1909] CHAMBERLAIN— SPERMATOGENESIS IN DIOON 219 



The behavior of the chromatin during the two mitoses by which 

 four spores are formed from the mother cell will be described at some 

 future time; at present I merely note that the number of chromo- 

 somes in both mitoses is 12. A hasty examination might lead one to 

 suspect that the number is much larger, since it is not difficult to find 

 prophases of the first mitosis, just before the disappearance of the 

 nuclear membrane, showing any number of chromosomes from 1 2 to 

 24. But when there are more than 1 2, some are always about half the 

 full size. Any number beyond 1 2 is due to the early separation of the 

 two parts of the chromosome, which, in most cases, are separated only 

 after the chromosomes have become arranged in the equatorial plate. 

 Even in prophase, there are occasional indications of the second 



splitting which is to be completed at the metaphase of the second 



mitosis. 



These mitoses are not simultaneous throughout the whole sporan- 

 gium, but begin at the periphery and proceed toward the center, so that 

 there may be a zone of dividing cells surrounding mother cells which 

 are still in the resting condition. 



POLLEN BEFORE SHEDDING 



Aue young pollen grains 

 flattened Dortion wVnVV. ™;rrV. 



(fig 



The exine is thickest in this basal region, while at the opposite end of 



l ^e grain where thp nnll^n tnko ;* irx 0™^™^ i't ic wrv thin. The 



very 



™ine is thinnest in the basal region where it is in contact with the 

 thickest portion of the exine. On the sides of the spore, the intine is 

 jay thick, often thicker than the exine. There is no trace of a 



rd s P° re coat, as described by Ferguson (4) for Pinus. 



The microspore germinates while still in the sporangium. A single 

 Persistent prothallial cell is formed, lenticular in shape and closely 

 applied to the base of the spore. Webber (2) described two prothal- 

 la cells in Zamia, the first formed being evanescent and the second 

 Persistent, but a reexamination by Miss Grace Smith (8) showed 

 °* 7 one. We find only one prot hallial cell in Dioon, Zamia, and 

 t ^ ce P hal artos, and it is' persistent. In later stages, after the pollen 

 U f C has be gun to form, it would be easy to misinterpret, for the line 



union of the stalk cell with the persistent prothallial cell often 

 ^es the impression of a small prothallial cell beneath the large 



