Urbanization disrupts latitude‐size rule in 17‐year cicadas

Many ectotherms show a decrease in body size with increasing latitude due to changes in climate, a pattern termed converse Bergmann's rule. Urban conditions—particularly warmer temperatures and fragmented landscapes—may impose stresses on development that could disrupt these body size patterns. To test the impact of urbanization on development and latitudinal trends in body size, we launched a citizen science project to collect periodical cicadas (Magicicada septendecim) from across their latitudinal range during the 2013 emergence of Brood II. Periodical cicadas are long‐lived insects whose distribution spans a broad latitudinal range covering both urban and rural habitats. We used a geometric morphometric approach to assess body size and developmental stress based on fluctuating asymmetry in wing shape. Body size of rural cicadas followed converse Bergmann's rule, but this pattern was disrupted in urban habitats. In the north, urban cicadas were larger than their rural counterparts, while southern populations showed little variation in body size between habitats. We detected no evidence of differences in developmental stress due to urbanization. To our knowledge, this is the first evidence that urbanization disrupts biogeographical trends in body size, and this pattern highlights how the effects of urbanization may differ over a species’ range.     

Plurimodal emergence and plurivoltinism of Central European populations of Nemurella pictetii (Plecoptera: Nemouridae)

Summary Nemurella pictetii Klapálek, 1900 (Plecoptera: Nemouridae) has bi-or trimodal emergence patterns in Central Europe. The emergence threshold temperature is about 8°C. Egg development is direct. The slopes of regressions describing its temperature dependence do not differ significantly between English, Norwegian and German populations, but the intercepts (i.e., the incubation period at 0°C) do. Regular sampling of a benthic population near Schlitz, in Hesse, West Germany, showed that part of the offspring of the first emergence group grew quickly and produced a second emergence peak. The remaining offspring of the first generation grew more slowly and overwintered, together with the offspring of the second emergence peak. This population exhibits partial bivoltinism. The potential for bivoltinism was also shown in laboratory rearings of a German population at a constant 14°C: many eggs yielded normal adults after about 120 days. Larval growth rates were up to 2.2% body length day^-1, much higher than for English Nemurella (unimodal emergence), or any other stonefly. The thermal sum (degree-days above 0°C) accumulated between the two emergence peaks, at the German sites studied, resembled cumulated degree-days in a laboratory culture. For an Austrian population with trimodal emergence, degree-days accumulated in the field were lower, suggesting a lower temperature demand and partial trivoltinism. This is the only case of confirmed plurivoltinism in Plecoptera. Nemurella pictetii seems to have lost seasonal life cycle cues, enabling the opportunistic use of favourable local conditions. Population synchrony appears to be regulated by the thermal demand for development and the emergence threshold temperature.     

Synanthrope Pflanzengesellschaften aus der Burgwallzeit (8.—10. Jh.) in der Tschechoslowakei

In der Burgwallzeit (8.—10. Jahrhundert) entfalteten sich im Gebiet der CSSR praktisch alle Unkraut- und Ruderalgesellschaften (incl. Ufersäume) wie in der Neuzeit (ohne Neophyten). Die archäobotanischen Funde sind leider meist an Getreidevorräte gebunden. Nur in Mikulcice (Süd-Mähren, 8. bis 9. Jahrhundert) wurden pflanzliche Makroreste der Ruderalsteilen und anthropogen beeinflußter Ufersaumgesellschaften gefunden. In Mikulcice müssen wir neben den allgemein bekannten Segetal-Gesellschaften auch mit weiteren Typen der Unkrautgsellschaften (Hackunkraut-Gesellschaften in Weinbergen, Gemüse-und Obstgärten) sowie mit Trittrasen rechnen. Die Unkrautgesellschaften können wir als Chenopodium album—Setaria viridis (glauca-)Assoziationen bezeichnen. Weiter behandelt werden die Arten, die im Osten von Mitteleuropa häufiger sind und Facies-Unterschiede (quantitativ und qualitativ) zwischen den synanthropen Gesellschaften im Raum der CSSR und in den westlicher gelegenen Regionen (Adonis aestivalis, Fumaria vaillantii, Glaucium corniculatum, Vaccaria pyramidata, Caucalis lappula, Galium tricorne, Papaver rhoeas usw.) aufgezeigt. Die Segetal-Gesellschaften der Burgwallzeit unterscheiden sich von denen aus älteren prähistorischen Funden nur wenig.     

