全文获取类型
收费全文 | 306篇 |
免费 | 21篇 |
专业分类
327篇 |
出版年
2016年 | 3篇 |
2012年 | 4篇 |
2011年 | 7篇 |
2010年 | 5篇 |
2009年 | 8篇 |
2008年 | 5篇 |
2007年 | 12篇 |
2006年 | 6篇 |
2005年 | 4篇 |
2004年 | 5篇 |
2003年 | 3篇 |
2002年 | 9篇 |
1999年 | 3篇 |
1998年 | 6篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1976年 | 6篇 |
1975年 | 3篇 |
1972年 | 5篇 |
1971年 | 4篇 |
1965年 | 3篇 |
1958年 | 11篇 |
1957年 | 9篇 |
1956年 | 9篇 |
1955年 | 7篇 |
1954年 | 6篇 |
1953年 | 10篇 |
1952年 | 7篇 |
1951年 | 14篇 |
1950年 | 7篇 |
1948年 | 8篇 |
1945年 | 3篇 |
1933年 | 2篇 |
1929年 | 2篇 |
排序方式: 共有327条查询结果,搜索用时 15 毫秒
291.
292.
293.
Novel Evidence for a Lack of Water Vapour Saturation Within the Intercellular Airspace of Turgid Leaves of Mesophytic Species 总被引:2,自引:0,他引:2
Ward, D. A. and Bunce, J. A. 1986. Novel evidence for a lackof water vapour saturation within the intercellular airspaceof turgid leaves of mesophytic speciesJ. exp. Bot. 37:504 By utilizing a dual-surface leaf chamber evidence was obtainedsuggesting that the water vapour pressure within the intercellularairspace of turgid leaves of mesophytic species can deviatesignificantly from the saturation vapour pressure at the leaftemperature. When the water vapour pressure of the air surroundingthe lower leaf surface of sunflower was maintained constantand high, suddenly exposing the upper leaf surface to air witha low water vapour content caused the lower leaf surface toexhibit a negative rate of transpiration (i.e. an apparent uptakeof water vapour). Since the vapour pressure of the air surroundingthe lower (moist) surface was less than the saturation vapourpressure at the leaf temperature, the occurrence of negativetranspiration indicated that the vapour pressure of the leafairspace deviated from saturation under the conditions of measurementused. For both soybean and sunflower it was also found that if thehumidity around the upper surface was maintained high and constant,a stepwise decrease in lower surface humidity caused substantialreductions in the transpiration rate and apparent conductanceof the upper surface without any concomitant change in its photosyntheticrate. In contrast, both the photosynthetic rate and conductanceof the lower surface were greatly reduced. The relative reductionsof photosynthetic rate and conductance at the lower surfacewere the same. These responses are most easily explained interms of a deviation from water vapour saturation within theintercellular airspace, which gives rise to spurious valuesof conductance. Key words: Intercellular space, water vapour pressure, turgid, leaves, mesophyte 相似文献
294.
295.
Tests for adaptive RAPD variation in population genetic structure of wild barley, Hordeum spontaneum Koch. 总被引:7,自引:0,他引:7
SERGEI VOLIS BAHTIYOR YAKUBOV † IRINA SHULGINA DAVID WARD ‡ VARDA ZUR SAMUEL MENDLINGER 《Biological journal of the Linnean Society. Linnean Society of London》2001,74(3):289-303
We examined the adaptive importance of RAPD variation in the population genetic structure of wild barley, Hordeu m spontaneum. The test involved (1) a nested sampling design with four population groups representing four distinct environments; and (2) a comparison of observed variation with that expected as a result of natural selection. Analyses of selection on fitness-related traits by reciprocal introductions served as guidelines for the expected pattern of RAPD variation. We found no concordance between the observed pattern of population genetic structure and that expected under the null hypothesis of environment-specific natural selection. There was no relationship between genetic distance and environmental similarity; none of 54 putative loci exhibited an allele distribution in accordance with that expected and no favoured epistatic allele combinations were detected across the four environments. The fact that environmentally induced adaptation, detected by fitness-related traits, was not reflected in inter-population RAPD structure (1) strongly enhances the neutralist viewpoint and (2) casts doubt on the notion that significant correlations between some environmental parameters and allele frequencies in one or more loci are evidence of selection on the latter. 相似文献
296.
297.
