GNETALES 3^5 



mation of parietal tissue, and centripetal growth until the sac is full 

 of tissue. The details have been given by Land for Ephedra trijurca 

 (i8). At the division of the megaspore nucleus, the two daughter 

 nuclei assume polar positions in the sac on account of the development 

 of a vacuole between them, which in fact surrounds the equatorial 

 region of the spindle like a ring. These two nuclei divide simultane- 

 ously, and the four resulting nuclei assume equidistant positions at 

 the periphery of the sac. Successive simultaneous divisions follow 

 rapidly (fig. 432), until approximately 256 free nuclei are formed, 

 free nuclear division extending through a period of about twenty 

 days. 



With the appearance of the walls the endosperm is differentiated 

 into distinct micropylar and antipodal regions, the former consisting 

 of very loosely arranged cells with extremely delicate walls, and the 

 latter of more compact cells. As the very rapid growth proceeds, 

 the antipodal tissue becomes differentiated into storage and haustorial 

 regions (fig. 423). The storage region comprises the greater bulk 

 of the endosperm and is full of starch and other foods. In the cen- 

 ter of it a column of thin-walled cells, richer in food than the others, 

 extends from the haustorial region to the base of the archegonia, 

 marking the path along which the embryo is to be thrust by the elon- 

 gating suspensor. The haustorial region consists of the one or two 

 outermost layers of endosperm cells at the antipodal end. The loosely 

 arranged cells of the micropylar region, with their delicate and some- 

 times belated walls, are of interest in connection with the same region 

 in Welwitschia, described below. 



About April (in New Mexico) the archegonium initials begin to 

 be differentiated, two being the usual number, although one or three 

 sometimes occur. After the primary- neck cell is cut off, it divides 

 at once periclinally, and other periclinal divisions follow before anti- 

 clinal walls are formed which divide the cell of each tier into four 

 cells. Eight such tiers of neck cells have been observed, and later 

 divisions may result in six or eight cells in a tier. The appearance 

 of regular tiers may be lost by the irregular planes of the later divisions, 

 and the neck may resemble ordinary tissue (figs. 433-435). The 

 minimum number of neck cells is probably thirty-two, but there may 

 be many more. In any event, the archegonium neck of Ephedra is the 



