

. V 



V 



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Figure 5-5. Electron Micrograph Showing Early State of Cell Plate 

 Formation in Telophase of Dividing Onion Root Tip Cell. A small portion 

 of each telophase nucleus is seen at the upper right and lower left. The de- 

 veloping cell plate extends diagonally from lower right to upper left. Mem- 

 brane elements of the endoplasmic reticulum with evidence of branching are 

 present on both sides of the cell plate. In the immediate vicinity of the cell 

 plate the elements of endoplasmic reticulum are shorter and form a reticula- 

 tion composed of a close lattice of tubules along the midline between the 

 two cells. Phragmosomes are seen concentrated on either side of the plate 

 region. Development of the cell plate is more advanced at the center (the 

 early phragmoplast) and grades out toward the margins, where fewer cyto- 

 plasmic elements are present. (From Porter, K. R. and Machado, R. D., 

 1960. "Studies on the Endoplasmic Reticulum. IV. Its Form and Distribu- 

 tion During Mitosis in Cells of Onion Root Tip," J. Biophys. Biochem. Cytol., 

 7, Fig. 16, Plate 94. Courtesy of Dr. Keith R. Porter, The Rockefeller 

 Institute.) 



MITOSIS AND MEIOSiS / 125 