Ecology of alpine streams 总被引:6,自引:0,他引:6
J.V. WARD 《Freshwater Biology》1994,32(2):277-294
1. This review examines ecological conditions and zoobenthic communities of kryal, krenal and rhithral streams of the alpine zone. Altitudinal and biogeographical faunal patterns are also analysed. 2. Kryal segments, fed by glacial meltwater, are characterized by low temperatures (Tmax4°C) and large diel flow fluctuations in summer. The water may be clear or turbid from suspended rock flour. Fishes and higher plants are absent. The macroalga Hydrurus foetidus may be abundant in kryal and other alpine stream types of the Holarctic. The highly restricted cosmopolitan fauna of glacial brooks consists of diamesine chironomids, sometimes accompanied by simuliids. Sparse food resources include algae and allochthonous (aeolian) organic matter. 3. Rhithral segments in alpine catchments are characterized by soft water, a hydrograph dominated by an extended period of snowmelt runoff, and a broader temperature range than kryal or krenal biotopes. Bryophytes, macroalgae (chrysophytes, chlorophytes, cyanophytes, rhodophytes) and epiphytic and epilithic diatoms constitute the flora. A relatively diverse zoobenthos includes four orders of insects (Flecoptera, Ephemeroptera, Trichoptera, Diptera), turbellarians, acarines, oligochaetes and nematodes. 4. Krenal segments, fed by groundwater, are typically calcareous with summer-cool and winter-warm thermal conditions, high water clarity, and constant flow regimes. Bryophytes and macroafgae are accompanied by a rich diatom flora. The zoobenthos consists of a composite of kryal and rhithral elements with few crenobionts. Zoobenthos species richness values are intermediate between those of kryal and rhithral segments, whereas densities in perennial, well-oxygenated springbrooks far exceed those in other alpine stream types. 5. Downstream faunal changes are most predictable in kryal segments where chironomids of the genus Diamesa are the predominant, if not sole, members of the zoobenthos in the upper zone of glacial brooks, the metakryal. Where Tmax exceeds about 2°C the transition to the hypokryal occurs and Diamesa is co-dominant with simuliids. These largely stenozonal headwater forms decline downstream where Tmax exceeds about 4°C, concomitant with a marked increase in the euryzonal mountain fauna. 6. Species occurring in alpine rhithral biotopes tend to be euryzonal forms at their upper alrirudinal limits, whereas the lower elevation mountain stream fauna consists of species with narrower distribution limits. There is, however, a precipitous drop in mean altitudinal range from the alpine rhithral to the kryat because of the stenozonal nature of the glacial brook fauna. 7. The view that effects of temperature on generation time and mutation rate determine the speed at which selection proceeds is consistent with altitudinal species richness patterns exhibited by zoobenthos along the alrirudinal gradient and may provide an evolutionary explanation for the low faunal diversity in alpine headwaters. 8. With increasing altitude, mountain ‘islands’ become progressively insular as area decreases and isolation increases. For a cold-adapted stream fauna the insular nature of mountain tops is greatest in the tropics. Nonetheless, alpine stream faunas generally 相似文献
298.
J. P. WARD 《Physiological Entomology》1980,5(3):301-306
ABSTRACT. In an investigation of the effect of mating on the behaviour and activity of female Stegobium paniceum (L.) the following factors were examined: initiation of oviposition, amount of walking activity, refractory behaviour, and tendency to fly. 2–3 min in copula were sufficient to induce initiation of oviposition, increase the amount of walking activity and produce refractory behaviour. Mating was also shown to increase the occurrence of female flight behaviour. The number of females flying increased after oviposition had started and decreased when oviposition was complete. The greatest behavioural changes were found in females allowed to separate from males in their own time after a mean period in copula of 63 min. The significance of these changes and possible causative stimuli are discussed. 相似文献
299.
Effects of ingested ivermectin on body mass 总被引:1,自引:0,他引:1
Abstract. Adults of Neomyia cornicina (F.) given continuous access to cow dung containing ivermectin at concentrations of 0.5.0.25 and 0.125 μg g-1 , the range found in the dung of injected cattle (Sommer et al. , 1992), were monitored every 12 h for changes in mass and excretory rates over a period of 3–5 days. Compared with the control insects, adult females that had ingested ivermectin showed disrupted patterns of daily mass change, an increase in dawn mass, and greatly reduced rates of excretion. Flies fed continuously with dung containing ivermectin at 0.25 μg g-1 and 0.125 μg g-1 in a regime identical to the above showed significantly increased abdominal mass compared with the controls. The observations are discussed in relation to other known effects of ivermectin on insect physiology. 相似文献
300.
RACHEL E. ADAMSON RICHARD D. WARD M. DORA FELICIANGELI RHAYZA MAINGON 《Medical and veterinary entomology》1993,7(3):203-207
Abstract. We have applied the recently developed Random Amplified Polymorphic DNA (RAPD) method to produce species-specific, DNA profiles for two sympatric, Venezuelan sandfly species, thought to be the vectors responsible for recent outbreaks of cutaneous and mucocutaneous leishmaniasis in the Andean State of Tachira. Moreover, within the profile, it was possible to identify a diagnostic DNA band for Lu.youngi of 0.32 kb. Results showed that the size of this diagnostic DNA band remained constant and did not vary with sex or geographical distribution. 相似文献