全文获取类型
收费全文 | 1293篇 |
免费 | 45篇 |
国内免费 | 1篇 |
出版年
2013年 | 22篇 |
2011年 | 26篇 |
2010年 | 41篇 |
2009年 | 35篇 |
2008年 | 32篇 |
2007年 | 40篇 |
2006年 | 25篇 |
2005年 | 18篇 |
2004年 | 19篇 |
1999年 | 12篇 |
1998年 | 16篇 |
1997年 | 22篇 |
1996年 | 21篇 |
1995年 | 20篇 |
1994年 | 25篇 |
1993年 | 23篇 |
1992年 | 19篇 |
1991年 | 18篇 |
1990年 | 17篇 |
1989年 | 23篇 |
1988年 | 22篇 |
1987年 | 21篇 |
1986年 | 15篇 |
1985年 | 15篇 |
1984年 | 14篇 |
1983年 | 20篇 |
1982年 | 14篇 |
1981年 | 16篇 |
1980年 | 19篇 |
1979年 | 28篇 |
1978年 | 14篇 |
1977年 | 27篇 |
1976年 | 19篇 |
1975年 | 18篇 |
1974年 | 24篇 |
1973年 | 23篇 |
1972年 | 25篇 |
1971年 | 21篇 |
1959年 | 31篇 |
1958年 | 50篇 |
1957年 | 45篇 |
1956年 | 37篇 |
1955年 | 37篇 |
1954年 | 35篇 |
1953年 | 35篇 |
1952年 | 37篇 |
1951年 | 27篇 |
1950年 | 19篇 |
1949年 | 21篇 |
1948年 | 24篇 |
排序方式: 共有1339条查询结果,搜索用时 15 毫秒
1.
Wei CHEN Liqing PENG Lichun JIANG David A.PIKE Christopher R.FRIESEN Gregory BROWN 《亚洲两栖爬行动物研究(英文版)》2018,(1)
Environmental unpredictability can influence strategies of maternal investment among eggs within a clutch. Models predict that breeding females should adopt a diversified bet-hedging strategy in unpredictable environments, but empirical field evidence from Asia is scarce. Here we tested this hypothesis by exploring spatial patterns in egg size along an altitudinal gradient in a frog species(Rana kukunoris) inhabiting the Tibetan Plateau. Within-clutch variability in egg size increased as the environment became variable(e.g., lower mean monthly temperature and mean monthly rainfall at higher altitudes), and populations in environments with more unpredictable rainfall produced eggs that were smaller and more variable in size. We provide support for a diversified bet-hedging strategy in high-altitude environments, which experience dynamic weather patterns and therefore are of unpredictable environmental quality. This strategy may be an adaptive response to lower environmental quality and higher unpredictable environmental variance. Such a strategy should increase the likelihood of breeding success and maximize maternal lifetime fitness by producing offspring that are adapted to current environmental conditions. We speculate that in high-altitude environments prone to physical disturbance, breeding females are unable to consistently produce the optimal egg size due to physiological constraints imposed by environmental conditions(e.g., duration of the active season, food availability). Species and populations whose breeding strategies are adapted to cope with uncertain environmental conditions by adjusting offspring size and therefore quality show a remarkable degree of ability to cope with future climatic changes. 相似文献
2.
ABSTRACT. An actograph is described for recording the behaviour of small insects, by using a standing-wave radar system in an enclosed metal cabinet. Experiments show that the system produces an output frequency dependent only on the speed of movement of the subject. By filtering the signal electrically into components below and above 5 Hz, walking and flying of adult Musca domestica could be separately recorded. Each filter is followed by a level-comparator with hysteresis. Activity causing signal to cross the hysteresis-band is recorded as an event on a digital counter, and also integrated or summed to be displayed as a stepped-integral or bar-chart of total distance covered by the subject in each counting period. The energy-density of the radar can be kept very low, so that possible influence on the subject is minimal. The detector is remote from the box containing the subject, giving great flexibility in container design, so that the apparatus is ideally suited to the study of environmental factors. 相似文献
3.
4.
