全文获取类型
收费全文 | 544篇 |
免费 | 41篇 |
国内免费 | 86篇 |
出版年
2024年 | 1篇 |
2023年 | 8篇 |
2022年 | 4篇 |
2021年 | 14篇 |
2020年 | 14篇 |
2019年 | 21篇 |
2018年 | 23篇 |
2017年 | 25篇 |
2016年 | 20篇 |
2015年 | 21篇 |
2014年 | 19篇 |
2013年 | 47篇 |
2012年 | 23篇 |
2011年 | 25篇 |
2010年 | 20篇 |
2009年 | 24篇 |
2008年 | 14篇 |
2007年 | 36篇 |
2006年 | 20篇 |
2005年 | 23篇 |
2004年 | 25篇 |
2003年 | 30篇 |
2002年 | 9篇 |
2001年 | 30篇 |
2000年 | 15篇 |
1999年 | 16篇 |
1998年 | 7篇 |
1997年 | 8篇 |
1996年 | 10篇 |
1995年 | 10篇 |
1994年 | 12篇 |
1993年 | 8篇 |
1992年 | 9篇 |
1991年 | 12篇 |
1990年 | 6篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 9篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 4篇 |
1974年 | 3篇 |
排序方式: 共有671条查询结果,搜索用时 15 毫秒
1.
The functioning of the vertebrate eye depends on its absolute size, which is presumably adapted to specific needs. Eye size variation in lidless and spectacled colubrid snakes was investigated, including 839 specimens belonging to 49 genera, 66 species and subspecies. Variations of adult eye diameters (EDs) in both absolute and relative terms between species were correlated with parameters reflecting behavioral ecology. In absolute terms, eye of arboreal species was larger than in terrestrial and semiaquatic species. For diurnal species, EDs of terrestrial species do not differ from semiaquatic species; for nocturnal species the ED of terrestrial species is larger than fossorial species but not different from semiaquatic species. In relative terms, ED did not differ significantly by habitat for diurnal species. Although the ED of terrestrial species is larger than fossorial species there were no differences for nocturnal species between semiaquatic and fossorial snakes. In contrast to other vertebrates studied to date, colubrid EDs in absolute and relative terms are larger in diurnal than in nocturnal species. These observations suggest that among colubrid snakes, eye size variation reflects adaptation to specific habitats, foraging strategies and daily activities, independently of phylogeny. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc. 相似文献
2.
3.
Day–night cycle is the main zeitgeber (time giver) for biological circadian rhythms. Recently, it was suggested that natural diurnal geomagnetic variation may also be utilized by organisms for the synchronization of these rhythms. In this study, life-history traits in Daphnia magna were evaluated after short-term and multigenerational exposure to 16 h day/8 h night cycle, 32 h day/16 h night cycle, diurnal geomagnetic variation of 24 h, simulated magnetic variation of 48 h, and combinations of these conditions. With short-term exposure, the lighting mode substantially influenced the brood to brood period and the lifespan in daphnids. The brood to brood period, brood size, and body length of crustaceans similarly depended on the lighting mode during the multigenerational exposure. At the same time, an interaction of lighting mode and magnetic variations affected to a lesser extent brood to brood period, brood size, and newborn's body length. The influence of simulated diurnal variation on life-history traits in daphnids appeared distinctly as effects of synchronization between periods of lighting mode and magnetic variations during the multigenerational exposure. Newborn's body length significantly depended on the lighting regime when the periods of both studied zeitgebers were unsynchronized, or on the interaction of light regime with magnetic variations when the periods were synchronized. These results confirm the hypothesis that diurnal geomagnetic variation is an additional zeitgeber for biological circadian rhythms. Possible mechanisms for these observed effects are discussed. Bioelectromagnetics. © 2020 Bioelectromagnetics Society 相似文献
4.
