ANALYSIS OF THE GEOTROPIC ORIENTATION OF YOUNG RATS. VII

The intraperitoneal injection of standard young rats of race A with 2/5 cc. of adrenalin chloride 1:50,000 results in increased speed of geotropically oriented creeping upon an inclined surface. It was expected that the effect of such increased frequency of stepping must be analogous to that due to imposition of added loads carried by the rats during geotropic progression. This is verified. The curve connecting θ with log sin α is distorted, under adrenalin, so as to be comparable to that obtained with an added mass of approximately 2.5 gm. upon the young rat''s saddle; the threshold slope of surface for orientation is accordingly lowered, from α = 20° to α = 12.5°; at the new threshold slope of surface the mean orientation angle θ is the same as in the absence of adrenalin at the corresponding threshold slope of surface. The total variation of performance is significantly increased in the injected rats, and at given slope of surface the variation is slightly increased. The proportionate modifiable variation of response is quite unaffected by the distortion of the θ – α curve, and is the same as in standard young A rats untreated or carrying additional loads. It is pointed out that for the consideration of the problem as to whether a given experimental treatment, or a given natural situation, affects in any way the variation of performance of a living system, it is necessary to obtain indices of variability which involve the expression of variation of performance as a function of measured conditions governing the performance.     

GEOTROPISM AND MUSCLE TENSION IN HELIX

1. The snail Helix aspersa Müller, is negatively geotropic during the daytime, but positive or indifferent at night. 2. The precision of geotropic orientation is a function of the gravity component acting on the body. 3. The rate of geotropic locomotion is also determined by the gravity component (sine of the angle of inclination). 4. The rate of upward movement is increased 1.51 times at 45° inclination by loading the snail with one-half its weight. No such increase is seen in loaded snails creeping on a horizontal surface. 5. Moderate centrifugation results in orientation and locomotion towards the center of rotation. 6. A response analogous to the homostrophic reflex occurs when a backward pull to right or to left is exerted on the shell. Bilaterally equal tension applied to the shell causes locomotion along a path parallel and opposite to the direction of the pull. 7. All the observations go to show that the stimulus for geotropic orientation and locomotion is tension of the body muscles produced by the downward pull of gravity, and that the stimulus is received by the proprioreceptors of these muscles. Otolith apparatus and analogous organs, when present, may assist in the response, but they do not seem to be requisite in all cases. Since the precision of orientation and the rate of locomotion are functions of the gravity component acting on the body, the muscle tension theory of the geotropic reactions accords fully with Loeb''s tropism doctrine for animals.     

GEOTROPIC ORIENTATION IN ARTHROPODS : III. THE FIDDLER CRAB UCA.

On an inclined surface the fiddler crab Uca pugnax, during sidewise progression, orients upward through an angle θ on the surface. The extent of negatively geotropic orientation (θ) is a rectilinear function of sin α, where α is the inclination of the surface to the horizontal. This equation differs from that describing the geotropic orientation of various other animals. The difference is traced to the fact that from an initial position with the transverse axis of the body horizontal the crab is required to turn upward to an extent such that the vertical line from its center of gravity pierces the inclined surface within the base of support provided by the legs. This leads to the equation sec θ/tan α = const., which is obeyed within the limits of precision of the measurements. This type of control of geotropic orientation represents an extension of the "muscle tension theory," and is in no sense in conflict with this view. The assumptions underlying the analytical expression connecting θ and α are verified by the asymmetry in the orientation of male fiddlers, which is shown to be due to the presence of the enlarged chela and which disappears when the claws are removed.     

GEOTROPIC ORIENTATION IN ARTHROPODS : I. MALACOSOMA LARV?.

The geotropic orientation of caterpillars of Malacosoma americana during progression upon a surface inclined at angle α to the horizontal is such that the path makes an average angle θ upward on the plane, of a magnitude proportional to log sin α. More precisely, the product (sin α) (sin θ) is constant. This is traced to the fluctuation of the pull of the head region upon the lateral musculature of the upper side during the side to side swinging implicated in progression.     

ON THE EQUILIBRATION OF GEOTROPIC AND PHOTOTROPIC EXCITATIONS IN THE RAT

The intensity of light required to just counterbalance geotropic orientation of young rats, with eyelids unopened, is so related to the angle of inclination (α) of the creeping plane that the ratio log I/log sin α is constant. This relationship, and the statistical variability of I as measured at each value of α, may be deduced from the known phototropic and the geotropic conduct as studied separately, and affords proof that in the compounding of the two kinds of excitation the rat is behaving as a machine.     

