Optomotorische Untersuchungen am visuellen System der Stubenfliege Musca domestica L

The compound eye of the housefly Musca domestica L. contains two different types of receptors. The visual acuity of the eye is determined by the divergence angle Δ? between the optical axes of neighbouring ommatidia. Δ? and its dependence on the mean pattern brightness is determined by an evaluation of the optomotor responses elicited from various test patterns. Based on the assumption that the visual fields of both types of receptors approximate the shape of a spatial Gaussian distribution they can be characterized by their half-width, designated as the acceptance angle ΔQ. The contrast transfer from the optical environment onto the receptor cells is limited by ΔQ. It is shown experimentally that ΔQ depends on the mean environmental brightness. The characteristic values Δ? and ΔQ constitute the limiting factors for the light flux received by the receptors. The light flux Φ exciting the receptor cells is proportional to (ΔQ·Δ?)². If the product ΔQ·Δ? is kept constant, there exists a certain ratio \(\frac{{\Delta _\rho }}{{\Delta _\varphi }}\) that leads to an optimal combination of both, contrast transfer and resolution. The ratio \(\frac{{\Delta _\rho }}{{\Delta _\varphi }}\) is experimentally determined and compared with the optimal condition. The torque exerted by fixed flying Muscae has been used as a measure of the reaction strength of the optomotor response elicited by the rotation of cylindrical patterns consisting of periodic distributions of surface brightness. The responses were investigated under different spatial wavelengths, contrasts, contrast frequencies and mean pattern brightness. Detailed results are:

1. The visual acuity (optical resolution power) of the compound eye of Musca is determined by the divergence angle Δ _? between the optical axes of those adjacent ommatidia which are not positioned in the same horizontally oriented row but — closer together — in neighboured rows.
2. Δ? and consequently also the visual acuity do not depend on the mean environmental brightness.
3. The acceptance angle ΔQ changes with the mean brightness of the environment. According to experimental conditions only the minimal acceptance angle Δ _min can be experimentally determined. Δ _min decreases with increasing mean pattern brightness from 3.6°–4.1° to 1.7°.
4. The decrease of ΔQ _min with increasing mean pattern brightness is not caused by a change of the acceptance angles of single receptors. The present tentative explanation is that the centrally located receptors No. 7 and 8 are participating in the uptake of relevant visual information at a critical brightness level.
5. Near the optomotor threshold the large acceptance angle ΔQ _min=3.6° at very dim light would thus be associated with the receptors No. 1 to 6, whereas the smaller acceptance angle ΔQ _min=1.7° with the receptors No. 7 and 8.
6. Due to a sample spacing of Δ?=2°, the acceptance angles of neighbouring receptors No. 1 to 6 show a considerable overlap.
7. Based on anatomical data, the difference in absolute light sensitivity for both receptor systems is calculated. It is predicted that the absorption rate of light quanta in the less sensitive system of the receptors No. 7 and 8 should be reduced by a factor of 24–48 compared to the more sensitive system of the receptors No. 1 to 6. This factor nicely meets the experimentally determined brightness thresholds of both receptor systems.
8. The optimal condition \(\frac{{\Delta _\rho }}{{\Delta _\varphi }}\) is nearly fulfilled by the receptor system No. 7 and 8 of Musca. The experimentally determined ratio amounts to \(\frac{{\Delta _\rho }}{{\Delta _\varphi }}\) =0.83. For the receptor system No. 1 to 6 one finds \(\frac{{\Delta _\rho }}{{\Delta _\varphi }}\) =1.86; in that system the transfer of spatial wavelengths is mainly limited by the reduced contrast transfer which drops to low values before the optical resolution limit is reached.
9. Based on the hypothesis that movement perception of the fly Musca is due to a correlation of sensory data one would expect an optomotor peak reaction at a spatial wavelength of λ _max=8° and a decrease of the optomotor response towards longer spatial wavelengths. The experimental data are in conflict with these predictions. The present notion is that the absence of the expected reaction decrease is not likely to be caused by a saturation effect in the reaction but rather is explainable in terms of a receptor system consisting of larger numbers of receptor types No. 1 to 6 whose excitations being summed before a correlation evaluation takes place.

     

Unsymmetrische Projektion der Retinulazellen auf die Lamina ganglionaris bei der Fliege Musca domestica

Ohne ZusammenfassungHerrn Prof. Werner Reichardt gebührt mein herzlichster Dank für die gewährte Gastfreundschaft am Max Planck-Institut für Biologie, Tübingen. Herr Dr. G. F. Meyer sei für die Überlassung einiger Präparate sehr bedankt.     

