DEVELOPMENT OF POLLEN-GRAINS. 



379 



1000. Development of polleii -grains. This affords some of the 

 most instructive examples of cell-division, and owing to the 

 facility with which material can be procured and studied, has 

 received much attention. 



(1) Superficial phenomena. These, which can be easily 

 traced without the employment of staining agents, are in brief 

 as follows : At the period when the locnli of the anthers begin 

 as minute elevations at the end of the stamen, the external 

 layer of cells, which is to serve as epidermis, is underlaid by 



m 



166 



a group of small cells which give rise to the mother-cells of the 

 pollen and to the lining of the anther itself. This group is 

 termed the archesporium ; by division of its inner layer, large 

 mother-cells are produced which divide to form the pollen- 

 grains. The division of a mother-cell may give rise to two, three, 

 or four pollen-grains, and in some cases more, according to the 



FIG. 166. Fritillaria Persica. Division of the mother-cells of pollen, a, early stage, 

 in which the threads are confused; b, the segments in course of longitudinal division; 

 c, the nuclear spindle in profile; d, the same seen from its extremity or pole; e, division 

 of the nuclear plate ; ./", separation of the derivative or daughter-segments ; g, formation 

 of the derivative tangles and the cell plate; h, the course of the nuclear threads in the 

 derivative nuclei; i, longitudinal extension; A', nuclear spindle, on the right, in profile, 

 on the left, from its extremity; /, separation of the segments, on the left seen in pro- 

 file, on the right from the extremity ; m, formation of the cell-plates. (Straslmrger.) 