ROGER M. EVANS 《Ibis》1995,137(3):340-344
Gannets (Sulidae) and some other pelecaniforms incubate their eggs under the webs of their totipalmate feet. These species have a wide latitudinal distribution from tropics to subarctic, but little is known of the incubation temperatures attained. I measured egg temperatures of the Australasian Gannet Morns serrator at Cape Kidnappers, New Zealand, employing a data logger to obtain records every 15 min for day-long sample periods at undisturbed nests. Egg surface temperatures were relatively stable and little affected by ambient temperature. Mean surface temperature of natural eggs was 34.9o C for samples taken during the first 4 days of incubation, but this then increased and stabilized at 36.5o C. Internal temperature of pipped eggs was about 1o C higher, attributable to embryonic heat production. Upper surface temperature of eggs kept in a fixed position was about 2o C below deep adult body temperature (40.3o C), suggesting heat is transferred directly from the body through the feet. Clutch size does not appear to be limited by an inability to warm two eggs. These results are in general agreement with measurements from other web incubators and are well within the range for species with conventional brood patch incubation. 相似文献
5.
6.
T. R. FRASIER T. RASTOGI M. W. BROWN P. K. HAMILTON S. D. KRAUS B. N. WHITE 《Molecular ecology resources》2006,6(4):1025-1029
A North Atlantic right whale (Eubalaena glacialis) genomic library was developed and screened with a (GATA)8 probe to identify tetranucleotide microsatellite loci. Sixteen characterized loci were polymorphic in North Atlantic and/or South Atlantic (Eubalaena australis) right whales, 12 being polymorphic in E. glacialis, and 15 in E. australis. Fourteen of these were combined with 21 other previously identified loci for a suite of 35 loci which can be used to increase resolution of genetic analyses of these species. Multiplex reactions were developed for genotyping samples at these loci, providing a method that is rapid, reliable and cost‐effective. 相似文献
7.
8.
Intra- and extracellular Cd uptake were investigated in themoss Rhytidiadelphus squarrosus. Intracellular Cd uptake displayedMichaelisMenten kinetics and showed that the moss hada high uptake rate and high affinity for Cd. Extracellular Cdbinding capacity was also high. The anion used had little effecton Cd uptake to either location but both intra- and extracellularCd uptake were reduced by equimolar concentrations of Ca, Mgor Zn, although inhibition was not wholly competitive. IntracellularCd uptake was not significantly affected by the supply of energygenerating reserves but, like extracellular Cd uptake, showedaltered characteristics following growth under controlled laboratoryconditions. The results are compared with data on Cd uptakeby the lichen genus Peltigera. Rhytidiadelphus squarrosus, moss, cadmium, intra- and extracellular uptake characteristics, kinetics 相似文献
9.
Mechanism of iron uptake by plants 总被引:5,自引:0,他引:5
JOHN C. BROWN 《Plant, cell & environment》1978,1(4):249-257
Abstract. Green plants require a continuous supply of Fe as they grow, because Fe does not not move from the older to the newer leaves. Soils do not lack Fe per se , but it may not be available to plants grown in alkaline soils. Plants are classed 'Fe-efficient' if they respond to Fe-deficiency stress by inducing biochemical reactions that make Fe available in a useful form, and 'Fe-inefficienT' if they do not. Iron uptake induced in response to Fe stress involves release of hydrogen ions and reductants by the root. The lowered pH and presence of reductant at the root zone, along with reduction of Fe3+ to Fe2+ at the root surface, enables Fe2+ to be taken up primarily through the young lateral roots. Ferrous iron is present throughout the protozylem and may or may not have entered the root by a carrier. The root-absorbed Fe2+ is oxidized to Fe3+ at the junction of the protoxylem and the metaxylem, chelated by citrate, and then transported in the metaxylem to the plant top. In the plant, the chemical reactions injuced by Fe-deficiency stress may affect nitrate reductase activity, use of Fe from Fe3+ phosphate and chelating agents, and tolerance to heavy metals. An efficient mechanism for Fe uptake in roots appears to be important for the efficient use of Fe in plant tops. 相似文献
10.