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R D ALLEN

Publications and source records attributed to R D ALLEN.

At least 19 recordsLinked to original sources
PubMed1965

THE CHANGING PATTERN OF BIREFRINGENCE IN PLASMODIA OF THE SLIME MOLD, PHYSARUM POLYCEPHALUM.

Plasmodia of the acellular slime mold, Physarum polycephalum, reveal a complex and changing pattern of birefringence when examined with a sensitive polarizing microscope. Positively birefringent fibrils are found throughout the ectoplasmic region of the plasmodium. In the larger strands they may be oriented parallel to the strand axis, or arranged circularly or spirally along the periphery of endoplasmic channels. Some fibrils exist for only a few minutes, others for a longer period. Some, particularly the circular fibrils, undergo changes in birefringence as they undergo cyclic deformations. In the ramifying strand region and the advancing margin there is a tendency for fibrils of various sizes to become organized into mutually orthogonal arrays. In some plasmodia the channel wall material immediately adjacent to the endoplasm has been found to be birefringent. The sign of endoplasmic birefringence is negative, and its magnitude is apparently constant over the streaming cycle. The pattern of plasmodial birefringence and its changes during the shuttle streaming cycle of Physarum are considered in the light of several models designed to explain either cytoplasmic streaming alone or the entire gamut of plasmodial motions. The results of this and other recent physical studies suggest that both streaming and the various other motions of the plasmodium may very likely be explained in terms of coordinated contractions taking place in the fibrils which are rendered visible in polarized light.

Animals↗
PubMed1963

SHUTTLE-STREAMING: SYNCHRONIZATION WITH HEAT PRODUCTION IN SLIME MOLD.

Two small blobs and a channel excised from a slime mold plasmodium were allowed to fuse into a dumbbell-shaped mass in a thermally insulated Kamiya double chamber equipped with naked bead thermistors in contact with the blobs. Cyclic temperature differences of from 1 X 10(-4) to 5 X 10(-2) deg Celsius were recorded by a sensitive lock-in amplifier method with a basal noise level of less than 2 X 10(-2)deg Celsius and a time constant of 0.5 second. The temperature differences were caused by periodic bursts of heat production synchronized perfectly with the shuttle-streaming cycle and invariably localized at the source rather than the destination of the streaming cytoplasm. The results support the theory that the motive force for cytoplasmic streaming in the slime mold is pressure, probably generated by contraction of elements in the channel walls.

Cytoplasm↗
PubMed1963

A NEW METHOD OF POLARIZATION MICROSCOPIC ANALYSIS. I. SCANNING WITH A BIREFRINGENCE DETECTION SYSTEM.

A new method of polarized light analysis is described in which a highly sensitive electronic detector specific for birefringence is used to identify the crystalline axes of an object and then measure its phase retardation due to birefringence. The microscopic system employed in the method consists of an electronic birefringence detection system (BDS), a microscope with strain-free lenses, and a driven stage for passing the specimen at appropriate velocities across the image of an aperture placed at the field stop and imaged in the specimen plane by the condenser. The detector registers retardations directly as voltage at a constant deflection sensitivity of ca. 1.1 v per angstrom unit over a range of 120 angstrom units. The basal rms noise level is 0.002 A for a spot 36 micro in diameter formed by a 95 x, N. A. 1.25 objective pair, and increases in proportion to the reciprocal of the diameter of the scanning spot. The increase in noise with high resolution scanning can be offset by increasing the instrumental time constant, which is adjustable in decades between 0.004 and 0.4 seconds. A number of difficult problems in high extinction polarization microscopy are avoided by the use of modulated light and a rapid electronic detector. For example: (a) The measured distribution of birefringence is unaffected by the usual diffraction anomaly; therefore polarization rectifiers are not required. (b) The detector is selective for birefringence, so that there is no problem in separating contrast due to different optical properties (e.g. dichroism, light scattering). (c) The speed and sensitivity are both increased by between one and two orders of magnitude over that attainable by visual or photographic methods, thereby rendering a vast number of weakly birefringent, light-scattering, and motile objects readily analyzable for the first time with polarized light.

Biophysical Phenomena↗
PubMed1960

The consistency of ameba cytoplasm and its bearing on the mechanism of ameboid movement. II. The effects of centrifugal acceleration observed in the centrifuge microscope.

Three species of common, free-living amebae, Amoeba proteus, Amoeba dubia, and Chaos chaos were directly observed and photographed while exposed to a range of centrifugal accelerations in two types of centrifuge microscopes. Cytoplasmic inclusions in all three species are displaced discontinuously (at a variable velocity) in apparently all parts of the cell, suggesting non-Newtonian behavior and/or heterogeneous consistency. The ectoplasm of all species shows the highest yield point of any region in the cell; the posterior ectoplasm is less rigid than that in the anterior part of the cell. The axial part of the endoplasm shows evidence of structure (a sharp viscosity transition if not a true yield point) by its: (a) resistance to the displacement of particles carried in that region of the cell, (b) hindrance to the passage through the cell of inclusions displaced from other regions, and its (c) support without visible back-slip of inclusion being resuspended in the axial endoplasm in a centripetal direction at accelerations as high as 170 g. At this acceleration, each crystal "weighs" the equivalent reduced weight of seven times its volume in gold at 1 g. The only regions of the normal, moving cell which show clear evidence of low apparent viscosity are the "shear zone" (see Fig. 8) and the "recruitment zone." Possible reasons for low apparent viscosity in these regions are discussed. A new scheme of ameba "structure" is presented on the basis of the combined results of velocity profile analysis and the present centrifugation study.

Acceleration↗
PubMed1958

An anterior-posterior gradient of refractive index in the ameba and its significance in ameboid movement.

Sustained locomotion in Amoeba proteus and Amoeba dubia results in the establishment of a measureable gradient of refractive index along the anterior-posterior axis of the cell, provided thickness of the specimens is kept constant by even compression under a selected coverglass supported by quartz beams of uniform diameter. The tail region of the ameba develops a higher refractive index, indicative of from 6 to 40 per cent more organic matter (expressed as protein) there than present in the front. This gradient fades on cessation of movement. The average protein concentrations in the crystal-free tails and fronts of 15 A. proteus were 3.9 and 3.4 per cent, respectively. In individual experiments, the tail-front difference ranged from one to eight times the accuracy of the method. Since the gradient of refractive index was shown not to result from extraction of water from the tail by the contractile vacuole, it was interpreted as displacement of water toward the anterior part of the cell during movement. It is suggested that contraction of the ectoplasm drives forward a "tide" of syncretic fluid, the anterior border of which is visible as the hyaline cap, which contains less than 1 per cent protein. The movement of the granular endoplasm into the hyaline cap would then complete the cycle by imbibition of the fluid tide. The theoretical positions of Pantin and of Dellinger have been combined in the proposal that ameba cytoplasm consists of a network of a contractile phase which is able to expel (by syneresis) a highly mobile fluid phase. Some other possible interpretations are discussed.

Amoeba↗