ERCC1‐XPF endonuclease—positioned to cut

To counteract damage to our genomes, numerous endo‐ and exonucleases incise the DNA backbone to remove damaged and aberrant DNA structures. It is imperative that such incisions be very tightly controlled, as unwanted DNA breaks are a key source of genome instability. Two new papers in The EMBO Journal shed light on how the activity of one such nuclease—ERCC1‐XPF, an enzyme involved in various DNA repair pathways—is regulated to perform incision in the vicinity of DNA interstrand crosslinks.     

Stoichiometry of interaction between soluble (—)-dopa melanin and enantiomers of ephedrine by NMR spectroscopy

Synthetic soluble (—)-dopa melanin was prepared in deuteriated buffer, pH 8, by autooxidation of the precursor. At 6 mM of the precursor, the incorporation was over 90%. The changes in the line width measurements of N-CH₃ protons of enantiomers of ephedrine in the soluble melanin were quantified by NMR spectroscopy. The dissociation constants of (—)-1R,2S-ephedrine, (+)-1S,2R-ephedrine, (—)-1R,2R-ψ-ephedrine, and (+)-1S,2S-ψ-ephedrine were 11.7, 4.20, 3.60, and 4.80 mM, respectively. Since the concentration of (—)-dopa was known and since the conversion of (—)-dopa to indole units of melanin was considered as 1:1, the stoichiometry of the interaction between the drug and the indole unit was calculated. Based on the dissociation constants of the enantiomers, it appears that up to four molecules of (—)-ephedrine can interact with one indole unit of the melanin, while such a ratio for other isomers appear to be 2:1. The preference by indole units of melanin is stereoselective. © 1992 Wiley-Liss, Inc.     

Ecophysiology of embryo development and seed germination of the European woodland herbaceous perennial Corydalis cava (L.) Schweigg. & Körte subsp. cava (Fumariaceae)

In this study we examined the germination ecology with special reference to the temperature requirements for embryo development and germination of Corydalis cava subsp. cava, under both outdoor and laboratory conditions. Corydalis cava is a spring flowering woodland tuberous geophyte widely distributed across Europe. Germination phenology, including embryo development and radicle and cotyledon emergence, was investigated in a population growing in northern Italy. Immediately after harvest, seeds of C. cava were sown both in the laboratory under simulated seasonal temperatures and naturally. Embryos, undifferentiated at the time of seed dispersal, grew during summer and autumn conditions, culminating in radicle emergence in winter, when temperatures fell to ca 5°C. Cotyledon emergence also occurred at ca 5°C, but first emergence was delayed until late winter and early spring. Laboratory experiments showed that high (summer) followed by medium (autumn) and low temperatures (winter) are needed for physiological dormancy loss, embryo development and germination respectively. Unlike seeds of C. cava that germinated in winter, in other Corydalis species radicle emergence occurred in autumn (C. flavula) or did not depend on a period of high summer temperature to break dormancy (C. solida). Our results suggest that subtle differences in dormancy and germination behavior between Corydalis species could be related to differences in their geographical distribution.     

Pflanzengesellschaften in Papua — New Guinea

In the highland of Papua — New Guinea there are meadows, in which several Graminees and Cyperacees exist together, without either of them dominating; on the other hand, there are meadows in which Imperata cylindrica dominates. On the banks of streams and rivers there is growth of grass 2–5 m high, in which one of the following species is dominant: Miscanthus floridulus, Saccharum spontaneum, Pennisetum macrostachyum or Themeda gigantea. In the highest altitudes there are meadows of Poa gymnohypsos. In swampy areas Cyperacees dominate. Moreover, associations are described, which have developed on an abandoned coffee-plantation, where mainly Eragrostis pilosa plays an important part.     

