2o6 GYMNOSPERMS 



pollen chamber, the inline protrudes and becomes anchored in the 

 tissue of the nucellus, where it acts as a haustorium, the original 

 function of a pollen tube. As in the cycads, the pollen grain end of 

 the tube advances toward the female gametophyte, breaking down 

 and absorbing the tissue before it, so that the pollen chamber is 

 enlarged until it reaches entirely through the nucellus, and nothing 

 remains between the pollen tubes and the female gametophyte. 



HiRASE^s3 traced the development of the pollen tube from its 

 earliest stages up to the mature sperms (fig. 223), After reaching the 

 pollen chamber, the generative cell divides, producing a stalk cell and 

 body cell. Hirase's figures show the nuclei, but not differentiated 

 cells. The blepharoplasts appear only in the body cell, where they 

 increase in size but apparently take no part in the division, unless 

 they may function in the orientation of the nuclear figure. In each 

 of the two sperms formed by the division of the body cell, the 

 blepharoplast becomes attached to the nucleus, a small portion of 

 which seems to be attracted so that it forms a beak. The blepharo- 

 plast is then drawn out into a spiral band, which develops hundreds 

 of cilia. With the exception of the sperms of cycads, these are the 

 largest swimming sperms which have ever been recorded, about 80 mi- 

 crons in length, as estimated from Hirase's figures. The sperms are 

 more elongated than in the cycads, and the spiral, ciliated band, with 

 fewer turns, is more confined to the apical region. Ginkgo and the 

 cycads are the only known living seed plants which have retained the 

 swimming sperm of their very remote ancestry. 



The Jcmale gametophyte. — The megaspore is the first cell of the 

 female gametophyte. Although four megaspores are usually formed 

 from the megaspore mother-cell, only the lower one develops. The 

 four spores are formed about the first of May, the time of pollina- 

 tion, so that the development of the pollen tube and the develop- 

 ment of the female gametophyte start together. 



The megaspore enlarges rapidly, its elongated shape changing to 

 nearly spherical. A period of free nuclear division follows, like that 

 in the cycads, with the protoplasm, containing the nuclei, pressed in 

 a thin sheet against the megaspore membrane by the turgidity of the 

 fluid in the large central vacuole. After hundreds of free nuclei have 

 been formed, wall formation begins and progresses from the periph- 



