THE PHOTOCHEMICAL BASIS OF ANIMAL HELIOTROPISM

1. Experiments on the heliotropic orientation of Limulus were made which confirmed Loeb''s photochemical theory of animal heliotropism proposed first in 1888 and 1889 in experiments on insects, and later in experiments on other forms of animals. 2. It is shown that these animals are oriented by light in such a way that the product I x t x cos α is the same for the symmetrical photosensitive elements of the eyes or the skin, where I is the intensity of the light, t the duration of illumination, and α the angle of incidence of the light at the surface element of the photosensitive organ. 3. When this equation holds, the products of decomposition by light must be the same in symmetrical elements of the eyes or skin, and the influence of these products of decomposition on the tension of symmetrical muscles of the locomotor organs of the animal must be the same. As a consequence the animal must move in the path of light, either to or from the source of light.     

BRIGHTNESS DISCRIMINATION AS A FUNCTION OF THE DURATION OF THE INCREMENT IN INTENSITY

1. This investigation has been concerned with an analysis of brightness discrimination as it is influenced by the duration of ΔI. The durations used extend from 0.002 second to 0.5 second. 2. ΔI/I values at constant intensity are highest for the shortest duration and decrease with an increase in duration up to the limits of a critical exposure time. At durations longer than the critical duration the ratio ΔI/I remains constant. 3. The Bunsen-Roscoe law holds for the photolysis due to ΔI. This is shown by the fact that, within the limits of a critical duration, the product of ΔI and exposure time is constant for any value of prevailing intensity, I. 4. At durations greater than the critical duration the Bunsen-Roscoe law is superseded by the relation ΔI = Constant. This change of relation is considered in the light of Hartline''s discussion (1934). 5. The critical duration is a function of intensity. As intensity increases the critical duration decreases. 6. Hecht''s theory (1935) accounts for the data of this experiment if it be assumed that brightness discrimination is determined by a constant amount of photolysis.     

THE DARK ADAPTATION OF THE EYE OF THE HONEY BEE

Bees which are held in a fixed position so that only head movements can be made, respond to a moving stripe system in their visual field by a characteristic motion of the antennae. This reflex can be used to measure the bee''s state of photic adaptation. A curve describing the course of dark adaptation is obtained, which shows that the sensitivity of the light adapted bee''s eye increases rapidly during the first few minutes in darkness, then more slowly until it reaches a maximum level after 25 to 30 minutes. The total increase in sensitivity is about 1000 fold. The adaptive range of the human eye is about 10 times greater than for the bee''s eye. The range covered by the bee''s eye corresponds closely to the adapting range which is covered by the rods of the human eye.     

