THE PHYSICAL BASIS OF THE POSITIVE PHOTOTROPISM OF PHYCOMYCES

A physical basis is demonstrated, in the case of a cylindrical cell illuminated with parallel light from one side, for greater photochemical action in the half of the cell farthest from the source of light, when the cell is surrounded by a medium of refractive index less than that of the cell. Factors governing the balance and magnitude of unequal action of light in the two halves of the cell are: the refractive index of the cell, the cell radius, and the absorption coefficient of the intracellular pigment. A limiting value of absorption coefficient is deduced which cannot be exceeded in cells of a particular size showing positive phototropism. In terms of this mechanism the positive phototropism of Phycomyces in air is explained.     

Physiological Studies in the Mucorales: PART I THE PHOTOTROPISM OF SPORANGIOPHORES OF PHYCOMYCES BLAKESLEEANUS

Previous views on the physical basis of phototropism in Phycomycessporangiophores are briefly discussed.

1. It was confirmed thatunilaterally illuminated sporangiophoresimmersed in liquidparaffin show strong negative phototropism.
2. Elongation growthceased and no phototropic response took placeunder anaerobicconditions.
3. By focusing a fine beam of light on to one edgeof the growingzone of a sporangiophore, leaving the other sidein darkness,it was established that greater elongation tookplace in theilluminated zone, the sporangiophore tending tobend out ofthe beam. Rapid reversal of the curvature followedwhen theillumination was transferred to the opposite edge ofthe sporangiophore.

Wassink and Boumann's suggestion that phototropism can be initiatedby a one-quantum-per-cell process is criticized in the lightof this result and other work by Castle.     

ORIENTATION IN COMPOUND FIELDS OF EXCITATION; PHOTIC ADAPTATION IN PHOTOTROPISM

During upward geotropic orientation upon a vertical plate the slug Agriolimax creeps vertically, in darkness. Horizontal light from one side produces orientation of dark-adapted slugs away from the vertical path, through an angle (β). The magnitude of this angle is a function of the light intensity and of time. The moderately rapid course of light adaptation is followed by measurements of β at fixed intervals. Simple assumptions as to the nature of the orienting forces lead to the conclusion that the logarithm of the tangent of β should decrease linearly with time, and that the rate of the decrease should vary directly with the logarithm of the light intensity. Both expectations are adequately realized. Certain implications of these results for behavior analysis are pointed out.     

INTRACELLULAR CENTRIFUGAL SEPARATION OF ORGANELLES IN PHYCOMYCES

Live sporangiophores of Phycomyces blakesleeanus were centrifuged at 35,000 rpm. The cell contents sedimented into distinct layers, and each layer was studied with an electron microscope and with cytochemical methods. The following layers were found (their volumes and their densities are shown in Fig. 3): 1. polyphosphates; 2. polyphosphates and protein crystals; 3. glycogen; 4. yellow layer with ferritin; 5. ribosomes; 6. protein crystals; 7. mitochondria; 8. mitochondria and fibrils; 9. nuclei; 10. endoplasmic reticulum; 11. vesicles, membranes, and reticulum; 12. vacuole; 13. lipoproteins, membranes; 14. fat droplet. The densities of the various layers were determined by the injection of droplets of inert oils of known density into the sporangiosphores before centrifugation. Sedimented cell organelles could be isolated. Centrifuged nuclei of a lycopene-producing mutant were injected into the intact sporangiophore of an albino host where they induced color formation. The ensuing spores, when plated, gave a mixture of white and colored colonies. It was concluded that cell organelles, sedimented by centrifugation of living sporangiophores, remain alive and can be used for biochemical studies. Microspectrophotometric examination of the layers indicated the presence of cytochromes and flavines in the mitochondria and of cytochromes in the nuclei. No pigments corresponding to the action spectrum for the light growth response were found.     

TEMPORARY ABOLITION OF PHOTOTROPISM IN LIMAX AFTER FEEDING

When fed upon cooked potato the slug Limax maximus becomes for a day or more indifferent to light—its natural negative phototropism is suppressed. This effect is not produced by raw potato or other diets; it can be duplicated by injection of sugar solutions into the stomach or into the body fluids, and seems to be due to sugar absorbed during the digestion of the cooked starch. The fact is of interest particularly for the suggestion which it affords as to the explanation of fluctuations in conduct.     

