Preprophase microtubules and stomatal differentiation; some effects of centrifugation on symmetrical and asymmetrical cell division

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Young wheat seedlings were centrifuged; premitotic and dividing cells of the leaf epidermis were examined. During the normal symmetrical, and also the first asymmetrical division (forming future guard mother cells), preprophase nuclei resisted sedimentation to a varying extent, dependent somewhat on the size of the preprophase band of microtubules. At other stages of these divisions, the spindle invariably sedimented, with accompanying structural disruption. During the second asymmetrical division (forming the subsidiary cell of the stomatal complex), all stages of division were highly polarized, and one pole remained adjacent to the GMC. Evidence is also presented which indicates that preprophase microtubules move intact into the spindle. The preprophase band is not considered functional specifically in orienting or positioning the premitotic nucleus; it appears to represent a gathering of future spindle structures, prior to these being incorporated, many intact, into the mitotic apparatus. Most spindle microtubules appear structurally altered following the centrifugal procedures. Telophase structures resist disruption more than other stages of division. Some evidence suggests that the subsidiary cell nucleus and cytoplasm at telophase are primarily responsible for the formation of the new, highly curved cell wall. Nucleolar material appeared between chromosome arms in the disrupted spindles, even at prophase and metaphase.

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