THE KINETICS OF THE BACTERIUM-BACTERIOPHAGE REACTION

The above data relating to the reaction between 16 hour cultures of S. aureus and antistaphylococcus bacteriophage in nutrient broth of pH 7.6 at 36°C. and with mechanical shaking to maintain a uniform B suspension, bring out the following points: (a) B growth in P-B mixtures does not differ from growth in controls without P except in the case of a very high initial P/B ratio as noted below. There is no evidence that lytic destruction of B begins shortly after mixing P and B nor that B growth is stimulated by P, for the B growth curves in the presence of ordinary [P]''s and in controls are identical. Only at the sudden onset of the rapid lytic process does the B curve of a P-B mixture deviate from the control curve. (b) B growth is an essential conditioning factor for P formation. (c) Both B growth and P production exhibit short lags. During this time P diffuses into or becomes adsorbed to B so rapidly that by the end of the lag period only 10 to 30 per cent of the total P present is extracellular, the remainder being associated with the B. (d) During the logarithmic B growth phase, P formation is also logarithmic but proceeds at a much faster rate. That is, d P/d t is proportional to a power of d B/d t. Consequently the statement that each time a B divides a certain amount of P is formed is not correct. (e) As B growth enters the phase of positive acceleration equilibrium between the extracellular and intracellular P fractions becomes established and is maintained up to the onset of lysis, extracellular [P] representing a small constant percentage of total [P]. The distribution of P on a constant percentage basis suggests the manner in which a relatively simple chemical compound would be distributed and is not at all typical of the distribution one would expect if P were a complex organized parasite. (f) When the value of log P/B = 2.1 lysis begins. Obviously, this limiting value for any initial [B] is reached sooner the higher the initial [P]. When log P/B at the time of mixing P and B is already 2.1 or greater, there is no growth of B and lysis soon occurs. (g) While there is good evidence that lysis is brought about by the attainment of a particular [P] per B and not by a certain [P] per ml., it is not clear at this time which of the ratios intracellular P/B, extracellular P/B or total P/B is the major conditioning factor for B lysis. (h) Experimentally the maximal [P]''s of lysates made by mixing a constant initial [B] with widely varying Po''s fall within a relatively narrow range. This fact is explained by the large value of d log P/d t as compared to d log B/d t. That is, the loci of points at which log P = 2.1 + log B (maxima-lysis begins) on the curves of log P against t originating in various [Po]''s will lie at a nearly constant level above the abscissa. Because of this same relationship the maximal [P]''s of such a series will be in the reverse order of magnitude of the Po''s, i.e., the larger the Po the smaller will be the maximal [P] attained during the reaction (cf. Fig, 16). (i) The lytic destruction of B is logarithmic with time, in this respect being similar to most death rate processes. The value -d log B/d t for a particular initial [B] is constant for various initial values of [P]. There is good evidence that cells need not be growing in order to undergo lysis. (j) During B lysis a considerable percentage of the total maximal P formed is destroyed, the chief loss probably occurring in the intracellular fraction. The major portion (70 to 90 per cent) of the final P present after the completion of bacteriophagy is set free during the brief phase of bacterial dissolution. (k) When the entire process of bacteriophagy is completed the lysates are left with certain [P]''s determined by the foregone P-B reaction. The destruction of P during lysis is sufficiently regular to maintain the relationship established at the maximal [P]''s. Therefore the final [P]''s have the same points in common that were noted in "h" as applying to the maximal [P]''s. That is, they all are grouped within a narrow range of [P] values, those having been made with high Po''s being of lower titre than those made with low initial [P]''s. (1) There is a significant difference in the temperature coefficients of P and B formation. Further, the temperature coefficients of P and B destruction during lysis differ in almost the same ratio. Consequently, while all experimental evidence postulates B growth as an essential conditioning factor for P formation, the temperature coefficient data suggest that the two processes are basically separate reactions. A similar interpretation holds in the case of B dissolution and P inactivation. (m) The major events in the complete process of "bacteriophagy" are mathematically predictable. The [B] at which lysis occurs under certain standard conditions for given values of Bo and Po may be calculated from the equation: See PDF for Equation Substitution of this value for log B in the equation: See PDF for Equation gives satisfactory agreement with observed values for t _(lysis). (n) The kinetic analysis of the P-B reaction predicts that the values of log Po plotted against t _(lysis) for a constant Bo will give a straight line. This plot is employed in a method for the quantitative estimation of P described in an earlier paper on the basis of experimental observation alone. Its use is made more rational by the facts given above.     

