164: MORPHOLOGY OF SPERMATOPHYTES 



FERTILIZATION 



It is now known in Cycadales and Ginkgoales, forms with 

 ciliated male cells or sperms, that these cells are not trans- 

 ferred through the pollen tubes. Branching tubes penetrate in 

 every direction through the loose nucellar tissue which lies be- 

 tween the beak and the embryo sac, and seem to be purely absorp- 

 tive in function. As the nucellar tissue is broken down, the 

 pollen grains containing the body cell are brought into close 

 proximity to the archegonia, so that when the ciliated male 

 cells or sperms are organized and discharged they are able to 

 reach the necks by swimming. As the Cordaitales and Ben- 

 nettitales show nucellar structures identical with those of Cyca- 

 dales and Ginkgoales, it seems reasonable to suppose that their 

 pollen tubes, if any existed, were also rhizoidal in character, 

 and that their sperms were ciliated. The conclusion seems to be 

 inevitable that pollen tubes appeared primarily as absorbing 

 organs, and that their function as sperm carriers is one that 

 should be regarded as secondary. 



In the other groups of Gymnosperms, so far as known, the 

 pollen tube, while retaining more or less of its rhizoidal char- 

 acter, is also distinctly a sperm carrier, and this function seems 

 to be associated with the development of nonciliated male cells. 

 In these cases the whole of the contents of the swollen tip of the 

 tube, consisting prominently of the two male cells, tube nucleus, 

 and stalk-cell nucleus, has been observed to be discharged into 

 the cytoplasm of the egg. 



The development of two male cells seems to be thoroughly 

 established throughout the Spermatophytes. In the grouped 

 archegonia of Cupresseae both male cells may function, and the 

 same may be true in the case of branching tubes which are sperm 

 carriers. In Taxus, however, the inequality of the male cells 

 indicates that one of them has long since ceased to be functional. 



The discharge of the contents of the pollen grain or pollen 

 tube is in every case closely preceded by the cutting off of the 

 ventral canal cell. Occasionally the nucleus of this cell becomes 

 as highly organized as that of the egg, and there is reason to 

 believe that in some instances it may be fertilized. The cyto- 

 plasm of the egg receives the various contents of the grain or 

 tube, but everything remains near the periphery of the cyto- 