Polymorphism of the Drosophila melanogaster — sigma virus system

In natural populations of D. melanogaster, a minority of flies are usually infected by the sigma Rhabdovirus, which is not contagious but vertically transmitted. A few parameters have been identified that affect the evolution of the Drosophila — sigma system. Various sets of values of these parameters in males mutually adapted to one another have been established in different populations. This study analyses female characteristics that were previously reputed not to vary much between populations. The newly collected data show that these characteristics (mainly transovarial transmission here) may also be polymorphic in the wild, and may contribute to the diversity of equilibria established between the virus and its host. The observations presented here, obtained from various populations, confirm the complexity of this system of two coevolving organisms: as already noted there seem to be as many different sets of values of parameters as there are populations.     

Cold‐adapted species in a warming world – an explorative study on the impact of high winter temperatures on a continental butterfly

Increasing evidence suggests that global warming significantly alters the range and phenology of plants and animals. Whereas thermophilous species usually benefit from rising temperatures, the living conditions of taxa adapted to cooler or continental climates are deteriorating. The woodland ringlet butterfly, Erebia medusa Fabricius (Lepidoptera: Nymphalidae) is one of the continental species that are supposed to be adversely affected by climate change, especially by rising winter temperatures. Here, we conduct an explorative study on the effects of low, moderate, and high winter temperatures on the pre‐adult and adult stages of E. medusa in a laboratory experiment. Compared to the two other temperature regimes, the warm winter treatment led to an earlier termination of diapause and higher larval weights at the end of the winter, but significantly lower survival rates. The after‐effects of the warm treatment included lower weight of the pupae and adult females, shorter forewings of adult males, and earlier emergence of both adult males and females. In natural environments, which are characterized by a much greater thermal variability and a much higher frequency of soil freeze‐thaw events compared to our experiment, the effects of rising winter temperatures might be stronger than in this study. Thus, we conclude that warmer winters pose a non‐negligible long‐term threat to E. medusa.     

Effects of temperature on the immature development and emergence of five species of <Emphasis Type="Italic">Trichogramma</Emphasis>

Five constant temperatures between 14 and 30°C were used to evaluate their effect on the development time and adult emergence of five Trichogramma species found parasitizing eggs of the velvetbean caterpillar Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae) on soybeans in subtropical Southern Brazil. Host eggs were parasitized at 20°C and then transferred to the study temperatures to follow development and emergence of parasitoids. All five species were able to develop and emerge within the range of temperatures evaluated, and the effect of temperature on development rates could be described by linear regression. Trichogramma acacioi Brun, Moraes & Soares and T. rojasi Nagaraja & Nagarkatti were the most cold-tolerant species, with lower threshold temperatures of 8.1 ± 0.16°C and 9.2 ± 0.16°C, respectively. Trichogramma atopovirilia Oatman & Platner was the least cold-adapted species, with a lower threshold of 10.2 ± 0.13°C. Degree-day accumulation ranged from 153.8 DD for T. atopovirilia to 190.7 DD for T. acacioi. Adult emergence was higher than 90% for T. atopovirilia and T. pretiosum at all temperatures, whereas T. lasallei Pinto emergence dropped to 71.3% at 14°C and to 58.3% at 26°C, both significantly lower than the emergence of T. pretiosum and T. atopovirilia. Significantly less T. acacioi adults emerged at 30°C than either T. pretiosum or T. atopovirilia. The sex-ratio was not affected within the range of temperatures studied, and varied from 0.65 to 0.88 (female/(male + female)). Differences among Trichogramma spp. densities in the field can be attributed to slower development rates and/or reduced emergence of adults, both at low and high temperatures.