Ordnung und Orientierung der Elemente im Sehsystem der Fliege

Summary The neural elements in the visual ganglia of insects show an uncommonly high degree of order. The mapping of the array of sensory elements in the periphery (of sampling points in the visual space) onto four successive levels of the ganglionic chain can be quite precisely described, each neuron in the ganglia being related to a point, or a set of points, in the visual field. Also some of the fibers which connect neurons related to different visual-space-points are very precisely oriented. One of these sets of fibers oriented obliquely appear to match the interactions postulated on the basis of one of Götz's (1968) models of movement perception in flies. Some embryological questions are also raised by the high degree of order and by the curious mirror symmetry with respect to the mid-sagittal plane on one hand and to the equatorial plane on the other, which pervades the whole system.     

Survey of Mariner-Like Elements in the Housefly, Musca Domestica

The presence of mariner-like elements in four strains of the housefly, Musca domestica, was surveyed by PCR. Using the inverted terminal repeat (ITR) sequences of the Mos 1element as primers, DNAs were successfully amplified from all strains of the housefly. Southern blot analysis indicated that these amplified DNAs were repetitive sequences in the genome of M. domestica. Sequence analyses of cloned PCR products showed that they were 45% identical to the Mos 1element. These fragments appeared to be nonfunctional, because they contained no intact open reading frame (ORF) capable of encoding transposase. We conclude that these DNAs are degraded mariner-like elements (MLEs) in M. domestica. Because these endogenous MLEs in M. domesticado not encode any functional proteins, they probably would not affect the behavior of mariner-based vectors if such were introduced into this species as transformation vectors.     

The Y-Chromosomal and Autosomal Male-Determining M Factors of Musca Domestica Are Equivalent

In Musca domestica, male sex is determined by a dominant factor, M, located either on the Y, the X or on an autosome. M prevents the activity of the female-determining gene F. In the absence of M, F becomes active and dictates female development. The various M factors may represent translocated copies of an ancestral Y-chromosomal M. Double mutants and germ line chimeras show that M(Y), M(I), M(II), M(III) and M(V) perform equivalent functions. When brought into the female germ line, they predetermine male development of the offspring. This maternal effect is overruled by the dominant female-determining factor F(D). M(I) and M(II) are weak M factors, as demonstrated by the presence of yolk proteins in M(I)/+ males and by the occurrence of some intersexes among the offspring that developed from transplanted M(I)/+ and M(II)/+ pole cells. The arrhenogenic mutation Ag has its focus in the female germ line and its temperature-sensitive period during oogenesis. We propose that M(I) and Ag represent allelic M factors that are affected in their expression. Analysis of mosaic gonads showed that in M. domestica the sex of the germ line is determined by inductive signals from the surrounding soma. We present a model to account for the observed phenomena.     

Hermes, a Functional Non-Drosophilid Insect Gene Vector from Musca Domestica

Hermes is a short inverted repeat-type transposable element from the house fly, Musca domestica. Using an extra-chromosomal transpositional recombination assay, we show that Hermes elements can accurately transpose in M. domestica embryos. To test the ability of Hermes to function in species distantly related to M. domestica we used a nonautonomous Hermes element containing the Drosophila melanogaster white (w(+)) gene and created D. melanogaster germline transformants. Transgenic G(1) insects were recovered from 34.6% of the fertile G(0) adults developing from microinjected w(-) embryos. This transformation rate is comparable with that observed using P or hobo vectors in D. melanogaster, however, many instances of multiple-element insertions and large clusters were observed. Genetic mapping, Southern blotting, polytene chromosome in situ hybridization and DNA sequence analyses confirmed that Hermes elements were chromosomally integrated in transgenic insects. Our data demonstrate that Hermes elements transpose at high rates in D. melanogaster and may be an effective gene vector and gene-tagging agent in this species and distantly related species of medical and agricultural importance.     

家蝇幼虫差异表达基因的克隆筛选与分析

目的是利用mRNA差异显示(DDRT-PCR)技术对诱导后家蝇幼虫差异表达基因进行克隆分析。取诱导和未诱导家蝇(Musca domestica vicina)三龄幼虫总RNA进行mRNA差异显示反应,产物经6％非变性聚丙烯酰胺凝胶电泳展开,银染分析后,回收差异显示条带。随机选取4条诱导家蝇三龄幼虫中上调表达的基因条带,进行Northern杂交验证,有2条被验证为真实带,命名为YD1和YD2。对这2条基因片段进行T-A克隆和测序,长度分别为495bp和265bp,登录NCBI运用Blastn程序进行同源性比较,发现两者与任何已知基因同源性都低于70％,提示为两个未报道的新基因,在家蝇免疫反应中可能起着一定的作用。     

Die Feinstruktur der tarsalen Schmeckhaare der Fliege Phormia terraenovae Rob.-Desv.

