46 



MORPHOLOGY OP SPERMATOPHYTES 



At the end of July the body cell increases still more in size, 

 becoming rich in contents, and its nucleus becoming very large. 

 At this stage the blepharoplasts appear, and finally assimie their 

 polar positions at the extremities of the long axis of the ellip- 

 soidal nucleus. This stage persists for three weeks — that is, 

 until the third week before fertilization. 



At the beginning of August the tube nucleus begins to pass 

 back to the grain end of the tube, which it reaches in about two 

 weeks, consorting with the body cell, or with the male cells, 

 until fertilization. During this retreat of the tube nucleus and 

 abandonment of the absorbing tube system, the endosperm be- 

 gins to develop a beak between the two archegonia. Hirase sug- 

 gests that its significance is to be explained by the fact that the 



tissue between the nucellar 

 beak and the embryo sac has 

 been broken down by the 

 •deepening of the pollen cham- 

 ber and by the absorptive work 

 of the pollen-tube system, and 

 that the heavy beak settles 

 down upon the sac. In about 

 two weeks the endosperm beak 

 looks like a small column, 

 with its summit against the 

 nucellar beak, " like a tent 

 supported by its center pole," 

 in the shelter of which there is freedom for the final processes 

 connected with fertilization. Pressure upon the grain ends of 

 the pollen tubes which happen to lie under " the tent " is thus 

 avoided, and these ends become very turgid and are directed 

 toward the near-lying archegonia (Fig. 33). 



During the third week before fertilization— that is, about 

 the last of August— the body cell begin^ division, and the cili- 

 ated male cells are organized. The swollen tip of the pollen, 

 tube, capped by the old wall of the pollen grain, now contains 

 in a group the two male cells, the tube nucleus, and the vege- 

 tative cell, together with whatever may remain of the stalk 

 cell. At the time of fertilization the capped tip of the tube 

 turns toward the archegonial chamber and discharges the con- 

 tents into it. The exact position of the sac wall in relation to 



Fig. ZB.— Ginkgo liloha, upper part of fe- 

 male gametophyte, showing two arche- 

 gonia, the bealjlike process of the en- 

 dosperm supporting the remains of the 

 nucellus, and the swollen tips of two 

 pollen tubes, September 9th, x 24.— After 



HiBASE. 