THE VISUAL INTENSITY DISCRIMINATION OF THE HONEY BEE

1. Bees respond by a characteristic reflex to a movement in their visual field. By confining the field to a series of parallel stripes of different brightness it is possible to determine at any brightness of one of the two stripe systems the brightness of the second at which the bee will first respond to a displacement of the field. Thus intensity discrimination can be determined. 2. The discriminating power of the bee''s eye varies with illumination in much the same way that it does for the human eye. The discrimination is poor at low illumination; as the intensity of illumination increases the discrimination increases and seems to reach a constant level at high illuminations. 3. The probable error of See PDF for Equation decreases with increasing I exactly in the same way as does See PDF for Equation itself. The logarithm of the probable error of ΔI is a rectilinear function of log I for all but the very lowest intensities. Such relationships show that the measurements exhibit an internal self-consistency which is beyond accident. 4. A comparison of the efficiency of the bee''s eye with that of the human eye shows that the range over which the human eye can perceive and discriminate different brightnesses is very much greater than for the bee''s eye. When the discrimination power of the human eye has reached almost a constant maximal level the bee''s discrimination is still very poor, and at an illumination where as well the discrimination power of the human eye and the bee''s eye are at their best, the intensity discrimination of the bee is twenty times worse than in the human eye.     

ON THE QUANTITY OF ELECTRICITY AND THE ENERGY IN ELECTRICAL STIMULATION

Weiss''s and Hoorweg''s laws are discussed with respect to the dynamics of the excitatory process. The former is shown to have a simple basis which is inadequate, however, because it implies a constant rate of subsidence of the state of excitation. Hoorweg''s law does not follow logically from the same basis so the two laws do not represent the same excitatory mechanism. Experimental data do not give minimal energies at 2 rheobases as predicted by each law. The experimental minima with direct currents are at 1.5 or more rheobases, while with condenser stimuli they are from 2.5 to 3.0 rheobases. These minima conform to the predictions of the writer''s equations which give the direct current minima as variable with a lower limit at 1.5 rheobases and the condenser minima as constant at e = 2.718 rheobases. The reasons for these differences are discussed and it is concluded that considerations of the quantity of electricity and the energy, per se, do not lead to any simple concepts with regard to the excitatory mechanism. The existing quantity and energy relations are, however, easily correlated in terms of the dynamics of the excitatory mechanism.     

THE OXIDATION-REDUCTION POTENTIAL OF THE LUCIFERIN-OXYLUCIFERIN SYSTEM

The oxidation-reduction potential of the Cypridina luciferin-oxyluciferin system determined by a method of "bracketing" lies somewhere between that of anthraquinone 2-6-di Na sulfonate (E_o ^'' at pH of 7.7 = –.22) which reduces luciferin, and quinhydrone (E_o ^'' at pH of 7.7 = +.24), which oxidizes luciferin. Systems having an E_o ^'' value between –.22 and +.24 volt neither reduce oxyluciferin nor oxidize luciferin. If the luciferin-oxyluciferin system were truly reversible considerable reduction and oxidation should occur between –.22 and +.24. The system appears to be an irreversible one, with both "apparent oxidation" and "apparent reduction potentials" in Conant''s sense. Hydrosulfites, sulfides, CrCl₂, TiCl₃, and nascent hydrogen reduce oxyluciferin readily in absence of oxygen but without luminescence. Luminescence only appears in water solution if luciferin is oxidized by dissolved oxygen in presence of luciferase. Rapid oxidation of luciferin by oxygen without luciferase or oxidation by K₃Fe(CN)₆ in presence of luciferase but without oxygen never gives luminescence.     

THE GROWTH OF DUCKWEEDS IN MINERAL NUTRIENT SOLUTIONS WITH AND WITHOUT ORGANIC EXTRACTS

1. Detmer''s solution and a modified Knop''s solution are unfavorable culture media for the growth of Spirodela polyrhiza. 2. When the modified Knop''s solution was diluted to 10 times its volume, Spirodela polyrhiza and Lemna valdiviana grew and reproduced for periods of 26 months and 21 months, respectively. 3. Growth in the dilute Knop''s solution, which alone can support the growth of Spirodela indefinitely, was considerably stimulated over a period of 23 days by adding to every liter the water-soluble material of 0.4 gm. autolyzed yeast, or the material of 2.5 gm. peat soluble in a 1 per cent solution of NaHCO₃. 4. The nature of the stimulus or of the protection afforded by the organic material is as yet unknown. 5. The necessity of organic accessory foods (auximones) in the nutrition of green plants cannot be accepted as an established fact.     

