SPERMATOPHYTES 



201 



a Cuban cycad (Microcycas) recently 

 studied, as many as sixteen mother 

 cells and sperms have been found in a 

 single pollen tube. Within each mother 

 cell a large, spirally grooved, multi- 

 ciliate sperm is formed (figs. 453, 454), 

 which is discharged and swims freely 

 (figs. 455, 45 6 )- The blepharoplast 

 has received its name from the fact 

 that it develops the cilia. The dis- 

 covery of these swimming sperms of 

 Cycadales was quite unexpected, since 

 it had been supposed that all seed plants 

 had abandoned swimming sperms ; but 

 the discovery served to emphasize the 

 fern connections of the cycads. 



Fertilization. The pollen tube pene- 

 trates the tissue of the nucellus in vari- 

 ous directions, often branching, and 

 always functioning as an absorptive 

 structure (haustorium) . Finally it col- 

 lapses, and the tube nucleus may return 

 to the grain end of the tube. In the 

 meantime the tissue of the nucellus 

 lying between the pollen chamber and 

 the archegonial chamber has broken 

 down, and the two chambers become . 

 continuous. Then the sperms are dis- 

 charged into the archegonial chamber, 

 and finding their way into the arche- 

 gonium fertilize the egg. It is evi- 

 dent that the pollen tube in these 

 primitive seed plants is not a means of 



m 



FIG. 459. Differentiation of proembryo of 

 Zamia by elongation of cells of suspensor region; 

 unelon gated cells above suspensor forming the 

 "rosette"; apical group of small meristematic 

 cells (note spindles) to form the embryo. After 

 COULTER and CHAMBERLAIN. 



459 