Zusammenfassung Die Schmeckhaare der Fliege Phormia terraenovae enthalten fünf Rezeptorzellen (vier Kontaktchemo- und eine Mechanorezeptorzelle), die von zwei Hüllzellen unter Aussparung von Lymphräumen (Vakuole) umgeben sind. Die Rezeptorzellen gliedern sich in Axon, Zellkörper und Dendrit, der aus Innenglied, Übergangsabschnitt und Außenglied besteht. Der Übergangsabschnitt hat die 9+0-Struktur der modifizierten Cilien mit zwei Basalkörpern. Im Plasma der Außenglieder sind nur Mikrotubuli zu erkennen. Die Außenglieder der Kontaktchemorezeptoren ziehen durch die röhrenförmige Dendritenscheide und den Kanal I des Haarschafts bis zu einem Porus in der Haarspitze, durch den sie mit den Schmeckstoffen in Kontakt kommen (Ort der Primärprozesse). Das Außenglied der Mechanorezeptorzelle enthält einen tubulären Körper und setzt an der Haarbasis an. Die Hüllzelle II (tormogene Zelle) bildet zum großen Sensillenlymphraum hin ein stark gefaltetes Membransystem aus. Der Feinbau wird in Beziehung zur Funktion der Schmeckhaare gesetzt.

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The fine structure of the tarsal taste hairs of the blowfly, Phormia terraenovae Rob.-Desv.

Summary The tarsal taste hairs of the blowfly contain five receptor cells: four contact chemoreceptors and one mechanoreceptor. These cells are surrounded concentrically by two sheath cells with fluid-filled spaces (vacuoles) between them. The receptor cell parts are the axon, the cell body, and the dendrite. The latter has an inner segment, a transitional region, and an outer segment. The transitional region shows the 9+0 configuration of a modified cilium with two basal bodies. In the outer segment only microtubules are found. The outer segments of contact chemoreceptors pass through the tube-like dendrite sheath, then through canal I of the hair shaft, to reach the pore at the tip of the hair. The outer segment of the mechanoreceptor contains a tubular body and terminates at the hair base. The sheath cell II (tormogene cell) forms a folded membrane system toward the main liquor space. The fine structure of the tarsal taste hairs is discussed in relation to their function.

Den Herren Dr. W. Pflumm, Prof. Dr. U. Thurm, Prof. Dr. E. Schnepf und Dr. K. E. Kaissling sei für anregende Diskussionen und der Deutschen Forschungsgemeinschaft für eine Sachbeihilfe gedankt. Frau R. Föhr danken wir für ausgezeichnete technische Mitarbeit.     

Zur Reizwirksamkeit von Monosacchariden bei der Fliege Phormia terraenovae

Zusammenfassung Verschiedene Monosaccharide (Tabelle 1) wurden durch Berühren der Tarsen hungriger Fliegen und Beobachten der Rüsselreaktion auf ihre Reizwirksamkeit geprüft. Diese wird zur Konformation der Zucker in Beziehung gesetzt.Nach der vorgeschlagenen Hypothese werden Monosaccharide mit C1-Konformation am Glucose-(d.h. C1-)Bindungsort, die in 1C-Konformation vorliegenden am Fructose-(d.h. 1C-) Bindungsort angelagert.Die Bedeutung der an den verschiedenen C-Atomen des pyranoiden Rings sitzenden Gruppen für die Reizwirksamkeit des Moleküls wird diskutiert. Bei den Monosacchariden mit C1-Konformation ist die Position der OH-Gruppe an C-3 für die Reizwirksamkeit entscheidend, deren Stärke hängt jedoch auch vom Vorhandensein oder Fehlen anderer Gruppen und deren Stellung ab. Bei den Monosacchariden mit 1C-Konformation kommt die Schlüsselstellung der OH-Gruppe am zweiten Ring-C-Atom zu.Die für den Geschmackssinn von Phormia aufgestellte Hypothese wird durch Heranziehen von Literaturangaben auf den anderer Insekten angewendet.

