652 



Journal of Agricultural R,esearch 



Vol. XII. No. 10 



DEGENERATION OF THE MICROSPORE 



Liberation of the microspore from the tetrad places these individuals 

 upon their own resources for future development. It is true that the 

 spores are surrounded by a nourishing medium furnished by the sporo- 

 ph3rte, but the abihty to use this material depends upon the individual 

 cell metabolism of the microspores. 



A study of the actual change in cytoplasmic content from the pollen 

 mother cell stage to the stage of complete formation of the pollen grains 

 indicates the degree to which the young spores are dependent upon 

 their own metabolism from the time of liberation from the tetrad to 

 maturity of the male gametophyte or mature pollen grain. Measure- 

 ments were made of the diameter of the spherical cytoplasmic mass at 

 the following stages during pollen formation: Rounded pollen mother 

 cell at diakenesis, young microspores which have just been liberated 

 from the tetrad, microspores which have completed growth and in 

 which the nuclei are either dividing or are about to divide, and the 

 mature pollen grains. A summary of the measurements is given in 

 Table XI. It will be seen by comparing the volume of the pollen mother 

 cells and of the microspores at liberation that the latter show slightly 

 less than one-fourth the volume of the former, indicating that during 

 reduction division and the period subsequent to the liberation of the 

 microspores no increase in cytoplasm has occurred. Following libera- 

 tion from the tetrad, however, when the microspores are floating in 

 the nourishing medium of the loculus as independent units, very rapid 

 wall growth takes place. From <the time of liberation to division of 

 the microspores nucleus the volume of the cells increases about 6.4 

 times, but shows no corresponding increase in cytoplasm. At the 

 mature pollen stage the cells have increased to nearly 7.5 times that 

 of the liberated microspore. The changes in volume and cytoplasmic 

 content from the pollen mother-cell stage to mature pollen are also 

 well illustrated by Plate B, figure 14; Plate C, figures '4 and 8; and 

 Plate D, figure 2, all of which are drawn to one scale. 



Table XI. — Volume of pollen mother cells and microspores at various stages of 



development . 



Stage. 



Volume. 



Pollen mother cell at diakenesis 



Liberated microspores 



Microspores, division stage 



Mature pollen 



Cm. /I. 

 1,859.99 

 436. 28 

 2, 777. 40 

 3.255-39 