THE KINETICS OF PENETRATION : XVI. THE ACCUMULATION OF AMMONIA IN LIGHT AND DARKNESS

The accumulation of ammonia takes place more rapidly in light than in darkness. The accumulation appears to go on until a steady state is attained. The steady state concentration of ammonia in the sap is about twice as great in light as in darkness. Both effects are possibly due to the fact that the external pH (and hence the concentration of undissociated ammonia) outside is raised by photosynthesis. Certain "permeability constants" have been calculated. These indicate that the rate is proportional to the concentration gradient across the protoplasm of NH₄ X which is formed by the interaction of NH₃ or NH₄OH and HX, an acid elaborated in the protoplasm. The results are interpreted to mean that HX is produced only at the sap-protoplasm interface and that on the average its concentration there is about 7 times as great as at the sea water-protoplasm interface. This ratio of HX at the two surfaces also explains why the concentration of undissociated ammonia in the steady state is about 7 times as great in the sea water as in the sap. The permeability constant P'''''' appears to be greater in the dark. This is possibly associated with an increase in the concentration of HX at both interfaces, the ratio at the two surfaces, however, remaining about the same. The pH of sap has been determined by a new method which avoids the loss of gas (CO₂), an important source of error. The results indicate that the pH rises during accumulation but the extent of this rise is smaller than has hitherto been supposed. As in previous experiments, the entering ammonia displaced a practically equivalent amount of potassium from the sap and the sodium concentration remained fairly constant. It seems probable that the pH increase is due to the entrance of small amounts of NH₃ or NH₄OH in excess of the potassium lost as a base.     

Simultaneous Measurement of Amyloid Fibril Formation by Dynamic Light Scattering and Fluorescence Reveals Complex Aggregation Kinetics

An apparatus that combines dynamic light scattering and Thioflavin T fluorescence detection is used to simultaneously probe fibril formation in polyglutamine peptides, the aggregating subunit associated with Huntington''s disease, in vitro. Huntington''s disease is a neurodegenerative disorder in a class of human pathologies that includes Alzheimer''s and Parkinson''s disease. These pathologies are all related by the propensity of their associated protein or polypeptide to form insoluble, β-sheet rich, amyloid fibrils. Despite the wide range of amino acid sequence in the aggregation prone polypeptides associated with these diseases, the resulting amyloids display strikingly similar physical structure, an observation which suggests a physical basis for amyloid fibril formation. Thioflavin T fluorescence reports β-sheet fibril content while dynamic light scattering measures particle size distributions. The combined techniques allow elucidation of complex aggregation kinetics and are used to reveal multiple stages of amyloid fibril formation.     

Measuring Spatially- and Directionally-varying Light Scattering from Biological Material

Light interacts with an organism''s integument on a variety of spatial scales. For example in an iridescent bird: nano-scale structures produce color; the milli-scale structure of barbs and barbules largely determines the directional pattern of reflected light; and through the macro-scale spatial structure of overlapping, curved feathers, these directional effects create the visual texture. Milli-scale and macro-scale effects determine where on the organism''s body, and from what viewpoints and under what illumination, the iridescent colors are seen. Thus, the highly directional flash of brilliant color from the iridescent throat of a hummingbird is inadequately explained by its nano-scale structure alone and questions remain. From a given observation point, which milli-scale elements of the feather are oriented to reflect strongly? Do some species produce broader "windows" for observation of iridescence than others? These and similar questions may be asked about any organisms that have evolved a particular surface appearance for signaling, camouflage, or other reasons.In order to study the directional patterns of light scattering from feathers, and their relationship to the bird''s milli-scale morphology, we developed a protocol for measuring light scattered from biological materials using many high-resolution photographs taken with varying illumination and viewing directions. Since we measure scattered light as a function of direction, we can observe the characteristic features in the directional distribution of light scattered from that particular feather, and because barbs and barbules are resolved in our images, we can clearly attribute the directional features to these different milli-scale structures. Keeping the specimen intact preserves the gross-scale scattering behavior seen in nature. The method described here presents a generalized protocol for analyzing spatially- and directionally-varying light scattering from complex biological materials at multiple structural scales.     