Acarbose, 17‐α‐estradiol,and nordihydroguaiaretic acid extend mouse lifespan preferentially in males

Four agents — acarbose (ACA), 17‐α‐estradiol (EST), nordihydroguaiaretic acid (NDGA), and methylene blue (MB) — were evaluated for lifespan effects in genetically heterogeneous mice tested at three sites. Acarbose increased male median lifespan by 22% (P < 0.0001), but increased female median lifespan by only 5% (P = 0.01). This sexual dimorphism in ACA lifespan effect could not be explained by differences in effects on weight. Maximum lifespan (90th percentile) increased 11% (P < 0.001) in males and 9% (P = 0.001) in females. EST increased male median lifespan by 12% (P = 0.002), but did not lead to a significant effect on maximum lifespan. The benefits of EST were much stronger at one test site than at the other two and were not explained by effects on body weight. EST did not alter female lifespan. NDGA increased male median lifespan by 8–10% at three different doses, with P‐values ranging from 0.04 to 0.005. Females did not show a lifespan benefit from NDGA, even at a dose that produced blood levels similar to those in males, which did show a strong lifespan benefit. MB did not alter median lifespan of males or females, but did produce a small, statistically significant (6%, P = 0.004) increase in female maximum lifespan. These results provide new pharmacological models for exploring processes that regulate the timing of aging and late‐life diseases, and in particular for testing hypotheses about sexual dimorphism in aging and health.     

Conformational features of 2′-O-methyl-adenosylyl-adenosine

In order to obtain information about the conformational features of a 2′-O-methylated polyribonucleotide at the nearest neighbor level, a detailed nuclear magnetic resonance study of AmpA was undertaken. AmpA was isolated from alkali hydrolysates of yeast RNA, and proton spectra were recorded at 100 MHz in the Fourier transform mode in D₂O solutions, 0.01 M, pH 5.4 and 1.5 at 25°C. ³¹P spectra were recorded at 40.48 MHz. Complete, accurate sets of nmr parameters derived for each nucleotidyl unit by simulation iteration methods. The nmr data were translated into conformational parameters for all the bonds using procedures developed in earlier studies from these laboratories. It is shown that AmpA exists in aqueous solution with a flexible molecular framework, which shows preferences for certain orientations. The ribose rings exist as a ²E ? ³E equilibrium with the —pA ribose showing a bias for the ³E pucker. The C(4′)—C(5′) bonds of both nucleotidyl units show significant preference (75–80%) to exist in gg conformation. The dominant conformer (80%) about C(5′)—O(5′) of the 5′-nucleotidyl unit is g′g′. Even though an unambiguous determination of the orientation of the 3′-phosphate group cannot be made, tentative evidence shows that it preferentially occupies g⁺ domains [O(3′)—P trans to C(3′)—C(2′)] in which the H(3′) —C(3′)—O(3′)—P(3′) dihedral angle is about 31°. There is reasonable evidence that the 2′-O-methyl preferentially occupies the domain in which the O(2′)—CH₃ bond is trans to C(2′)—C(1′). Lowering of pH to 1.5, which results in protonation of both the adenine moieties, causes destacking of AmpA. Such destacking is accompanied by small, but real, perturbations in the conformations about most of the bonds in the backbone. A detailed comparison of the solution conformations of ApA and AmpA clearly shows that 2′-O-methylation strongly influences the conformational preference about the C(3′)—O(3′) bond of the 3′-nucleotidyl unit, in addition to inducing small changes in the overall ribophosphate backbone conformational equilibria. The effect of 2′-O-methylation is such that the C(3′)—O(3′) is forced to occupy preferentially the g⁺ domain rather than the normally preferred g^? domain [O(3′)—P trans to C(3′)—C(4′)] in ApA. The data on ApA and AmpA further reveal that the extent of stacking interaction is less in AmpA compared to ApA. It is suggested that stacked species of AmpA exist as right-handed stacks where the magnitude of ω and ω′ about O(5′)—P and P—O(3′) is about 290°. The reason for the lesser degree of stacking in AmpA compared to ApA is intramolecular interaction between 2′-O-methyl and the flexible O(3′)—P—O(5′) bridge, the interaction causing some perturbation in the magnitudes of ω/ω′, causing destacking. The destacking will lead to an increase in _χCN by a few degrees, causing an increase in ²E populations; the latter in turn will shift the 3′ phosphate group from g^? to g⁺ domains. In short, a coupled series of conformational events is envisioned at the onset of destacking, made feasible by the interaction between the 2′-O-methyl group and the swivel O(3′)—P—O(5′) bridge.     

