CYCADALES 



119 



lar (fig. 117). Long before the cellular stage is reached, a nutritive 

 layer, one or two cells thick, is developed in contact with the gameto- 

 phyte. It has been called the endosperm jacket, and is so conspicu- 

 ous that it can be seen with the naked eye. It is shown in both the 

 preceding figures. It functions like a tapetum, passing nutritive ma- 

 terial from the cells next to it into the growing gametophyte. 



The arrangement of cells in the young gametophyte is extremely 

 regular, radiating from the center to the periphery, as shown in 

 fig- 117- 



b 



Fig. 115. — Zamia floridana: First division of megaspore; a, actively nutritive cells; 

 b, tissue of closely packed cells; c, flattened cells; X930. — After Dr. F. Grace Smith.s^^ 



Soon after this stage, when the gametophyte has reached a diam- 

 eter of about 3 mm., some of the cells at the micropylar end become 

 larger, and their nuclei move from their central position to the pe- 

 ripheral end of the cell (figs. 1 18-122). These are archegonium ini- 

 tials. They are very numerous in comparison with the number which 

 develop up to the fertilization stage. In most of them the nucleus 

 does not divide at all, and, in Dioon edule, not more than ten ever 

 reach the fertilization stage, and the usual numbers are only three, 

 four or five. In other cycads there may be a couple more or a couple 

 less. Microcycas is exceptional. Caldwell^^ found scores, and some- 