THE ACTION OF SULFONAMIDES ON THE RESPIRATION OF BACTERIA AND YEAST : INHIBITION OF BACTERIAL AND YEAST CARBOXYLASES BY SULFONAMIDE DRUGS STRUCTURALLY RELATED TO COCARBOXYLASE

The inhibiting effects of sulfonamide drugs and their derivatives on the anaerobic decarboxylation of pyruvic acid by Staphylococcus aureus, Escherichia coli, baker''s and brewer''s yeast, and a carboxylase preparation from brewer''s yeast have been investigated. These drugs are: sulfanilamide, sulfapyridine, sulfadiazine, sulfamethyldiazine, sulfathiazole, sulfamethylthiazole, sulfanilamido-5-ethyl-4-thiazolone, 2-aminopyrimidine, 2-aminothiazole, and 2-aminopyridine. The sulfathiazole ring appears to exercise decidedly greater specific inhibiting effect on the carboxylases of Staph. aureus and E. coli. The inhibiting effect on yeast carboxylase is non-differentiable among all the substances tried, except sulfamethyldiazine which is completely ineffective on the carboxylases of the organisms studied. The specific inhibitory effect of sulfathiazole on the carboxylases of Staph. aureus and E. coli in comparison to sulfanilamide, sulfapyridine, and sulfadiazine is in harmony with in vivo and in vitro experimental results of other investigators. The results of the present investigation appear to support the hypothesis (1) that sulfonamides exert their bacteriostatic action through chemical affinity for the carrier proteins of certain respiratory enzymes of the bacterial cell, and that this affinity may in part be related to structural similarity between components of the drugs and the corresponding respiratory coenzymes.     

THE FLICKER RESPONSE CONTOURS FOR GENETICALLY RELATED FISHES. II

The flicker response contour has been determined for several species and types of the teleosts Xiphophorus (X.) and Platypoecilius (P.) under the same conditions. The curve (F vs. log I_m) is the same for representatives of each generic type, but is different for the two genera. Its duplex nature is analyzable in each instance by application of the probability integral equation to the rod and cone constituent parts. The parameters of this function provide rational measures of invariant properties of the curves, which have specific values according to the genetic constitution of the animal. The F ₁ hybrids (H'''') of X. montezuma x P. variatus show dominance of the X. properties with respect to cone F_max. and σ'' _{log I}, but an intermediate value of the abscissa of inflection (τ''). The rod segment shows dominance of σ'' _{log I} from P., but an intermediate value of F_max. and of τ''. The composite flicker curve involves the operation of two distinct assemblages of excitable elements, differing quantitatively but not qualitatively in physicochemical organization, probably only secondarily related to the histological differentiation of rods and cones because almost certainly of central nervous locus, but following different rules in hereditary determination, and therefore necessarily different in physical organization. The interpretation of the diverse behavior of the three parameters of the probability summation is discussed, particularly in relation to the physical significance of these parameters as revealed by their quantitative relations to temperature, retinal area, and light time fraction in the flash cycle, and to their interrelations in producing the decline of rod effects at higher intensities. It is stressed that in general the properties of the parameters of a chosen interpretive analytical function must be shown experimentally to possess the physical properties implied by the equation selected before the equation can be regarded as describing those invariant properties of the organic system concerned upon which alone can deduction of the nature of the system proceed. The importance of genetic procedures in furthering demonstration that the biological performance considered in any particular case exhibits constitutionally invariant features provides a potentially powerful instrument in such rational analysis.     

THE KINETICS OF TRYPSIN DIGESTION : V. SCHUTZ'S RULE.