C. EDUARDO VALLEJOS 《Plant, cell & environment》1991,14(1):105-112
Abstract. Comparisons were made between the changes in mRNA levels induced by low night temperatures in the cold–sensitive tomato and two altitudinal ecotypes of the wild species L. hirsutum. Changes in mRNA levels were detected by resolving in vitro translation products of poly(A)+ RNA by 2-D PAGE. The treatment was applied by first growing plants in a thermoperiod of 25/18°C and then switching to 25/6°C. All tomatoes displayed a diurnal cycling in which a set of mRNAs accumulated at the end of the 18°C nights, whereas another accumulated at the end of the 25°C days. The accumulation of night specific mRNAs was inhibited by 6°C nights in the cold sensitive tomatoes while that of the tolerant one was only marginally affected. All tomatoes showed a similar reduction in the apparent turnover rate of the day specific mRNAs during the 6°C nights. Finally, low night temperatures induced the accumulation of six to eight mRNAs in all genotypes. This number increased by 15 in L. esculentum after the seventh night and are likely involved in stress response rather than acclimation/tolerance. The tomato is proposed as a genetic model to discriminate genes involved in acclimation/tolerance from those involved in stress response. 相似文献
5.
CO2 fixation was studied in a lichen, Xanthoria parietina, kept in continuous light, and with cyclic changes in light intensity, dark period or temperature. The diurnal and seasonal courses of CO2 exchange were followed. The rate of net photosynthesis was observed to fall from morning to evening, and this decline was more pronounced in winter than in summer. The maximal net photosynthetic rate, 223 ng CO2g-1dws-1, occured in winter and the minimum, 94 ng CO2g-1dws-1, late in spring. The light compensation point in summer was four times as high as in winter. In continuous light (180 or 90 mol photons m-2s-1, 15°C) net photosynthesis decreased noticeably during one week, falling below the level maintained in a 12 h light: 12 h dark cycle. Photosynthetic activity did not decrease, however, in lichens held in continuous light (90 mol photons m-2s-1) with cyclic changes of temperature (12 h 20 °C: 12 h 5 °C). Active photosynthesis was also maintained in light of cyclically changing intensity (12 h: 12 h, 15 °C) when night-time light was at least 75% lower than illumination by day. A dark period of 4 hours in a 24-h light:dark cycle was sufficient to keep CO2 fixation at the control level. It seems that plants need an unproductive period during the day to survive and this can be induced by fluctuations in light and/or temperature. 相似文献
6.
Gas exchange studies in two Portuguese grapevine cultivars 总被引:8,自引:0,他引:8
M. Manuela Chaves Peter C. Harley John D. Tenhunen Otto L. Lange 《Physiologia plantarum》1987,70(4):639-647
Gas exchange characteristics of leaves of Vitis vinifera L. cvs Tinta Amarela and Periquita, two grapevine cultivars grown in distinct climatic regions of Portugal, were studied under natural and controlled conditions. Daily time courses of gas exchange were measured on both a hot, sunny day and a cooler, partly cloudy day. Responses of net photosynthesis to irradiance and internal partial pressure of CO2 , were also obtained. A strong correlation between net photosynthesis (PN ) and leaf conductance (gs ) was found during the diurnal time courses of gas exchange, as well as a relatively constant internal partial pressure of CO2 (Pi ), even under non-steady-state conditions. On the cloudless day, both PN and gs were lower in the afternoon than in the morning, despite similar conditions of leaf temperature, air to leaf water vapor deficit and irradiance. The response curves of net photosynthesis to internal CO2 showed linearity up to pi values of 50 Pa, possibly indicating a substantial excess of photosynthetic capacity. When measured at low partial pressures of O2 (1 kPa), PN became inhibited at high CO2 levels. Inhibition of PN at high CO2 was absent under normal levels of O2 (21 kPa). Significant differences in gas exchange characteristics were found between the two cultivars, with T. Amarela having higher rates under similar measurement conditions. In particular, the superior performance of T. Amarela at high temperatures may represent adaptation to the warmer conditions at its place of origin. 相似文献
7.
