全文获取类型
收费全文 | 3030篇 |
免费 | 433篇 |
国内免费 | 1篇 |
出版年
2021年 | 37篇 |
2018年 | 30篇 |
2017年 | 28篇 |
2016年 | 37篇 |
2015年 | 70篇 |
2014年 | 80篇 |
2013年 | 101篇 |
2012年 | 137篇 |
2011年 | 108篇 |
2010年 | 83篇 |
2009年 | 73篇 |
2008年 | 127篇 |
2007年 | 126篇 |
2006年 | 114篇 |
2005年 | 99篇 |
2004年 | 108篇 |
2003年 | 125篇 |
2002年 | 128篇 |
2001年 | 119篇 |
2000年 | 117篇 |
1999年 | 86篇 |
1998年 | 43篇 |
1997年 | 29篇 |
1996年 | 23篇 |
1994年 | 43篇 |
1993年 | 43篇 |
1992年 | 99篇 |
1991年 | 80篇 |
1990年 | 88篇 |
1989年 | 73篇 |
1988年 | 65篇 |
1987年 | 84篇 |
1986年 | 71篇 |
1985年 | 66篇 |
1984年 | 54篇 |
1983年 | 55篇 |
1982年 | 24篇 |
1981年 | 38篇 |
1980年 | 33篇 |
1979年 | 50篇 |
1978年 | 30篇 |
1977年 | 23篇 |
1976年 | 37篇 |
1975年 | 38篇 |
1974年 | 30篇 |
1973年 | 38篇 |
1972年 | 44篇 |
1970年 | 27篇 |
1969年 | 22篇 |
1968年 | 26篇 |
排序方式: 共有3464条查询结果,搜索用时 15 毫秒
81.
Summary Histological and fine structural studies of adrenal cortices were performed on male Sprague-Dawley rats which had been given
intravenous injections of prostaglandin E1 or E2. It was found that there were increased numbers of intracellular lipid droplets, free ribonucleoprotein particles, cholesterol
ester clefts and coated vesicles of both the small and large varieties. A reorganization of the internal mitochondrial membranes
and the appearance of protrusions of parenchymal cytoplasm into the sinusoidal lumina accompanied by vasodilation were also
observed. These alterations are not typical of those observed following exogenous ACTH administration and are therefore considered
to be prostaglandin-induced.
Supported in part by N.I.H. Grant AM-09561. 相似文献
82.
D. M. Olson J. R. Ruberson A. R. Zeilinger D. A. Andow 《Entomologia Experimentalis et Applicata》2011,139(2):161-169
Producers of Bt cotton, Gossypium hirsutum L. (Malvaceae), in the southeastern USA face significant losses from highly polyphagous stink bug species. These problems may be exacerbated by crop rotation practices that often result in cotton, peanut, Arachis hypogaea L., and soybean, Glycine max (L.) Merrill (both Fabaceae), growing in close proximity to one another. Because all of these crops are hosts for the major pest stink bug species in the region, we experimentally examined colonization preference of these species among the crops to clarify this aspect of their population dynamics. We planted peanut, soybean, Bt cotton, and glyphosate‐tolerant (RR) non‐Bt cotton at three sites over 3 years in replicated plots ranging from 192 to 1 323 m2 and calculated odds ratios for colonization of each crop for Nezara viridula (L.) and Euschistus servus (Say) (both Hemiptera: Pentatomidae). In four of five experiments, both E. servus and N. viridula preferred soybean significantly more often than Bt cotton, non‐Bt cotton, and peanut. Neither N. viridula nor E. servus showed any preference between non‐Bt and Bt cotton in any experiment. Both species had higher numbers in Bt and non‐Bt cotton relative to peanut; this was not significant for any single experiment, but analyses across all experiments indicated that N. viridula preferred Bt and non‐Bt cotton significantly more often than peanut. Our results suggest that soybean in the landscape may function as a sink for stink bug populations relative to nearby peanut and cotton when the soybean is in the reproductive stage of development. Stink bug preference for soybean may reduce pest pressure in near‐by crops, but population increases in soybean could lead to this crop functioning as a source for later‐season pest pressure in cotton. 相似文献
83.
Robert L. Pitman John W. Durban Michael Greenfelder Christophe Guinet Morton Jorgensen Paula A. Olson Jordi Plana Paul Tixier Jared R. Towers 《Polar Biology》2011,34(2):303-306
Studies have shown that killer whale (Orcinus orca) communities in high latitudes regularly comprise assemblages of sympatric ‘ecotypes’—forms that differ in morphology, behavior,
and prey preferences. Although they can appear superficially similar, recent genetic evidence suggests that breeding is assortative
among ecotypes within individual communities, and species-level divergences are inferred in some cases. Here, we provide information
on a recently recognized ‘type D’ killer whale based on photographs of a 1955 mass stranding in New Zealand and our own six
at-sea sightings since 2004. It is the most distinctive-looking form of killer whale that we know of, immediately recognizable
by its extremely small white eye patch. Its geographic range appears to be circumglobal in subantarctic waters between latitudes
40°S and 60°S. School sizes are relatively large (mean 17.6; range 9–35; n = 7), and although nothing is known about the type D diet, it is suspected to include fish because groups have been photographed
around longline vessels where they reportedly depredate Patagonian toothfish (Dissostichus eleginoides). 相似文献
84.
85.
