GNETALES— WELWITSCHIA 



403 



finally nearly all of the cells become uninucleate. In their further 

 development, the two regions continue to be different. 



A remarkable feature in the development of this gametophyte is 

 that there are no archegonia. Cells which might be regarded as 

 archegonial initials, elongate, 

 break through the megaspore 

 membrane, and grow up into the 

 nucellus, looking like pollen 

 tubes, and growing up into the 

 nucellus just as pollen tubes 

 grow down . They are called ' 'pro- 

 thalhal tubes." They grow about 

 half-way through the nucellus, 

 where they meet the pollen tubes 

 coming down. Nuclear details 

 have not been worked out, but if 

 the nucleus in a prothallial tube 

 has been formed by the fusion of 

 several nuclei, fertilization and 

 chromosome numbers would pre- 

 sent an interesting problem. 



After the cells in the lower part 

 of the gametophyte have become 

 uninucleate through the exten- 

 sive nuclear fusions, mitosis, with 

 the formation of cell walls, is re- 

 sumed. The chromosome num- 

 bers, at first very large, become 

 smaller as divisions continue, 

 and the cells also become smaller. This behavior stops before ferti- 

 lization, but is resumed as soon as fertilization has taken place, a 

 feature quite characteristic of angiosperms. 



While the gametophyte is characterized by the x number of 

 chromosomes, and the sporophyte is characterized by 2x chromo- 

 somes, we should still regard the prothallium of Welwitschia as 

 gametophyte in which the normal x number of chromosomes has 



Fig. 378. — Welwitschia mirabilis: upper 

 part of nucellus with prothallial tubes 

 going up to meet the pollen tubes which 

 will be coming down: /, prothallial tubes; 

 s, sterile cells of upper region of female 

 gametophyte; n, nucellus; m, megaspore 

 membrane; X140. — After Pearson. «' 