M. W. A. Verstegen A. Siegerink W. Van der Hel R. Geers C. Brandsma 《Journal of thermal biology》1987,12(4):257-261
The aim of the experiments was to investigate the effect of air velocity on the temperature preferred by growing pigs 12–14 weeks old. Pigs displayed a temperature preference by means of operant supplemental heating. They pushed a button connected to heating lamps. Six experiments of three weeks each and with two treatments with a group of 8 pigs each were made. Animals were housed in groups and weighed 14–20 kg at the start of the experiments. Air velocity was 0.08, 0.25 and 0.40 m/s. At each air velocity four replicates were made. Mean temperatures preferred were 17.9°C at 0.08 m/s, 20.5°C at 0.25 m/s and 21.7°C at 0.40 m/s. Within a day temperature preference fluctuated with 5.7 K at 0.08 m/s, 4.3 K at 0.25 m/s and 4.2 K at 0.4 m/s. Temperatures preferred were highest during day time. 相似文献
8.
Alan Scaife 《Plant and Soil》1989,114(1):139-141
A simple simulation model is described to account for the rates at which plants take up nitrate and reduce it to protein. It is based on the pump and leak principle, with the pump working at a constant rate per unit sap volume provided that there is an adequate concentration of nitrate at the root surface. The rate of leakage is assumed to be proportional to the concentration difference between the inside and the outside of the plant. Nitrogen is removed from the plant nitrate pool (the buffer) at a constant fraction of the photosynthesis rate. When applied to data for the diurnal variation in nitrate uptake by ryegrass, the model predicts an uptake pattern similar to that actually observed, with a time lag of about 5 hours between photosynthesis and uptake. 相似文献
9.
While diurnal cycles in nitrogen fixation rates are sometimes assumed to result from diurnal variation in photosynthetically active radiation, contradicting evidence exists that indicate soil temperature is the primary environmental influence. These studies assessed the significance of temperature on soybean nitrogen fixation under field conditions. Two groups of intact field-grown soybean plants, one at ambient and the other exposed to a 10°C diurnal variation in soil temperature, were nondestructively assayed for acetylene reduction rates. Activity was closely associated with soil temperature (R2=0.85), even when temperature was 12 h out of phase with ambient. Data were also obtained to determine if the effects of rhizosphere temperature on nitrogen fixation are mediated through an effect on the nodule oxygen permeability. Nodule oxygen permeability of intact, aeroponically grown soybean was closely correlated with the diurnal changes in temperature (R2=0.90). 相似文献
10.
Horizontal distribution of crustaceans in Lake Glubokoe 总被引:2,自引:2,他引:0
O. S. Boikova 《Hydrobiologia》1986,141(1-2):113-123
A study has been made on the distribution of crustaceans along a profile of 6–9 stations from depths of less than 2 m to 20
m, in summer, autumn and early spring. The species composition, abundance, size-frequency distribution, biomass and diurnal
dynamics were estimated. Many species abundant in the pelagic zone were found to be absent or scarce out to depths of 5 m.
In the daytime, at depths out to 3 m, crustaceans greater than 1.0 mm were seldom found, while those 0.15–0.40 mm long were
most abundant. At depths out to 5 m, not only was the total average size of crustaceans found to be smaller, but also that
of individual species, including small ones: Ceriodaphnia and Chydorus. The biomass of crustaceans in shallow water and in the open lake differs in summer by more than 100 times, and in late autumn
and early spring by more than 10 times. At the depth of 1 m, in none of the seasons did it exceed 0.026 mg l-1. All the crustaceans except for nauplii and Chydorus considerably increased in numbers in the shallow water at night.
The impoverishment of the littoral plankton in Lake Glubokoe is regarded as a specific case of the well-known phenomenon of
‘avoidance of the littoral’ by euplankton (Hutchinson, 1967; Kiselyov, 1980). It is pointed out that there is a possible relationship
between diurnal horizontal migrations of plantivorous fish and the impoverishment of plankton near the shore. An attempt has
been made to substantiate the assumption that the ‘avoidance of the littoral’ by pelagic crustaceans is due to high numbers
of fish in shallow water. 相似文献