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The stimulation effectiveness of monosaccharides in the fly Phormia terraenovae

Summary The stimulation effectiveness of different monosaccharides (Table 1) was studied by touching the tarsi of hungry flies with solutions of the monosaccharides and observing the extension of the proboscis. The effectiveness was related to the conformation of the sugars.According to the given hypothesis, monosaccharides in the C1 conformation are attached at the glucose (i.e. C1) site, those in the 1C conformation at the fructose (i.e. 1C) site.The importance of the groups on the different carbon atoms of the pyranose ring for the stimulation effectiveness of the molecule is discussed. For monosaccharides in the C1 conformation, the stimulating effectiveness depends on the orientation of the hydroxyl group on C-3 and also on the presence and position of other groups. As far as the monosaccharides in the 1C conformation are concerned, the orientation of the hydroxyl group on the second carbon atom of the ring is of major importance.The hypothesis given for the sense of taste of Phormia is applied to that of other insects using data given by other workers in the literature.

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Mit Unterstützung durch die Deutsche Forschungsgemeinschaft.     

The Mutation Masculinizer (Man) Defines a Sex-Determining Gene with Maternal and Zygotic Functions in Musca Domestica L

In Musca domestica, the primary signal for sex determination is the dominant factor M, which is assumed to regulate a postulated female-determining gene F. Presence of M prevents expression of F so that male development ensues. In the absence of M, F can become active, which dictates the female pathway. The existence of F is inferred from F(D), a dominant factor that is epistatic to M. We describe a new mutation masculinizer, which has all the properties expected for a null or strongly hypomorphic allele of F: (1) it maps to the same chromosomal location as F(D), (2) homozygous man/man animals develop as males, (3) homozygous man/man clones generated in man/+ female larvae differentiate male structures, (4) man has a sex-determining maternal effect. About a third of the morphological males synthesize yolk proteins, which indicates that they are intersexual in internal structures. The maternal effect of man is complete in offspring that derive from homozygous man/man pole cells transplanted into female hosts. In this case, all man/+ progeny become fertile males that do not produce yolk proteins. A sex-determining maternal effect has previously been demonstrated for F(D). Like F, maternal man(+) is needed for zygotic man(+) to become active, providing further evidence that man is a loss-of-function allele of F.     

Farbensinn der Tiere und optomotorische Reaktionen

Ohne ZusammenfassungHerr Prof. v.Buddenbrock, Kiel, hatte die Güte das Manuskript durchzusehen. Ich danke ihm auch an dieser Stelle herzlich dafür.     

Developmental Analysis of Two Sex-Determining Genes, M and F, in the Housefly, Musca Domestica

In the housefly, Musca domestica, a single dominant factor, M, determines maleness. Animals hemior heterozygous for M are males, whereas those without M develop as females. In certain strains, however, both sexes are homozygous for M, and an epistatic dominant factor, F(D), dictates female development. The requirement for these factors was analyzed by producing, with mitotic recombination, mosaic animals consisting of genetically male and female cells. Removal of F(D) from an M/M;F(D)/+ cell at any time of larval development, even in the last larval instar, resulted in sex-reversal, i.e., in the development of a male clone in an otherwise female fly. In contrast, when M was removed from M/+ cells, the resulting clones remained male despite their female genotype, even when the removal of M happened at embryonic stages. The occurrence of spontaneous gynandromorphs, however, shows that the loss of M in individual nuclei prior to blastoderm formation causes the affected cells to adopt the female pathway. These results are consistent with the hypothesis that M is the primary sex-determining signal which sets the state of activity of the key gene F at around the blastoderm stage. Parallels and differences to the sex-determining system of Drosophila are discussed.     

