GINKGOALES 45 



even the genus Cycas, and it is not inconceivable that this differ- 

 ence between a very ancient type and miach more modern types 

 may occur. 



FEETILIZATION 



We have reserved for this topic all of the events associated 

 with the development of the pollen tubes, as well as the imme- 

 diate act of fertilization. When the pollen grains reach the 

 pollen chamber, about the first of May, the male gametophyte is 

 in its three-celled stage, the cells being the persistent vegetative 

 cell, the generative cell, and the large tube cell. The pollen 

 chamber now begins to enlarge at the expense of the nucellar 

 tissue beneath, carrying the grains deep into the nucellus. By 

 the beginning of June the chamber has greatly enlarged, form- 

 ing a large irregular cavity, which practically reaches the 

 embryo sac. TMien the grains begin to put out their tubes, the 

 opening to the pollen chamber becomes closed and remains so, 

 the surrounding tissue becoming brown and forming a solid 

 beak which long persists as a cap on the sac. The deep placed 

 grains, carried by the enlarging chamber well below the beak, 

 send out their tubes in every direction into the adjacent nucellar 

 tissue, but chiefly away from the embryo sac, and often directly 

 toward the beak. Into these young tubes the tube nuclei pass, 

 and remain there as long as the tube system is developing. 



About the beginning of July the tubes have penetrated the 

 nucellar tissue more deeply and branch freely, the tube nucleus 

 remaining where a tube begins to branch. This extensive sys- 

 tem of branching tubes seems to function only as an absorbing 

 system, as in the Cycads. At this stage the generative cell has 

 the appearance of being pushed away from contact with the 

 vegetative cell, on account of the development of vacuoles in the 

 latter (Fig. 32, D); it increases very much in size, its nucleus 

 keeping pace with it; and these relative positions are main- 

 tained until just before fertilization, which occurs about ten 

 weeks later. 



About the middle of July the nucleus of the generative cell 

 divides rapidly, forming two daughter nuclei which represent 

 the stalk and body cells. One of these nuclei enlarges very 

 much, and comes to occupy a central position in the cell, crowd- 

 ing the other aside into the space between the generative cell 

 and the vegetative cell. In this way the body cell is organized. 



