THE GYMNOSPERMAE : CYCADALES, ETC. 



765 



cells of this tissue form radiating rows and develop while the prothallus is 

 still in an early free-nuclear stage (Fig. 766). 



The outstanding fact is that no archegonia are formed and that apparently 

 any of the nuclei of the embryo sac may be fertilized. Several pollen tubes 



STALK CELL 



BODY CELL 



TUBE NUCLEUS 



ANTHERIDIAL NUCLEUS 



BODY 

 NUCLEUS 



STALK 

 NUCLEUS 



TUBE NUCLEUS 



Fig. j6 S-—Gnetum gnenion. Development of the 

 male gametophyte. A, Microspore. B, First 

 division. C, Division of antheridial nucleus. 

 D, Organization of antheridial stalk and body 

 cells. E, Division of body cell to form male 

 gametes. {After Thompson.) 



may enter each embrvo sac, and each of the two male gametic nuclei unites 

 with a female nucleus (double fertilization) so that many zygote nuclei are 

 produced, which enlarge and begin embryo development. Each surrounds 

 itself with a cell wall and may unite with the tip of the pollen tube (cf. 

 Welwitschia) or may remain free and eventually develop a tubular suspensor 

 which pushes the embryo down into the antipodal end of the embryo sac. 

 Some unfertilized female nuclei now divide to produce a prothallial tissue, 

 while others disappear (Fig. 767). This tardy development of endosperm is, 

 however, so vigorous that it invades and destroys the nucellus, only leaving 

 an apical residue below the micropyle. At this stage the seed is shed and the 

 further development of the embryo takes place on the ground, but its details 

 are unknown. 



Welwitschia mirabilis 



The third genus of Gnetales is in many respects one of the strangest plants 

 in the world. There is only one species, Wehvitschia mirabilis, which is 

 limited in its distribution to South West Africa (Fig. 768). 



