 all 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 like 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 Ephedra: three .of fus j on nudeus (with j n the 
the eight free proembryonal cells. After 
LAND, thalhal 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 
division in embryo formation has disappeared, and the first division of 
the fertilized egg is accompanied by wall formation, which is an 
angiosperm 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- 