The rate of digestion of concentrated casein solutions by low concentrations of trypsin at 0° has been followed. Under these conditions the enzyme is inhibited by the product of the reaction and under certain conditions this effect should lead to Schütz''s rule, i.e. the amount of hydrolysis should be proportional to the square root of the product of the time into the enzyme concentration. This is the result obtained. Both Schütz''s rule and Arrhenius'' equation fail to hold accurately owing to the incorrect relation assumed to hold between the rate of hydrolysis and the substrate concentration.     

THE SENSIBILITY OF THE NOCTURNAL LONG-EARED OWL IN THE SPECTRUM

Infrared radiation (750–1500 mµ) produces no iris contraction in the typically nocturnal long-eared owl even when the energy content is millions of times greater than that of green light which easily elicits a pupil change. The energies in different parts of the visible spectrum required for a minimal iris response yield a spectral visibility curve for the owl which is the same as the human visibility curve at low light intensities. Functionally, the owl''s vision thus corresponds to the predominantly rod structure of its retina, and the idea that nocturnal owls have a special type of vision sensitive to infrared radiation for seeing in the woods at night is erroneous.     

FURTHER STUDIES ON THE INHIBITION OF CYPRIDINA LUMINESCENCE BY LIGHT,WITH SOME OBSERVATIONS ON METHYLENE BLUE

1. Eosin, erythrosin, rose bengale, cyanosin, acridine, and methylene blue act photodynamically on the luminescence of a Cypridina luciferin-luciferase solution. In presence of these dyes inhibition of luminescence, which without the dye occurs only in blue-violet light, takes place in green, yellow, orange, or red light, depending on the position of the absorption bands of the dye. 2. Inhibition of Cypridina luminescence without photosensitive dye in blue-violet light, or with photosensitive dye in longer wave-lengths, does not occur in absence of oxygen. Light acts by accelerating the oxidation of luciferin without luminescence. Eosin or methylene blue act by making longer wave-lengths effective, but there is no evidence that these dyes become reduced in the process. 3. The luciferin-oxyluciferin system is similar to the methylene white-methylene blue system in many ways but not exactly similar in respect to photochemical change. Oxidation of the dye is favored in acid solution, reduction in alkaline solution. However, oxidation of luciferin is favored in all pH ranges from 4 to 10 but is much more rapid in alkaline solution, either in light or darkness. There is no evidence that reduction of oxyluciferin is favored in alkaline solution. Clark''s observation that oxidation (blueing) of methylene white occurs in complete absence of oxygen has been confirmed for acid solutions. I observed no blueing in light in alkaline solution.     

THE KINETICS OF PENETRATION : XX. EFFECT OF pH AND OF LIGHT ON ABSORPTION IN IMPALED HALICYSTIS

The rate of entrance of electrolyte and of water into impaled cells of Halicystis Osterhoutii is unaffected by raising the pH of the sea water to 9.2 or lowering it to 7.0. It is quite possible that sodium enters by combining with an organic acid HX produced by the protoplasm. If the pK'' of this acid is sufficiently low the change in external pH would not produce much effect on the rate of entrance of sodium. The rate of entrance of electrolytes is affected by light. In normal light (i.e. natural succession of daylight and darkness) the rate is about twice as great as in darkness.     

Regulation of swelling of etiolated-wheat-leaf protoplasts by phytochrome and gibberellic acid

The effect of light on the size of intact protoplasts isolated from the primary leaves of etiolated Triticum aestivum was studied. A 2-min red-light irradiation in the presence of 1 mM KCl was sufficient to cause a swelling of protoplasts compared with those maintained in darkness. The effect was photoreversible by far-red light over two light cycles, indicating the involvement of phytochrome. At 4°C, escape from reversibility occurred between 2 and 5 min after the exposure to red light. In exposure-response experiments, 20 s red light at 27 μmol m^-2s^-1 was sufficient to saturate the response. Exogenous gibberellic acid added in darkness in the presence of KCl also induced protoplast swelling. Gibberellins may act as an intermediate in the phytochrome-induced swelling of protoplasts.