Die Reaktionen von vier Termitenarten in der Feuchtigkeitsorgel

Zusammenfassung Es wird über das Verhalten von Termitengruppen gegenüber einem Luftfeuchtigkeitsgefälle (15-90-15% rel. LF) in einer geraden Kammer berichtet. Die Kontrollversuche ohne Luftfeuchtigkeitsgradienten ergeben einen deutlichen Vorzug der beiden Orgelenden. Bedingt durch soziale Faktoren reagieren meist alle Versuchstiere als geschlossene Gruppe. Zur Auswertung der Feuchtigkeitsversuche wird deshalb bei größeren Serien die Verteilung der Termiten in den trockenen und in den feuchten Stufen der Orgel beobachtet. Als Maß für die Stärke der Reaktion wird der Überschußprozentsatz der Termiten in der feuchten Zone berechnet.Die dem feuchten Zuchtmilieu entnommenen Termiten verhalten sich — ausgenommenNasutitermes — anfänglich ähnlich wie die Kontrollen in der isohygren Kammer, und ihr Überschußprozentsatz erreicht erst später positive Werte. Eine deutlich positive Reaktion nach den hohen Luftfeuchtigkeitsstufen in der Orgelmitte kommt dann zustande, wenn das Feuchtigkeitsbedürfnis der Versuchstiere die thigmotaktische Reaktion nach den Enden der Orgel überwiegt. Dazu ist es notwendig, die Tiere vor dem Versuch in trockenem Milieu zu halten, wobei die Intensität der positiven Reaktion von der Dauer der vorausgegangenen Austrocknung abhängig ist; solche Tiere reagieren nach einer Wasseraufnahme wieder negativ (Zootermopsis).Die untersuchten Arten unterscheiden sich durch die zur Ausbildung der positiven Reaktion benötigte Zeitdauer. Die tropische und für Trokkenheit sehr anfällige ArtNasutitermes reagiert innerhalb 1 Stunde deutlich positiv, und zwar aus trockenem und aus feuchtem Milieu mit ähnlichem überschußprozentsatz. Die untersuchten südeuropäischen ArtenKalotermes undReticulitermes benötigen 72, bzw. 48 Stunden, bis die Mehrzahl der Tiere sich in den feuchten Stufen befindet. Die amerikanische FeuchtholztermiteZootermopsis reagiert innerhalb 3–6 Stunden. Positive Werte erzielen nur ausgetrocknete Larven und Soldaten. Da ausgetrocknete Larven ohne Antennen negativ reagieren, kann geschlossen werden, daß sich die Sinnesorgane für die Luftfeuchtigkeitswahrnehmung hauptsächlich auf den Antennen befinden.

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Summary A Report will be given of the behaviour of termite-groups at a gradient of air-humidity (15-90-15 per cent rel. air-humidity) in a regular moisture chamber. The control tests without gradients of air-humidity show a distinct preference for the two ends of the Feuchtigkeitsorgel. Depending on social factors mostly all termites used as objects of this scientific experiment react as one group. For evaluating the humidity tests with large series, therefore, the distribution of the termites in the dry as well as in the humid parts of the Orgel must be considered. For measuring the degree of reaction we estimate the percentage of surplus of termites in the humid zone.The termites taken from humid rearing-surroundings behave—exceptNasutitermes—in the beginning like those controlled in the isohygrical chamber, and their percentage of surplus reaches positive values only later. A clear positive reaction toward the high degrees of air humidity in the middle of the organ takes place, when the requirement of humidity by the test-insects overrates the thigmotactic reaction toward the ends of the Orgel. For this test it is necessary to keep the insects before the experiment within dry surroundings; the intensity of the positive reaction will depend on the duration of the previously effected desiccation; such insects, after having taken up water, again react negative ly (Zootermopsis).The tested species differ by the duration of time which is necessary to show positive reaction. The tropical formNasutitermes which is very sensitive to humidity, reacts within one hour clearly positive, to dry as well as to humid surroundings with a similar percentage of surplus. The tested southern European formsKalotermes andReticulitermes, need from 72 to 48 hours till the majority of the insects is within the humid grades. The American moist-wood termite Zootermopsis reacts within 3–6 hours. Positive values are only reached by desiccated larvae and soldiers. As desiccated larvae without antennae react negatively, we can conclude that the organs of sense for air-humidity are mainly situated on the antennae.