The freshwater turtle genus Mauremys (Testudines,Geoemydidae) — a textbook example of an east–west disjunction or a taxonomic misconcept?

Barth, D. Bernhard, D. Fritzsch, G. & Fritz, U. (2004): The freshwater turtle genus Mauremys— a textbook example of an east–west disjunction or a taxonomic misconcept? —Zoologica Scripta, 33, 213–221. We compare 1036 bp of the mitochondrial cytochrome b gene (cyt b) from all six Mauremys species with 16 other taxa, representing both currently recognized subfamilies of the Geoemydidae (Geoemydinae and Batagurinae) to contribute a comprehensive dataset towards resolving the conflicting Mauremys taxonomy and phylogeography. Mauremys, a representative of the Geoemydinae, is thought to be an example of a taxon with an east–west disjunction due to Pleistocene glacial extinction, with species occurring in the western Palearctic and species in the eastern Palearctic and Oriental regions. Our results contradict this traditional zoogeographical scheme and the current taxonomy of the Geoemydidae. Mauremys is paraphyletic with respect to two East Asian genera belonging to the Batagurinae: Chinemys and Ocadia. Therefore, Mauremys, as currently understood, clearly represents a taxonomic misconcept. Mauremys+Chinemys+Ocadia contains four well supported clades, two of which —M. japonica+Chinemys+Ocadia and M. annamensis+M. mutica— are confined to eastern Asia. The other two —M. caspica+M. rivulata and M. leprosa— occur in the western Palearctic. Mauremys leprosa may represent an ancient lineage which differentiated before the split between the other western and eastern species occurred. The patchy distribution of the four clades is likely the result of several ancient radiations rather than of a Pleistocene extinction. The sister‐group of Mauremys+Chinemys+Ocadia is Cuora, a morphologically highly specialized genus with a complicated shell hinging mechanism.     

Labyrinthula diatomea n. sp.—A Labyrinthulid Associated with Marine Diatoms

Labyrinthulomycetes are mostly fungus‐like heterotrophic protists that absorb nutrients in an osmotrophic or phagotrophic manner. Members of order Labyrinthulida produce unique membrane‐bound ectoplasmic networks for movement and feeding. Among the various types of labyrinthulids’ food substrates, diatoms play an important role due to their ubiquitous distribution and abundant biomass. We isolated and cultivated new diatom consuming Labyrinthulida strains from shallow coastal marine sediments. We described Labyrinthula diatomea n. sp. that differs from all known labyrinthulids in both molecular and morphological features. We provided strain delimitation within the genus Labyrinthula based on ITS sequences via haplotype network construction and compared it with previous phylogenetic surveys.     

γ‐Octalactone,an effective oviposition stimulant of Bactrocera tryoni

Insects commonly rely on olfactory, gustatory and visual cues when deciding where to lay eggs. The olfactory cues that stimulate oviposition in the Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), are not well understood. Here, we show that two known oviposition stimulants of the Oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae)—γ‐octalactone and benzothiazole—strongly elicit aggregation and oviposition in B. tryoni. Two other known oviposition stimulants of B. dorsalis—ethyl tiglate and 1‐octen‐3‐ol—elicit aggregation but not oviposition. Highlighting species overlap, but also differences, in oviposition stimulants, these findings have practical application for mass‐rearing in which vast numbers of flies are reared for sterile insect technique programs and may also have practical application in the development of pest management and monitoring tools.     

