388 MORPHOLOGY OF GYMNOSPERMS 



meet the pollen tubes in the lower half of the nucellar cap. It is 

 evident that archegonia are eliminated, and that this is associated 

 with the fact that the sexual nuclei are differentiated before a uninu- 

 cleate tissue is formed. An earlier differentiation is possible only 

 in the free nuclear stage of the endosperm, before there is any wall- 

 formation; and this condition has been reached by Gnetum. The 

 details of the formation of the permanent endosperm tissue of Wel- 

 witschia need critical study, but the general outline of the situation 

 is clear. 



The embryo sac of Gnetum has long been noteworthy for its sug- 

 gestion of that of angiosperms. Several species have been investi- 

 gated by Karsten (9, ii, 12), and Gnetum Gnemon has been described 

 by LoTSY (15). The embryo sac of the last species has been regarded 

 as illustrating a female gametophyte intermediate in structure between 

 the tissue-filled sacs of Ephedra and Welwitschia on the one hand, and 

 the sacs of the other species of Gnetum, which contain only free nuclei 

 at the time of fertilization. The antipodal end of the embryo sac 

 was described as being filled with a compact endosperm tissue before 

 fertilization, while the micropylar end contained free nuclei. Later 

 investigation (25) has shown that the tissue referred to is a nutritive 

 tissue developed in the chalaza in contact with the antipodal end of 

 the sac, and that all the species of Gnetum, so far as known, have 

 embryo sacs containing only free nuclei at the time of fertilization, 

 as described by Karsten. 



The endosperm of Gnetum begins with free nuclear division, and 

 eight nuclei have been observed (25) grouped near the center of the 

 sac before vacuolation. Later the vacuolation of the sac results 

 in the imbedding of the nuclei in a thin parietal layer of cytoplasm 

 surrounding a large central vacuole. The number of free nuclei 

 has not been determined, but presumably it is quite indefinite. It 

 is in this stage that fertilization occurs, for the free nuclei are poten- 

 tial egg nuclei, although a group near the antipodal end of the sac 

 may be as distinctly vegetative as are the antipodal cells of angiosperms. 

 After fertilization, walls appear among the endosperm nuclei and 

 a tissue is formed, which finally invades and destroys all of the nucellar 

 tissue (p. 383). Under such circumstances the formation of arche- 

 gonia is impossible, free eggs being organized at the free nuclear stage 