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Résumé Nous rendrons compte du comportement de groupes de termites dans une chambre droite en présence d'une diminution de l'humidité de l'air (15-90-15% rel. d'humidité de l'air). Les expériences de contrôle faites sans graduations de l'humidité de l'air montrent une priorité très nette des deux extrémités de l'orgue hygrométrique. La plupart des insectes, sous l'influence de facteurs sociaux, réagissent d'une façon unitaire. Aussi, pour la mise en valeur des expériences sur l'humidité dans des séries plus importantes, on observe la répartition des termites dans les parties sèches et dans les parties humides de l' «orgue». Pour mesurer l'intensité de la réaction, on évalue le pourcentage de l'excédent des Termites dans les zones humides.Les termites arrachés à leur milieu de culture humide, à l'exception desNasutitermes, se comportent, au début, de la même façon que ceux que l'on examine dans la chambre isohygre, et le pourcentage de leur excédent n'atteint que plus tard des valeurs positives. Une réaction nettement positive vers les hauts degrés d'humidité de l'air au centre de l' «orgue» se produit quand le besoin d'humidité des animaux l'emporte sur la réaction thigmotactique à l'extrémité de l' «orgue». Pour cela, il est indispensable de garder les insectes, avant l'expérience, en milieu sec; au cours de celle-ci, l'intensité de la réaction positive dépendra de la durée de la dessiccation antérieure. Après nouvelle hydratation, ces, animaux réagissent de nouveau négativement (Zootermopsis).Les espèces examinées se différencient par le laps de temps nécessaire à la constitution de la réaction positive. L'espèce tropicale desNasutitermes, particulièrement sensible à la sécheresse, réagit de façon nettement positive en l'espace d'une heure en outre, en milieu sec ou humide, son pourcentage d'excédent est le même. Les espèces de l'Europe méridionaleKalotermes etReticulitermes exigent respectivement soixante-douze et quarante-huit heures jusqu'à ce que la majorité des insectes se trouve dans les parties humides. L'espèce américaine des termites de bois humide,Zootermopsis réagit en l'espace de trois à six heures. Des valeurs positives ne sont atteintes que par les larves et des soldats desséchés. Étant donné que des larves desséchées dépourvues d'antennes réagissent négativement, on a pu conclure que les organes sensoriels sensibles à l'humidité de l'air sont essentiellement les antennes.

     

Topo-optische Reaktionen an der Membran von menschlichen Erythrocytenschatten

Zusammenfassung Frühere Untersuchungen erwiesen, daß die Thiazinfarbstoffe Toluidinblau, Azur A, Azur B, 1:9-Dimethyl-Methylenblau, sowie die Chinolinfarbstoffe N,N-Diäthylpseudoisocyaninchlorid und N,N-6,6-Dichlorpseudoisocyaninchlorid für die topo-optische Reaktion an der menschlichen Erythrocytenmembran geeignet sind. In dieser Studie wird die Verwendbarkeit der Thiazin- und Chinolinfarbstoffe an den menschlichen Erythrocytenschatten gezeigt. Auf Grund der optischen Analyse sind die Thiazinfarbstoffmoleküle radiär zur Membran ausgerichtet, während sich die Chinolinfarbstoffmoleküle membranparallel anlagern.

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Topo-optical reactions on the membrane of the human red cell ghost

Summary Previous studies have proved that the thiazin dyes toluidine blue, azure A, azure B, 1.9-dimethyl methylene blue and the quinolin dyes N,N-diethylpseudoisocyanine chloride, N,N-6,6-dichlorpseudoisocyanine chloride are suitable for topo-optical reaction on the membrane of the red blood cells. In the present study the applicability of the thiazin and quinolin dyes on the membrane of the human red cell ghost was examined. Optical analysis revealed that the thiazin dyes are bound in radial position to the membrane, while the quinolin dyes are bound parallel to the membrane's plane.

     

Versuche zur Analyse der positiven und der negativen geotropischen Reaktionen von Keimwurzeln

Summary In the present work the influence of moist air, of sand and of several solutions on the geotropic behaviour of primary roots is studied. The course of the geotropic movement is the result of a concerted action of positive and negative reactions the intensity and duration of which differ in roots of various species.In pea roots the negative movement appears only during a short stage of development. No direct relation exists between the speed of elongation and the appearance of the negative reaction.Primary roots of Zea mays and of Pisum arvense are indifferent to thigmotropic stimuli. The negative movement has, at least in pea roots a smaller mechanically effective force than the positive movement. Therefore the negative reaction does not appear in sand because it cannot overcome the mechanical resistance of the granular medium.In liquid media pea roots react in another way than in air: here the negative reaction begins later, but it has then more influence on the course of the geotropic curvature.The influence of different cations on the geotropic behaviour and on the elongation of the roots can be understood as a combined action of the osmotic effect and of the specific ionic permeability.In pea roots the negative reaction, which appears during the time from 3 to 6 hours after the induction also depends on a definite level of turgor.Primary roots of Zea mays, which grow in a relatively large angle to the vertical line do not lose their geotropic sensibility. They react like plagiotropic organs.In pea roots relations exist between the development of the positive and the negative reaction and the presence of the cotyledons and the tip of the root.Both reactions are induced at the same time by the gravitional stimulus. Their reaction times, however, are different.The root tip is necessary for the induction of both reactions. The negative curvature also appears when the tip is cut off before the end of the reaction time.The course of the geotropic movement of primary roots is compared with the geotropic behaviour of rhizomes. As a possible explanation of both kinds of reactions a two-hormone-hypothesis is discussed.