Non-climatic thermal adaptation: implications for species' responses to climate warming

There is considerable interest in understanding how ectothermic animals may physiologically and behaviourally buffer the effects of climate warming. Much less consideration is being given to how organisms might adapt to non-climatic heat sources in ways that could confound predictions for responses of species and communities to climate warming. Although adaptation to non-climatic heat sources (solar and geothermal) seems likely in some marine species, climate warming predictions for marine ectotherms are largely based on adaptation to climatically relevant heat sources (air or surface sea water temperature). Here, we show that non-climatic solar heating underlies thermal resistance adaptation in a rocky–eulittoral-fringe snail. Comparisons of the maximum temperatures of the air, the snail''s body and the rock substratum with solar irradiance and physiological performance show that the highest body temperature is primarily controlled by solar heating and re-radiation, and that the snail''s upper lethal temperature exceeds the highest climatically relevant regional air temperature by approximately 22°C. Non-climatic thermal adaptation probably features widely among marine and terrestrial ectotherms and because it could enable species to tolerate climatic rises in air temperature, it deserves more consideration in general and for inclusion into climate warming models.     

ANALYSIS OF THE GEOTROPIC ORIENTATION OF YOUNG RATS. IX

Adult hybrid rats from the cross of races A x B show a total capacity to vary their geotropic performance which is identical with that of their B parents. The proportionate modifiable variability of geotropic orientation also agrees quantitatively with that for the B parents. These relationships are not disturbed by the action of adrenin, which leads to a distortion of the θ vs. α curve. Young rats of the F ₁ generation show a greater proportion of unmodifiable variation of geotropic orientation. It is pointed out that the present findings support the conclusion that the capacity of rats to exhibit variation of geotropic orientation is limited by their genetically determined composition and that the special condition in the young hybrids may be understood as due to a kind of temporary disharmony of developmental events.     

GEOTROPISM OF AGRIOLIMAX

On an inclined glass plate the slug Agriolimax orients and creeps upward or downward. The angle of orientation on the plane (θ) is proportional to the logarithm of the component of gravity in the creeping plane. The coefficient of variability of the measured values of (θ) decreases linearly as the logarithm at the gravity component in the creeping plane increases. The cosine of the angle of orientation decreases almost directly in proportion to the sine of the angle of inclination of the creeping plane to the horizontal, as previously found for young rats (Crozier and Pincus). But a more satisfactory formulation for the present case shows that the sine of the angle of orientation (θ) decreases in direct proportion to the increase of the reciprocal of the sine of the angle of inclination of the creeping plane. This formulation is derived from the theory that the geotropic orientation is limited by the threshold difference between the pull of the body mass on the mutually inclined longitudinal muscles at the anterior end of the slug.     

GEOTROPIC CREEPING OF YOUNG RATS

The rate of upward creeping in negatively geotropic rats aged 13 to 14 days is a function of the gravitational stimulus. The rate of upward movement on the creeping plane, like the angle of orientation, is directly proportional to the logarithm of the gravity component. The variability in the speed of creeping decreases in proportion to the logarithm of the gravitational effect. When weights are attached to the animals'' tails the rate of upward creeping varies almost directly as the logarithm of the attached weight, and the speed of creeping is still proportional to the angle of upward orientation.     

ANALYSIS OF THE GEOTROPIC ORIENTATION OF YOUNG RATS. X

The inheritance of elements of geotropic performance in lines of rats (A and B) has been investigated by examining the orientation of young offspring produced in matings of F_1(AxB) with A: Previous studies had shown that the three recognizable groups of receptor elements concerned in geotropic orientation in each of these lines appeared to be inherited in such a way that B groups were dominant with respect to A groups, although this was to a minor extent complicated by influences affecting the variation of orientation as well as the exact form of the curve relating orientation angle (θ) to slope of surface. In the backcross F₁ x A, therefore, at least eight different types of curves were to be expected. These are in fact identifiable among the forty-one individuals carefully studied. Their classification is concordant with the behavior of the respective indices of variation of θ, for which an interpretation has been provided. The basic result is, therefore, that the three receptor groups of excitation units are inherited independently, and alternatively as regards the members of a homologous pair, and that rather simple dominance relations obtain between homologous groups from the two races, namely that a B effect is dominant over the homologous A effect. This interpretation has been tested in various ways, and is in principle completely consistent with the results of a similar experiment involving races A and K.     

THE GEOTROPIC RESPONSE IN ASTERINA

Upon a surface inclined at angle α Asterina gibbosa orients upward during negatively geotropic creeping until the average angle (θ) of the path is such that Δ sin θ/Δ sin α = const. This is true also in positively geotropic movement. The direction of orientation may be temporarily reversed by mechanical disturbance. The variation of θ is greater at low slopes. Tests with directed impressed pulls, due to an attached cork float, show that the pull upon the tube feet is of primary consequence for the determination of θ. When the component of gravitational pull in the direction of movement reaches a fraction of the total pull which is proportional to the gravitational vector parallel to the surface, the laterally acting component is ineffective. On this basis, it follows that Δ sin θ/Δ sin α = const.     

