HYDROSTATIC PRESSURE EFFECTS UPON THE SPINDLE FIGURE AND CHROMOSOME MOVEMENT. II. EXPERIMENTS ON THE MEIOTIC DIVISIONS OF TRADESCANTIA POLLEN MOTHER CELLS

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Abstract
Hydrostatic pressures have been applied to Tradescantia PMC as a technique for studying the structure of spindles and chromosomes and the mechanics of anaphase movement. Pressure increments progressively reduce gel rigidity, ultimately to the point of liquefaction. The effects are reversible. The first meiotic division spindle was seriously affected by 5,000 lbs./in2. pressure which also sufficed to block anaphase movement. Condensed chromosomes were significantly softened by even 1,000 lbs./in2. as indicated by an undue elongation of the kinetochore stalk. Fusion bridges became particularly obvious when 3,000 lbs. was applied. Shortening and rounding occurred at 4,000 lbs. Total fusion and rounding did not occur until pressures of 15,000 lbs. were employed. These effects were thought to be upon matrix material since un-condensed chromosomes were not affected. The fusion and rounding appeared to be a surface tension effect, and suggested the existence of a true interfaeial membrane. The presence of fusion bridges allowed only a very abnormal ana-phase movement at pressures of 3,000 and 4,000 lbs. Spin-dles re-formed de novo after the release of high pressure. To some extent the formation of traction fibers was independent of the growth of the rest of the spindle, and many abnormalities occurred. It was concluded that gel structure in the spindle is essential for anaphase movement. The traction fiber may serve as nothing more than a semi-elastic connection between the chromosome and the main mass of the spindle which, in turn, is in motion. Motion and force may be imparted by sol-gel-sol transformations, with gel being added to the central bulk of the spindle while a proportional solation goes on at the poles.