PISUM SATIVUM 365 



The nucleus of the microspore divides to form a tube cell and a 

 smaller generative cell, the former disintegrating early so that it 

 may disappear completely by the time of pollination. The genera- 

 tive nucleus divides during the growth of the pollen tube, and the 

 two microgametes which are surrounded by a narrow layer of 

 cytoplasm are separated from the cytoplasm of the tube by a thin 

 membrane. 



Development of the Ovule and Megasporogenesis. — As the 

 young carpel develops, the ovules arise on the adaxial margins of 

 the carpel along two parietal placentae. (Fig. 183, Z.) At this 

 time, the adaxial suture is still visible; but in the later development 

 of the carpel, it becomes less and less distinct. At maturity, the 

 carpellary margins are intimately fused and the ovules appear to be 

 arranged in a single row. This appearance is further emphasized 

 by the formation of an abaxial suture and the development of pa- 

 renchymatous ray tissue which bisects the large abaxial bundle. 

 The suture becomes more and more pronounced and finally con- 

 stitutes one of the lines of dehiscence in the mature fruit. 



The ovule first appears as a dome-shaped primordium of nucellar 

 tissue and at its base two integuments arise in close succession. 

 The outer integument precedes the inner one and by its rapid 

 growth soon encloses the latter. As the integuments are differen- 

 tiated around the nucellus, the ovule curves toward the stylar end 

 of the carpel and gradually becomes campylotropous. 



In megasporogenesis, a hypodermal cell at the apex of the ovule 

 becomes the archesporial cell, and divides to form a primary 

 parietal and a primary sporogenous cell. The latter functions as 

 the megaspore mother cell, enlarging and developing a finely vacu- 

 olated cytoplasm preceding diakinesis. Cooper (7) noted several 

 cases in which two or even three sporogenous cells developed in 

 the ovule. 



Four megaspores result from the meiotic divisions, and the one 

 nearest the chalaza is functional, enlarging and elongating prior 

 to further division. The usual sequence follows, three successive 

 nuclear divisions producing an eight-celled megagametophyte. 

 (Fig. 185.) Three of the nuclei remain at the chalazal end and 

 become the antipodals, three form the megagamete and synergids 

 toward the micropyle, and the other two serve as the polar nuclei 

 uniting at about the time of fertilization. The mature megagamet- 

 ophyte is elongated and curved, eight nucleate, and seven celled. 



