148 MORPHOLOGY OF GYMNOSPERMS 



known in other plants or in animals. They are usually slightly 

 longer than broad, in Zamia floridana being 222-332 fi in length 

 and 222-306/^ in width; in Dioon edule 300/* in length and 230/1* 

 in width; in Cycas revoluia 180-210 /a in width; and in Microcycas 

 calocoma about 60 /* in width. As shown in fig. 173, the nucleus is 

 very large, with only a thin sheath of cytoplasm. Such large bodies, 

 of course, are easily visible to the naked eye, and since the pollen 

 tubes are transparent, it is easy to observe the movements of the 

 sperms. 



The cilia begin to move while the sperms are still within the mother 

 cells, and their movement is accompanied by pulsating and amoeboid 

 movements of the cytoplasm and nucleus. The peripheral portion 

 of the partition between the sperms soon breaks down, and the two 

 sperms, still attached to one another, often begin to swim in the old 

 body cell before they escape into the general cavity of the tube. When 

 free from one another, the principal movement is straight ahead, with 

 a rotation on the long axis. The movements may continue for several 

 hours before the sperms are discharged from the tube. For some 

 time before the sperms are discharged, the portion of the pollen tube 

 which projects into the pollen chamber becomes more and more turgid, 

 until finally a rupture occurs at or near the wall of the pollen grain 

 which still caps the end of the tube. One sperm escapes in two or 

 three seconds, but the other may be half a minute in getting 

 out. Experiments have been made to determine whether the sperms 

 are chemotactic or not, but so far with only negative results. 



FERTILIZATION 



When the pollen chamber breaks through the base of the 

 nucellar cap, the portion of the megaspore membrane covering the 

 archegonial chamber is ruptured, so that the two chambers form 

 one continuous cavity. The archegonial chamber is moist, but con- 

 tains no liquid until the turgid pollen tubes begin to discharge. The 

 sperms swim in the liquid discharged from the pollen tubes and enter 

 the necks of the archegonia (fig. 176). 



Fertilization has been described in five cases: in Cycas revoluta 

 by Ikeno (27), in Zamia floridana by Webber (32), and by Cham- 

 berlain in Dioon edule (70), Ceratozamia mexicana (80), and Stan- 