GEOTROPIC ORIENTATION OF YOUNG MICE

The geotropic orientation of young mice, on a plane at angles between 20° and 50° to the horizontal, obeys the equations previously found for young rats by Crozier and Pincus (1926–27). When the individuals tested in such experiments are not of the utmost uniformity, the variability of the measured orientations is increased.     

Orientation and Geotropic Reaction of Seminal Roots of Wheat

The geotropic orientation of seminal roots of wheat has been studied on seedlings grown in five different positions, stationary and on clinostats. The roots perceive a geoinduc-tion before they have emerged from the grain and perform curvatures inside the grain. These are very sharp and transient, the following root growth is straight in any direction unless the positions are shifted. The roots are insensitive to a static gravi-induction but react to a change in gravitation with a geotropic curvature in positive direction. The roots may not reach or reach, or even pass the plumb-line. The orientation of a root depends upon the direction of its initiation and the geotropic curvature attained before the reaction has ceased. There is no nastic component in the reactions. The ‘plagiotropic’ orientation is explained by the limited positive reaction followed by an ageotropic state. Main root and adventitious roots react in the same way. Reactions to later stimuli give likewise limited curvatures which are weaker but of longer duration. — The effect of temperatures from 10°C to 25°C has been studied and compared to the temperature effect on cell elongation. It is concluded that the whole reaction may be explained by the regular auxin effects on cell elongation. No other hormone should be required and no plagiotropic mechanism is necessary.     

ON THE GEOTROPIC ORIENTATION OF HELIX

The snail Helix nemoralis in negatively geotropic creeping orients upward upon an inclined surface until the angle of the path of progression (θ) is related to the tilt of the surface (α) as (Δ sin θ) (Δ sin α) = – const.; θ is very nearly a rectilinear function of log sin α. The precision of orientation (P.E._θ) declines in proportion to increasing sin α, P.E._θ/^θ in proportion to θ. These facts are comprehensible only in terms of the view that the limitation of orientation is controlled by the sensorial equivalence of impressed tensions in the anterior musculature.     

ANALYSIS OF THE GEOTROPIC ORIENTATION OF YOUNG RATS. VIII

The upward geotropic orientation (angle θ) of adult rats (race A) has been measured as a function of slope of substratum. The relative variation of orientation angle is a declining rectilinear function of θ. The fraction of the total observable variation of performance (θ) which is controlled by the intensity of excitation (56 per cent) is identical with that found for young rats of the same strain, although the total variation is a little greater. Injection of adrenin distorts the θ vs. α graph in a manner quite concordant with the effect obtained in young rats. With the adults the absolute magnitudes of the variations of θ, at corresponding intensities of excitation, are not affected by the action of adrenin, and, as with the young, the proportion of modifiable variation of θ is not altered. The variability of performance, considered as a function of the performance, must therefore be regarded as an organic invariant. Certain consequences of this finding are referred to.     

GEOTROPIC ORIENTATION IN ARTHROPODS : II. TETRAOPES.

The creeping of the beetle Tetraopes tetraopthalmus during negatively geotropic orientation shows the angles of orientation (θ) on a surface inclined at α° to the horizontal to be proportional to sin α. The direction of orientation easily suffers temporary reversal to positive as result of handling. Mechanical stability during upward progression should be just possible when K ₁ cot α = K ₂ sin θ + K ₃ cos θ, the weight of the body being supported on the tripod formed by the legs on either side and by the posterior tip of the abdomen. Lack of this stability produces tensions on the legs through (1) the bilaterally distributed pull of the body mass on the legs, and (2) the torque on the legs due to the weight of the abdomen. The downward gravitational displacement of the tip of the abdomen causes K ₂ and K ₃ in the preceding formula to be functions of α. These relations have been tested in detail by shifting the location of the center of gravity, by attaching additional masses anteriorly and posteriorly, and by decreasing the total load through amputation of the abdomen; the latter operation changes the conditions for stability. Different formulæ are thus obtained (cf. earlier papers) for the orientation of animals in which the mechanics of progression and the method of support of the body weight on an inclined surface are not the same. This demonstrates in a direct way that the respective empirical equations cannot be regarded as accidents. The results are in essence the same as that already obtained with young mammals. The diversity of equations required for the physically unlike cases merely strengthens the conception of geotropic orientation as limited by the tensions applied to the musculature of the body (caterpillars, slugs) or of appendages (beetles, and certain other forms) when the body is supported upon an inclined surface, since equations respectively pertaining to the several instances, and satisfactorily describing the observations, are deduced on this basis.     

