Gravitropism in Higher Plant Shoots : III. Cell Dimensions during Gravitropic Bending; Perception of Gravity.

Cross and longitudinal sections were prepared for light microscopy from vertical control plants (Xanthium strumarium L. Chicago strain), free-bending horizontal stems, plants restrained 48 hours in a horizontal position, and plants restrained 48 hours and then released, bending immediately about 130 degrees . Top cells of free-bending stems shrink or elongate little; bottom cells continue to elongate. In restrained stems, bottom cells elongate some and increase in diameter; top cells elongate about as much but decrease in diameter. Upon release, bottom cells elongate more and decrease in diameter, while top cells shorten and increase in diameter, accounting for the bend. During restraint, bottom cells take up water while tissue pressures increase; top cells fail to take up water although tissue pressures are decreasing.Settling of amyloplasts was observed in cells of the starch sheath.Removal of different amounts of stem (Xanthium; Lycopersicon esculentum Miller, cv Bonny Best; Ricinus communis L. cv Yolo Wonder) showed that perception of gravity occurs in the bending (elongation) zone, although bending of fourth and fifth internodes from the top was less than in uncut controls. Uniform application of 1% indoleacetic acid in lanolin to cut stem surfaces partially restored bending. Reversing the gradient in tension/compression in horizontal stems (top under compression, bottom under tension) did not affect gravitropic bending.

Subunit structure of Eutherian sperm chromatin.

The beta-mercaptoethanol induced decondensation of spermatozoon cell nuclei from several Eutherian species has been followed from the intact spermatazoon cell to the solubilized linear unit sperm chromosomal fiber using fluorescence and electron microscopy. Data from nuclease digestion studies in conjunction with electron microscopic evidence indicate that the gross structure of the unit Eutherian sperm chromosomal fiber consists of DNA folded around sperm specific histone multimers spaced regularly along the fiber generating a linear array of sperm nucleosomes connected by short stretches of uncomplexed DNA. The sperm nucleosomes, 80 A in diameter are separated by 20 A filaments of DNA. This structure is remarkably similar to the structure of somatic chromatin although the protein components of the two chromosomes are markedly different. It seems likely that chromosomal fibers, similar to those described herein, may be present in the male pronucleus following fertilization.