Influence of riparian cover on stream temperatures and the growth of the mayfly <Emphasis Type="Italic">Baetis rhodani</Emphasis> in an upland stream

This research aimed to assess whether stream temperature and the size of Baetis rhodani larvae (Insecta: Ephemeroptera) differed between sites with contrasting riparian vegetation cover. Stream temperatures and Baetis sizes were monitored at sites along a 5-km section of a small (30 km²) upland Scottish stream. Some sites had riparian tree cover dominated by mature broadleaved woodland, while others were located in areas of open Calluna moorland. The 13 month data period included two contrasting summers, one typical for the region and the other unusually warm. Daytime water temperatures were generally highest at the open moorland sites. The difference between moorland and broadleaved woodland sites was most apparent in the summer months, with the woodland shading apparently acting to reduce maximum temperatures and mediate temporal fluctuations. Size data suggest that Baetis has a bivoltine life-cycle in the stream. Significant differences in the mean size of Baetis were found between moorland and woodland sites, with differences occurring primarily during the summer months. Mean size of Baetis was significantly greater in the moorland during the time of summer adult emergence. During the months that followed (growth of the next generation), Baetis larvae in the woodland sites were significantly larger. Size distribution data for the population suggest earlier adult emergence in the moorland section and different egg-to-larvae hatch times. Overall, the results indicate that subtle differences in growth and life cycles, linked to differences in riparian cover and streamwater temperature, can occur over short distances in upland streams.     

Phenological responses in a sycamore–aphid–parasitoid system and consequences for aphid population dynamics: A 20 year case study

Species interactions have a spatiotemporal component driven by environmental cues, which if altered by climate change can drive shifts in community dynamics. There is insufficient understanding of the precise time windows during which inter‐annual variation in weather drives phenological shifts and the consequences for mismatches between interacting species and resultant population dynamics—particularly for insects. We use a 20 year study on a tri‐trophic system: sycamore Acer pseudoplatanus, two associated aphid species Drepanosiphum platanoidis and Periphyllus testudinaceus and their hymenopteran parasitoids. Using a sliding window approach, we assess climatic drivers of phenology in all three trophic levels. We quantify the magnitude of resultant trophic mismatches between aphids and their plant hosts and parasitoids, and then model the impacts of these mismatches, direct weather effects and density dependence on local‐scale aphid population dynamics. Warmer temperatures in mid‐March to late‐April were associated with advanced sycamore budburst, parasitoid attack and (marginally) D. platanoidis emergence. The precise time window during which spring weather advances phenology varies considerably across each species. Crucially, warmer temperatures in late winter delayed the emergence of both aphid species. Seasonal variation in warming rates thus generates marked shifts in the relative timing of spring events across trophic levels and mismatches in the phenology of interacting species. Despite this, we found no evidence that aphid population growth rates were adversely impacted by the magnitude of mismatch with their host plants or parasitoids, or direct impacts of temperature and precipitation. Strong density dependence effects occurred in both aphid species and probably buffered populations, through density‐dependent compensation, from adverse impacts of the marked inter‐annual climatic variation that occurred during the study period. These findings explain the resilience of aphid populations to climate change and uncover a key mechanism, warmer winter temperatures delaying insect phenology, by which climate change drives asynchronous shifts between interacting species.     

Applications——Influence of Biology on Engineering

Examples are presented showing the way in which biological systems produce a range of functions which can be implemented in engineering, such as feedback-control of stiffness （muscles and nervous system）, the design of fault-free structures （trees） and damage-tolerant materials （wood） and high performance insulation （penguin feathers） and shock absorbers （hedgehog spines）.     

