ELECTRON MICROSCOPE STUDIES ON THE NORMAL AND COLCHICINIZED MITOTIC FIGURES OF THE ONION ROOT TIP (ALLIUM CEPA)

¹ Department of Zoology, and Radiation Research Laboratory, State University of Iowa, Iowa City

1. The electron microscope was used to study normal and colchicinized mitotic figures in onion root tip cells.

2. The polar cap contains "spindle fibers" prior to the breakdown of the nuclear membrane. It is bounded at one margin by the nuclear membrane and on the other by a membrane similar to and apparently continuous with the nuclear membrane. No spindle material is distinguishable inside the nucleus prior to the breakdown of the nuclear membrane.

3. Chromosomal fibers are composed of several smaller unit fibers oriented in a longitudinal fashion along the main axis of the fiber. This compound structure was not observed in continuous fibers.

4. Spindle fibers differ somewhat in diameter with different fixation. Flemming's fluid fixation results in a slightly smaller fiber diameter than that obtained with fixation in Randolph's CRAF mixture. The general character of the fibers in both preparations is essentially the same.

5. The origin of the cell plate is briefly discussed in the light of the work of early investigators. Electron micrographs show an origin by thickenings in the equatorial region of the spindle.

6. The effect of short exposures to near-threshold concentrations of colchicine is a progressive swelling and solubilization of fiber material.

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