PHOTOTROPISM IN YOUNG RATS

Before the eyelids have opened young rats are negatively heliotropic. They behave very much as does the larva of the blow-fly. The angle of orientation by lights opposed at 180° may be calculated by an equation based upon the elementary requirement of phototropism, namely that orientation is attained when the illumination of bilaterally disposed photoreceptors is equal. The precision of orientation decreases very nearly in proportion to the sum of the logarithms of the acting light intensities, due to photokinetic head movements. When the eyelids are opened, the rats move toward a darkened place in the field of vision, usually toward the shaded region immediately to one side of the lamp house. Therefore, when heliotropic, the rat is not "seeking the dark". The phototropism of these animals may be brought into conflict with their pronounced stereotropism, and the resolution of such conflicts may perhaps be utilized for the investigation of central nervous states.     

INVESTIGATIONS ON AUXIN-GIBBERELLIN INTERACTION ON THE PHOTOTROPISM OF HELIANTHUS ANNUUS HYPOCOTYL

On applying IAA-GB and IAA-NAA mixtures to the hypocotyls ofHelianthus annuus, the plants showed an increase and a decrease,respectively, in their phototropic response compared with thecontrol treated with IAA-H₂O only. In the in vitro experimentsit was observed that the illumination (with 1,500 lux light,lasting for 24 hr) caused photoinactivation of IAA in the mixturesof IAA-H₂O, IAA-GB and IAA-NAA by 50%, 24% and 0%, respectively.In the presence of 0.01% riboflavin as a photosensitizer inthe same mixtures, the degree of photodestruction of IAA causedby the same illumination was 92%, 97% and 86%, respectively. The stronger positive phototropism in the IAA-GB treated hypocotylwas accounted for by the situation that on the illuminated sidethe photo-inactivation of IAA was accelerated by GB, while onthe shaded side IAA and GB acted synergistically. On the otherhand, the weaker phototropism in the IAA-NAA treated hypocotylwas explained as due to a partial inhibition of photoinactivationof IAA by NAA. (Received October 6, 1962; )     

PHOTOTROPISM OF DIXIPPUS MOROSUS

1. Local differences in the effects of stimulation of parts of the eye by light are expressed in Dixippus morosus by differential circus movements. 2. The angle of inclination of the body axis toward one source of light when the animal is on a vertical plane with light from one side is inversely proportional to the logarithm of the intensity of the light.     

Physiological Studies in the Mucorales: PART II SOME OBSERVATIONS ON GROWTH REGULATION IN THE SPORANGIOPHORE OF PHYCOMYCES

Lanolin emulsion paste incorporating indolylacetic acid in concentrationsof 10,000, 1,000, 100, and 10 µg./ml when applied witha micromanipulator to one side of Phycomyces sporangiophoreshad no clearly marked action on subsequent growth Griseofulvin, a metabolic product of Penicillium janczewskiiand certain other moulds, at a concentration of 1OO µg./ml.,produced a local increase in the rate of extension of the wallat or near the region to which it was similarly applied, withconsequent curvature from the normal erect growth habit. Previously proposed explanations of the regulation of sporangiophoregrowth, and of phototropism, in terms of the action of heteroauxinand auxin- are criticized, and attention directed to the dangerof assuming that substances active in regulating the growthof cellulose-walled times will have a similar action on chitin-walledfungal hyphae.     

THERMAL INCREMENTS FOR PULSATION-FREQUENCY IN "ACCESSORY HEARTS" OF NOTONECTA

The frequencies of pulsation of the "accessory hearts" in the isolated swimming-legs of Notonecta were studied in relation to temperature, with the idea that in such organs central nervous control is impossible, and that in an isolated system irreversibly proceeding toward death it might be expected that further evidence would be found regarding the supposed specific significance of critical thermal increments. A number of values of µ are found, commonly 8,200; 16,200; or 32,200; less frequently 11,400; 19,800; and 24,500. These values are definitely contrasted with that (12,300) typical for heart beat frequencies in arthropods. They exhibit interrelationships of the sorts already found in other cases. There occur also sharp irreversible changes in frequency of pulsation, which may or may not be accompanied by change of increment. The net result is held to be confirmatory of the interpretation of thermal relations proposed in earlier papers.