GROWTH AND CELL DIVISION 79 



.thus augments the size of the nuclear cavity. In the nuclei of 

 root tips and similar vegetative plant parts the two daughter 

 nuclei finally reach the stage of perfect resting nuclei, with 

 chromatin net, nuclear sap, and nucleolus. The nucleolus grows 

 gradually with the chromatin net, but its origin is still obscure. 

 It should not be overlooked by the student that the above proc- 

 esses, which lead to the formation of the new chromatin net of 

 the daughter nuclei by vacuolization and anastomosis of the 

 daughter chromosomes, is exactly the reverse of the processes by 

 which the chromatin net of a resting nucleus is transformed into 

 chromosomes. Condensation of a chromatin net to form the 

 chromosomes of the mother nucleus is always followed by expan- 

 sion of the chromosomes to form the resting net of the daughter 

 nuclei in the vegetative cells of plant organs. In germ cells a 

 slightly different procedure is usually manifested at certain 

 stages in their division processes. 



O A 



Cell division is initiated by the formation of a dense spindle (/) 

 which we have called the second spindle, in the space occupied 

 by the central spindle fibers of the first spindle during anaphase 

 and early telophase. It soon becomes barrel-shaped (&), with 

 very dense outer peripheral fibers, which stain heavily with 

 cytoplasmic stains. This second spindle unites the forming 

 daughter nuclei, and has for its function the formation of the 

 new cellulose wall, which completes the separation of daughter 

 nuclei and the protoplast of the mother cell into two daughter 

 cells. The new separating cellulose wall is secreted by a dense 

 cell plate of cytoplasm, which apparently arises as thickenings 

 of the fibers of the second spindle at the center of each fiber. 

 These fiber thickenings increase in size and finally unite to 

 form a solid cytoplasmic disk, or plate (/), extending across the 

 equator of the spindle. The spindle at the same time increases 

 in diameter and stretches entirely across the cell from wall to 

 wall, dividing its protoplast into two equal parts. The cell 

 plate then splits, the split beginning at its center and extending 

 to the junction of the cell plate with the cellulose walls of the 

 mother cell. The new cell wall is formed by the deposit or 

 secretion of cellulose particles between the halves of the cell 