The relationship between cell division and elongation during development of the nectar-yielding petal spur in Centranthus ruber (Valerianaceae)

Background and Aims Floral spurs are hollow, tubular outgrowths that typically conceal nectar. By their involvement in specialized pollinator interactions, spurs have ecological and evolutionary significance, often leading to speciation. Despite their importance and diversity in shape and size among angiosperm taxa, detailed investigations of the mechanism of spur development have been conducted only recently.Methods Initiation and growth of the nectar-yielding petal spur of Centranthus ruber ‘Snowcloud’ was investigated throughout seven stages, based on bud size and developmental events. The determination of the frequency of cell division, quantified for the first time in spurs, was conducted by confocal microscopy following 4'',6-diamidino-2-phenylindole (DAPI) staining of mitotic figures. Moreover, using scanning electron microscospy of the outer petal spur surface unobstructed by trichomes, morphometry of epidermal cells was determined throughout development in order to understand the ontogeny of this elongate, hollow tube.Key Results Spur growth from the corolla base initially included diffuse cell divisions identified among epidermal cells as the spur progressed through its early stages. However, cell divisions clearly diminished before a petal spur attained 30 % of its final length of 4·5 mm. Thereafter until anthesis, elongation of individual cells was primarily responsible for the spur’s own extension. Consequently, a prolonged period of anisotropy, wherein epidermal cells elongated almost uniformly in all regions along the petal spur’s longitudinal axis, contributed principally to the spur’s mature length.Conclusions This research demonstrates that anisotropic growth of epidermal cells – in the same orientation as spur elongation – chiefly explains petal spur extension in C. ruber. Representing the inaugural investigation of the cellular basis for spur ontogeny within the Euasterids II clade, this study complements the patterns in Aquilegia species (order Ranunculales, Eudicots) and Linaria vulgaris (order Lamiales, Euasterids I), thereby suggesting the existence of a common underlying mechanism for petal spur ontogeny in disparate dicot lineages.     

Investigations of the Geotropic Curvature of the Avena Coleoptile

The geotropic reaction in Avena coleoptiles is studied as a function of the stimulation time. The direction of the stimulation with respect to the vascular bundles must be defined when studying geotropic responses. It is found that the threshold time to evoke geotropic response is less than half a minute, i.e., at least ten times lower than the presentation time usually reported in the literature. An extrapolation procedure can be used to give a so-called extrapolated presentation time t_b, which is intimately related to the logarithmic part of the geotropic response curve and has a physical meaning in the reciprocity rule. The problem of the duration of the true threshold time for stimulation with 1 g is discussed. An experiment indicates that it is not necessary for mass particles (“statoliths”) to settle on the lateral cell wall in order to start the geotropic reaction chain. The slope of the logarithmic part of the geotropic response curve is independent of the transverse force applied to the coleoptiles. Support is given to the view that the slope is determined by the number of sedimenting mass particles.     

THE EFFECT OF EXPOSURE PERIOD AND TEMPERATURE ON THE PHOTOSENSORY PROCESS IN CIONA

1. Experiments are presented which show that the latent period in the photosensory response of Ciona is inversely proportional to the duration of the exposure period to light. From this it is found that the velocity of the chemical reaction which determines the latent period is directly proportional to the concentration of photochemical products formed during the exposure period. This is interpreted as showing that the two processes form a coupled photochemical reaction, of which the secondary reaction proceeds only in the presence of products from the primary reaction. This coupling may be a catalysis or a direct chemical relation. 2. Further experiments show that the relation between temperature and the latent period is accurately described by the Arrhenius equation in which µ = 16,200. The precise numerical value of µ tentatively identifies the latent period process as an oxidation reaction which is catalyzed by iron. 3. The photocatalytic properties of certain iron compounds are used as a model for the coupled photochemical reaction suggested for the photosensory mechanism of Ciona and Mya.     

Movements of genes between populations: are pollinators more effective at transferring their own or plant genetic markers?

The transfer of genes between populations is increasingly important in a world where pollinators are declining, plant and animal populations are increasingly fragmented and climate change is forcing shifts in distribution. The distances that pollen can be transported by small insects are impressive, as is the extensive gene flow between their own populations. We compared the relative ease by which small insects introduce genetic markers into their own and host-plant populations. Gene flow via seeds and pollen between populations of an Asian fig species were evaluated using cpDNA and nuclear DNA markers, and between-population gene flow of its pollinator fig wasp was determined using microsatellites. This insect is the tree''s only pollinator locally, and only reproduces in its figs. The plant''s pollen-to-seed dispersal ratio was 9.183–9.437, smaller than that recorded for other Ficus. The relative effectiveness of the pollinator at introducing markers into its own populations was higher than the rate it introduced markers into the plant''s populations (ratio = 14 : 1), but given the demographic differences between plant and pollinator, pollen transfer effectiveness is remarkably high. Resource availability affects the dispersal of fig wasps, and host-plant flowering phenology here and in other plant–pollinator systems may strongly influence relative gene flow rates.