全文获取类型
收费全文 | 2646篇 |
免费 | 262篇 |
出版年
2023年 | 8篇 |
2022年 | 12篇 |
2021年 | 33篇 |
2020年 | 27篇 |
2019年 | 25篇 |
2018年 | 44篇 |
2017年 | 29篇 |
2016年 | 63篇 |
2015年 | 116篇 |
2014年 | 115篇 |
2013年 | 132篇 |
2012年 | 184篇 |
2011年 | 186篇 |
2010年 | 121篇 |
2009年 | 93篇 |
2008年 | 150篇 |
2007年 | 123篇 |
2006年 | 134篇 |
2005年 | 158篇 |
2004年 | 119篇 |
2003年 | 145篇 |
2002年 | 115篇 |
2001年 | 69篇 |
2000年 | 52篇 |
1999年 | 59篇 |
1998年 | 35篇 |
1997年 | 31篇 |
1996年 | 37篇 |
1995年 | 19篇 |
1994年 | 39篇 |
1993年 | 30篇 |
1992年 | 29篇 |
1991年 | 33篇 |
1990年 | 35篇 |
1989年 | 28篇 |
1988年 | 20篇 |
1987年 | 22篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 16篇 |
1983年 | 22篇 |
1982年 | 11篇 |
1981年 | 17篇 |
1980年 | 10篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 10篇 |
1973年 | 6篇 |
1968年 | 10篇 |
1967年 | 9篇 |
排序方式: 共有2908条查询结果,搜索用时 15 毫秒
1.
Sukhraj S. Dhillon Graeme P. Berlyn Jerome P. Miksche 《American journal of botany》1978,65(2):192-196
Five populations of Pinus rigida growing in contrasting ecological situations ranging from North Carolina (35°53'N latitude) to Quebec (45°06'N latitude) showed no significant variation in amounts of nuclear DNA with respect to germinating seedlings. Nuclear volume of dormant nuclei also showed no significant variation between and within populations, a finding that is consistent with the concept that the basic 2C DNA value of Pinus rigida is uniform under all habitats. This finding is in contrast to numerous reports for other coniferous species. 相似文献
2.
Treatment of cut-throat trout broodfish Salmo clarki (Richardson) with Sulphamerazine at 220 mg/kg (10 g/100 1b) of fish/day for 14 days resulted in severe kidney histopathology and increased mortality among males. Experimental data presented showed that cut-throat trout broodfish were extremely sensitive to Sulphamerazine toxicity. Hydropic degeneration of renal tubule epithelium and haemorrhage into tubule lumens were observed in kidneys of both male and female trout, but was more severe in the former. Death, which occurred only in males, was correlated with spawning stress and impaired renal function. 相似文献
3.
Expanded thermal niche for a diving vertebrate: A leatherback turtle diving into near-freezing water
Michael C. James Graeme C. Hays 《Journal of experimental marine biology and ecology》2006,335(2):221-226
The global distribution of extant reptiles is more limited than that of mammals or birds, with low reptilian species diversity at high latitudes. Central to this limited geographical distribution is the ectothermic nature of reptiles, which means that they generally become torpid at cold temperatures. However, here we report the first detailed telemetry from a leatherback turtle (Dermochelys coriacea) diving in cold water at high latitude. An individual equipped with a satellite tag that relayed temperature-depth profiles dived continuously for many weeks into sub-surface waters as cold as 0.4 °C. Global warming will likely increase the foraging range of leatherback turtles further into temperate and boreal waters. 相似文献
4.
In many ecological situations, resources are difficult to find but become more apparent to nearby searchers after one of their numbers discovers and begins to exploit them. If the discoverer cannot monopolize the resources, then others may benefit from joining the discoverer and sharing their discovery. Existing theories for this type of conspecific attraction have often used very simple rules for how the decision to join a discovered resource patch should be influenced by the number of individuals already exploiting that patch. We use a mechanistic, spatially explicit model to demonstrate that individuals should not necessarily simply join patches more often as the number of individuals exploiting the patch increases, because those patches are likely to be exhausted soon or joining them will intensify future local competition. Furthermore, we show that this decision should be sensitive to the nature of the resource patches, with individuals being more responsive to discoveries in general and more tolerant of larger numbers of existing exploiters on a patch when patches are resource-rich and challenging to locate alone. As such, we argue that this greater focus on underlying joining mechanisms suggests that conspecific attraction is a more sophisticated and flexible tactic than currently appreciated. 相似文献
5.
Diel changes in the near-surface (050 m) abundance, prosomelength, and carbon and nitrogen content of the copepod Metridialucens were measured in Deep Cove, Doubtful Sound, New Zealand(45°27'S, 167°9'E) between 3 and 6 September 1996. Metridialucens showed maximal abundance a night, suggesting a patternof normal diel vertical migration (DVM). The change in abundancesuggested that the descent of the population occurred 1h prior to dawn and the ascent 1 h after dusk. However,a proportion of the population remained near the surface duringthe day. Although there was no diel pattern in the prosome lengthof M.lucens collected near the surface, there was a marked dielcycle in the measured carbon and nitrogen contents, with maximalvalues being measured towards the end of the night prior tothe downward migration. We suggest that this diet cycle in themeasured elemental content was caused by DVM occurring morestrongly in those animals which had a better body condition,i.e. a higher elemental content per unit length. 相似文献
6.
7.
M Kubel B Freistedt P Hammer V Thierbach W Helbig B Haustein W Schultze 《Folia haematologica (Leipzig, Germany : 1928)》1982,109(2):355-364
Thrombocyte substitution is an essential prerequisite for intensive cytoreductive therapy in acute leukemia. Evaluating 228 thrombocyte transfusions in 17 patients shows that the clinical effectiveness of thrombocyte concentrates can be increased by making the coordination of HLA antigens of donor and receiver as good as possible. When measured in the corrected increment (CI) 24 hours after transfusion, the effectiveness of A3/B1 match preparations (CI = 7.0 +/- 1.6) is significantly higher than that of random preparations (CI = 3.0 +/- 0.5). With the presence of HLA antibodies an effective substitution (CI24 greater than or equal to 4.5) can only be achieved by A3/B1 match thrombocytes. This can only be realized by applying the fourfold thrombapheresis of single donors. 相似文献
8.
Graeme J. Inglis† 《Journal of Ecology》2000,88(1):88-99
9.
10.
Injection of norepinephrine (NE) into the paraventricular nucleus of the hypothalamus has been shown to elicit eating in satiated rats. In the present study, the monoamine oxidase inhibitor tranylcypromine (TCP), which is known to release endogenous NE, was found to produce a similar effect in rats maintained and tested on a palatable milk-mash combination diet. The effect of this antidepressant drug was positively correlated in magnitude with the NE effect in the same animals. It was selectively antagonized by central injection of drugs which block α-adrenergic receptors. Local pretreatment with NE synthesis inhibitors similarly blocked the TCP eating response but had no effect on eating elicited by exogenous NE. It is suggested from these results that TCP is stimulating eating through the release of endogenous NE from adrenergic neurons innervating the region of the paraventricular nucleus. 相似文献