Organization of microtubules and endoplasmic reticulum during mitosis and cytokinesis in wheat meristems.

The fine-structural changes accompanying mitosis in meristematic cells of the roots and coleoptile tissue of wheat have been studied. A band of microtubules encircling the nucleus appeared in the cytoplasm before the cells entered prophase. These microtubules were oriented at right angles to the direction of the mitotic spindle and were located at the position on the mother cell wall where the future cell plate dividing the daughter cells would have joined it. During prophase the number of microtubules in this preprophase band decreased and eventually disappeared, while microtubules were found to be aligned along the spindle axis. These spindle microtubules appeared as a cone-shaped array of units radiating from the polar zones of the spindle and passing very close tangentially to the nucleus. At late prophase they penetrated the disintegrating nuclear envelope and were seen between the chromosomes. During metaphase and anaphase many microtubules were present running throughout the length of the spindle, and others were found to be attached to chromosomes. Paired sister chromosomes were found joined to microtubules from opposite poles of the spindle. The position and orientation of the lamellae of the endoplasmic reticulum which invaded the spindle from the two poles was closely related to the position and alignment of the microtubules. During the formation of the cell plate vesicles were seen to be collected between the microtubules. As the vesicles fused to form the plate the microtubules were found only at its growing edge, where the vesicles were still being aligned. At the initial stage of its formation the microtubules passed right through the plate, but as it extended they appeared to end at the plate region. The results of the investigation are discussed in relation to the descriptions of mitosis and cytokinesis based on optical microscopy of living cells.