AND THE Embryo-sac in Bignonia venusta 95 



Development of the Pollen Grain and of the DEFiNrnvE 



ArCHESI'ORIUM. 



^lop 



Very early in the development of the microsporangium the de- 

 finitive archesporium is differentiated, and the nuclei of the pollen 

 mother-cells enter upon a considerable period of rest and growth. 

 As previously mentioned, the few divisions which have occurred 

 in the primitive archesporium arc truly of the vegetative or homo- 



very 



; 



The chromosomes are then small, oval or oblong, and number 

 about fifty, the full number of the sporophyte generation. 



TJic Resting archcsporial Cell and its Nucleus 



In general, from the time of the last divisions in the primitive 

 archesporium until synapsis of the definitive cells, the pollen 

 mother-cells increase in size to at least twice their former diameter, 

 the nucleus is about two-thirds larger, and the nucleolus approxi- 

 mately one-half greater in diameter. The nucleolus, however, is 

 now larger than the original nucleus of the early differentiated 



periblem layer. 



In the resting cell the cytoplasm is closely netted and the nu- 

 clear membrane distinct. The nucleolus takes the gentian stain in 

 the Flemming combination. It is a striking feature in these nuclei 

 that there is very little chromatin on the reticulum, and the nucle- 

 olus takes the chromatin stain constantly. In the early period of 

 growth the nucleolus shows a deeper-strained outer zone. This 

 is more evident in the early stages, but before the time of synapsis 

 it is no longer noticeable. Small clear spaces in the nucleolus, 

 which have been termed vacuoles, are present. I have generally 

 been able to trace to these spaces linin strands of the nucleolus, 

 as in some other plants studied later ; and it seems highly probable 

 that the appearance of small vacuolations are often only projections 

 on the surface. This occurs not only during the reticulum stage, , 



but also later. 



The reticulum is more closely interlaced in the periphery of 



+ 



the nucleolus, and upon it are found slight thickenings and gran- 

 ules which give up their gentian readily. In addition, there are 



