TRITICUM 



173 



unequal growth of the nucellus and its investing integuments 

 results in the formation of a completely anatropous ovule. 



Megasporo GENESIS AND THE Megagametophyte. — The hypo- 

 dermal archesporial cell is wedge-shaped, and successive divisions 

 of it result in the formation of a linear tetrad of megaspore mother 

 cells. (Fig. 78, A-C) The three outer cells disintegrate, and 

 the innermost one enlarges greatly, functioning as the megaspore. 

 (Fig. 78, D.) The development of the megagametophyte proceeds 



B 



Fig. 78. A-D, median longisections of portions of young ovules : A, stage showing 

 primary archesporial cell ; B, after the first division of the archesporial cell ; C, the tetrad 

 stage; D, a later stage showing the collapse of the three outer cells. (Redrawn from 

 Percival, The Wheat Plant, Duckworth and Co.) 



to an eight-celled stage, during which time it increases in size and 

 absorbs some of the nucellar tissue. The two synergids and the 

 megagamete occupy the micropylar end of the megagameto- 

 phyte; and, prior to gametic union, the polar nuclei fuse. The 

 three antipodals increase in size and divide, the number ultimately 

 produced varying from six to ten according to Percival (10), 

 although Kornicke (7) suggests that 36 or more may be formed. 



Brenchley (4) states that double fertilization occurs between 

 2.4 and 48 hours after pollination; and, although there may be 

 some doubt as to a union of the primary endosperm nucleus with 