Gleichgewichtsbewegungen von Blättern als Reaktion auf asymmetrische Belastung — Isoklinotropismus nach Härdtl (1927) — und ihre Beziehung zum Gravitropismus

1. Die anscheinend in Vergessenheit geratenen älteren Angaben von HÄRDTL (1927 und spärer) und PRINGSHEIM (1931), nach denen einseitig belastete Blätter Gleichgewichtsbewegungen ausführen können (Isoklinotropismus nach HÄRDTL), wurden an mehreren Arten (Chelidonium majus, Aegopodium podagra ria, Ranunculus repens, Sambucus nigra, Coleus blumei und Hibiscus rosa sinensis) bestätigt,— Die einseitige Belastung erfolgte durch Einstecken von Nadeln in die eine Hälfte der Blattspreite bzw. in eine Blattfieder. 2. Bei Blättern, die in Spreite oder Stiel epinastisch gekrtimmt sind (die überwiegende Mehrzahl) oder auch durch die Belastung abwärts gebogen sind, kommt es dabei zu zwei verschiedenen Reaktionen: a) zu einer sofort eintretenden rein physikalischen reversiblen Reaktion, da die einseitige Last eine Torsion der Spreite bewirkt, die die Spreitenspitze nach der der Last gegenüberliegenden Seite [führt, und b) zu einer spärer eintretenden physiologischen Reaktion durch eine Wachstumskrümmung nach derselben Seite, - Beide Komponenten der Bewegung ftihren zu einer Verringerung oder einem Ausgleich des Ungleichgewichtes. 3. Die physiologische Reaktion war meist nach einem oder einigen Tagen erkennbar, bei Chelidonium mitunter schon nach einigen Stunden, bei Hibiscus gelegentlich erst nach etwa einer Woche. Mitunter blieb die Reaktion auch aus. Bei den gefiederten Blattern erfolgte sie in der Rhachis, ober- und unterhalb der belasteten Fieder, mitunter auch im Blattstiel, bei Coleus im Spreitengrund und im Blattstiel, bei Hibiscus im oberen Gelenkpolster (“Sekundargelenk”) des Blattstiels. Auch ältere Blätter reagierten oft noch überraschend gut. Vielfach führte die physiologische Reaktion zu einer vollständigen Ausbalancierung der einseitig belasteten Blattspreite. Auf nachtragliche Entfernung der eingesteckten Nadeln gingen auch die Krtimmungen in 2–3 Tagen wieder weitgehend zurück. 4. Als Ursache ftir die Gleichgewichtsbewegungen der Blätter kommen zwei verschiedene Mechanismen in Betracht: a) Infolge der durch die einseitige Belastung hervorgerufenen Schräglage der Blattspreite sammelt sich das Auxin auf der tiefer liegenden Flanke von Mittelnerv bzw. Rhachis und Blattstiel an, was zu einem stärkeren Wachstum dieser Seite und einer Gleichgewichtskrümmung führen muß. Nach dieser Auffassung ordnet sich der Isoklinotropismus dem Gravitropismus ein. b) Die nach der Belastung sofort eintretende auf der Torsionsspannung beruhende rein physikalische und zunächst reversible Gleichgewichtsreaktion des Blattes wird nach einigen Stunden oder Tagen teilweise irreversibel. (Für die spannungsfreien Abschnitte der Rhachis eines gefiederten Blattes oberhalb der Belastung kann diese Erklärung natürlich nicht gelten). Vermutlich sind beide Mechanismen, vor allern wohl der erstgenannte, bei den einzelnen Arten in verschiedenem Maße, als Ursache der Gleichgewichtsbewegungen wirksam. 5. Der biologische Sinn der Ausbalancierung eines (größeren) Blattes (PRINGSHELM 1931, HÄRDTL 1927, 1937 a) liegt darin, daß a) ein ausbalanciertes Blatt den geringsten Aufwand an mechanischen Elementen erfordert, und b) Photo- und Gravitropismus nur ein ausbalanciertes Blatt ohne Komplikationen in die angestrebte Lage fuhren konnen.