LIFE HISTORY OF PINUS 39 



SUMMARY. 



In Pinus rigida, P. austriaca and P. resinosa the primitive 

 archesporium is well developed before the approach of winter, 

 but the microspore-mother-cells do not arise until the end of the 

 following April. The male inflorescence does not appear in 

 Pimis Strobus, until the end of the April preceding pollination, 

 and the definitive archesporium is differentiated in this species 

 about the middle of May. The nuclei of the primitive arche- 

 sporium are characterized by several deeply staining nucleoli 

 and a fine, close-meshed reticulum which responds but slightly 

 to chromatic dyes. 



The wall of the pollen-sac consists in all cases of from three 

 to four layers of cells. The tapetum is not clearly distinguished 

 until spring and there are indications that it may be derived 

 from the outer layer of sporogenous tissue. The nuclei of 

 this tissue multiply mitotically and the cells reach their maxi- 

 mum size about the time when the microspores become free. 

 At this period each cell has from one to three nuclei which pre- 

 sent all stages of fusion. When the pollen-grains are mature 

 the tapetum has entirely disappeared and the wall of the micro- 

 sporangium consists of a single layer of cells, or at most of 

 not more than two. 



Synapsis is recognized as a normal stage in the prophase of 

 the heterotypical division in the pollen-mother-cell of Pinus. 

 It is not preceded by a definite spireme, but a broad skein con- 

 taining irregular masses of chromatin separated by clear portions 

 of the linin thread issues from the contracted nuclear mass. 



The chromatic spireme splits longitudinally and breaks up 

 by transverse fission into several segments. The loosely coiled, 

 delicate threads resulting from the longitudinal division soon 

 draw together and fuse, double threads also come into contact 

 at various points and fuse more or less perfectly. These threads 

 always anastomose most freely in the region of the nucleoli, 

 some of which still stain deeply while others stain but faintly 

 after synapsis. 



Fission occurs at various points in the now irregularly con- 

 tracted and anastomosed threads, and the separate chromosomes, 



