3i6 THE STRUCTURE OF ECONOMIC PLANTS 



sporogenous cells, which are commonly arranged side by side, but 

 may occur in a linear row. These cells function directly as mega- 

 spore mother cells producing tetrads of megaspores; and, fre- 

 quently, two or three tetrads are formed in a single ovule. In most 

 instances, only a single chalazal megaspore of one of the tetrads 

 develops to form a mature megagametophyte; but Cooper reports 

 that an ovule may contain two well-developed gametophytes. 

 The megagametophyte reaches the eight-nucleate stage as a result 

 of three equational nuclear divisions, and cell plates are formed in 



Fig. 158. A, longisection of young carpel showing initial stages in curvature of campylo- 

 tropous ovules ; one row of ovules has been removed ; B, longisection of portion of an ovule 

 showing two-nucleate megagametophyte and two integuments, the outer one completely 

 enclosing nucellus ; C, longisection of campylotropous ovule showing mature megagameto- 

 phyte ready for fertilization. (Redrawn after Reeves.) 



such a way that three nuclei at each end of the embryo sac are 

 enclosed by walls while the fourth nucleus of each group remains 

 in the central region. In this manner, a seven-celled gametophyte 

 is developed with a megagamete and two synergids at the micro- 

 pylar end, three antipodals at the chalazal end, and an elongated 

 endosperm mother cell in the central region. 



Fertilization and Embryogeny. — Fertilization occurs approx- 

 imately 2.4 hours after pollination, one microgamete uniting with 

 the megagamete while the other fuses with the two polar nuclei 

 to form the endosperm nucleus. The zygote divides transversely 

 to form a two-celled proembryo, and further transverse divisions 

 of the apical cell result in the formation of a proembryo consisting 



