STUDIES ON THE PLANT CELL. — IV. 



BRADLEY MOORE DAVIS. 



Section III. Highly Specialized Plant Cells and 

 Their Peculiarities {Continued). 



4. The Spore Mother-cell. 



The spore mother-cell and its homologues the pollen mother- 

 cell and certain embryo-sacs have furnished some of the most 

 interesting subjects for cell studies in the plant kingdom. 

 Sporogenesis in all plants above the thallophytes seems to be 

 a period when nuclear structures are especially clearly differ- 

 entiated and when the mechanism of mitosis reaches the highest 

 degree of complexity. These intricate conditions are only 

 equalled by processes in the development of the female game- 

 tophyte of some angiosperms, and during endosperm formation, 

 also in the events of spermatogenesis and with the segmentation 

 of the egg nucleus of certain gymnosperms. 



Sporogenesis is one of the critical periods in the life history 

 of a higher plant since it is the time when the asexual genera- 

 tion (sporophyte) passes over to the sexual (gametophyte). This 

 provides certain important features such as the reduction phe- 

 nomena concerned with chromosomes and greatly adds to the 

 interest in these cells. These matters will receive special atten- 

 tion in Section V, but they must be borne in mind to appreciate 

 fully the significance of many events of spore formation. 



The general history of the spore mother-cell may be described 

 as follows : It is the product of the last mitosis in the repro- 

 ductive tissue called the archesporium. This mitosis always 

 presents the number of chromosomes characteristic of the 

 sporophyte which is double the number found in the game- 

 tophyte. Therefore the nucleus that passes into the spore 

 mother-cell has the sporophyte number of chromosomes. Two 



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