GNETALES 387 



A zone of jacket cells (tapetum) is organized about the central 

 cell, the cells being at first isodiametric, but with the elongation of the 

 central cell they become much elongated and their walls become so 

 thin as to be seen with difTicultv. At the time of fertilization the walls 

 separating the jacket cells from the egg break down altogether, and 

 the contents of these cells are mingled with those of the egg. Miss 

 Berridge and Miss Sanday (22) have reported a remarkable behav- 

 ior of the jacket cells of Ephedra distachya, claiming that the nuclei 

 of the broken-down jacket cells escape into the egg, fuse in pairs, and 

 produce proembryos. Such beha\-ior was never observed in thousands 

 of ovules of E. trijurca (23), but occasionally jacket cells were found 

 resembling the isolated proembryonic cells in appearance but never in 

 function. 



The ventral nucleus of Ephedra trijurca (18) is cut off during the 

 first two weeks of April (in New Mexico), and there is no trace of a 

 wall separating it from the egg. It remains in the upper part of the 

 archegonium and becomes conspicuously enlarged. 



The earlier account of the female gametophyte of Welwitschia v/as 

 very perplexing, and although it is now found to have been inaccurate 

 and even misleading, it suggested an unusual situation, which has 

 been cleared up to some extent by Pearson (27). The endosperm 

 begins as usual with free nuclear division, and an embryo sac with 

 approximately 1,024 free nuclei was observed, representing ten suc- 

 cessive divisions rather than the usual eight. Attention has been 

 called to the fact, however, that in Pinus there may be as many as 

 eleven such divisions, and that the number may well vary widely 

 among gymnosperms (see p. 259). These nuclei are distributed 

 through the sac, so that when cleavage of the cytoplasm by walls occurs 

 the embryo sac is divided into multinucleate cells. Those in the micro- 

 pylar region contain fewer nuclei than do the others, and these nuclei 

 become the nuclei of free eggs. The cells of the lower three-fourths 

 of the endosperm contain many nuclei, which are said to fuse and 

 thus produce a uninucleate tissue, and this primary endosperm tissue 

 continues to grow both before and after fertilization. The multi- 

 nucleate cells of the micropylar region, which were formerly mistaken 

 for archegonium initials, send out tubes into the overlying nucellar 

 tissue, into which the free nuclei pass, and these prothallial tubes 