Anne Lorrain Heidi Pethybridge Nicolas Cassar Aurore Receveur Valrie Allain Nathalie Bodin Laurent Bopp C. Anela Choy Leanne Duffy Brian Fry Nicolas Goi Brittany S. Graham Alistair J. Hobday John M. Logan Frederic Mnard Christophe E. Menkes Robert J. Olson Dan E. Pagendam David Point Andrew T. Revill Christopher J. Somes Jock W. Young 《Global Change Biology》2020,26(2):458-470
Considerable uncertainty remains over how increasing atmospheric CO2 and anthropogenic climate changes are affecting open‐ocean marine ecosystems from phytoplankton to top predators. Biological time series data are thus urgently needed for the world's oceans. Here, we use the carbon stable isotope composition of tuna to provide a first insight into the existence of global trends in complex ecosystem dynamics and changes in the oceanic carbon cycle. From 2000 to 2015, considerable declines in δ13C values of 0.8‰–2.5‰ were observed across three tuna species sampled globally, with more substantial changes in the Pacific Ocean compared to the Atlantic and Indian Oceans. Tuna recorded not only the Suess effect, that is, fossil fuel‐derived and isotopically light carbon being incorporated into marine ecosystems, but also recorded profound changes at the base of marine food webs. We suggest a global shift in phytoplankton community structure, for example, a reduction in 13C‐rich phytoplankton such as diatoms, and/or a change in phytoplankton physiology during this period, although this does not rule out other concomitant changes at higher levels in the food webs. Our study establishes tuna δ13C values as a candidate essential ocean variable to assess complex ecosystem responses to climate change at regional to global scales and over decadal timescales. Finally, this time series will be invaluable in calibrating and validating global earth system models to project changes in marine biota. 相似文献
86.
Bray Rodney A. Waeschenbach Andrea Littlewood D. Timothy J. Halvorsen Odd Olson Peter D. 《Systematic parasitology》2020,97(3):285-296
Systematic Parasitology - Chimaeras, or ratfishes, are the only extant group of holocephalan fishes and are the sole host group of gyrocotylidean cestodes, which represent a sister group of the... 相似文献
87.
Tammy J. Westmoreland Sajith M. Wickramasekara Andrew Y. Guo Alice L. Selim Tiffany S. Winsor Arno L. Greenleaf Kimberly L. Blackwell John A. Olson Jr Jeffrey R. Marks Craig B. Bennett 《PloS one》2009,4(6)
The chemotherapeutic doxorubicin (DOX) induces DNA double-strand break (DSB) damage. In order to identify conserved genes that mediate DOX resistance, we screened the Saccharomyces cerevisiae diploid deletion collection and identified 376 deletion strains in which exposure to DOX was lethal or severely reduced growth fitness. This diploid screen identified 5-fold more DOX resistance genes than a comparable screen using the isogenic haploid derivative. Since DSB damage is repaired primarily by homologous recombination in yeast, and haploid cells lack an available DNA homolog in G1 and early S phase, this suggests that our diploid screen may have detected the loss of repair functions in G1 or early S phase prior to complete DNA replication. To test this, we compared the relative DOX sensitivity of 30 diploid deletion mutants identified under our screening conditions to their isogenic haploid counterpart, most of which (n = 26) were not detected in the haploid screen. For six mutants (bem1Δ, ctf4Δ, ctk1Δ, hfi1Δ,nup133Δ, tho2Δ) DOX-induced lethality was absent or greatly reduced in the haploid as compared to the isogenic diploid derivative. Moreover, unlike WT, all six diploid mutants displayed severe G1/S phase cell cycle progression defects when exposed to DOX and some were significantly enhanced (ctk1Δ and hfi1Δ) or deficient (tho2Δ) for recombination. Using these and other “THO2-like” hypo-recombinogenic, diploid-specific DOX sensitive mutants (mft1Δ, thp1Δ, thp2Δ) we utilized known genetic/proteomic interactions to construct an interactive functional genomic network which predicted additional DOX resistance genes not detected in the primary screen. Most (76%) of the DOX resistance genes detected in this diploid yeast screen are evolutionarily conserved suggesting the human orthologs are candidates for mediating DOX resistance by impacting on checkpoint and recombination functions in G1 and/or early S phases. 相似文献
88.
Nitrogen and water affect direct and indirect plant systemic induced defense in cotton 总被引:1,自引:0,他引:1
Plants have direct and indirect constitutively produced and inducible defenses against herbivores and pathogens, which can substantially aid in their ability to defend themselves. However, very little is known about the influence of agronomic factors on such defenses. Here, we tested the effects of nitrogen levels and water availability on the ability of cotton plants to deter feeding by Spodoptera exigua through induction of anti-feedants, and to attract Microplitis croceipes through systemic induction of volatile emission. Cotton plants were grown with various nitrogen levels and were either exposed to water stress or normal water before being exposed to S. exigua for 48 h for induction of defenses. Dual choices of various nitrogen and water treatments were provided to M. croceipes in flight tunnel bioassays. Dual choices of leaf tissue from the various nitrogen and water treatments were provided to S. exigua larvae. Both water stress and nitrogen levels under and over the recommended levels increased leaf tissue consumption and decreased attraction of M. croceipes to the plants. Analyses of induced volatiles released from herbivore damaged plants indicate that their concentrations differ among the nitrogen levels tested with plants receiving no nitrogen or twice the recommended dose having amounts much lower than plants receiving the recommended dose. Because both direct and indirect plant defense mechanisms are negatively affected by improper nitrogen and insufficient water, we argue that these factors should be considered for a better natural control of pests in cotton and most probably in other crops. 相似文献
89.
Daniel J. Conklin Harvey B. Lillywhite Barbara Bishop Alan R. Hargens Kenneth R. Olson 《Comparative biochemistry and physiology. Part A, Molecular & integrative physiology》2009,152(3):389-397
Terrestrial, but not aquatic, species of snakes have hepatic portal veins with a corkscrew morphology immediately posterior of the liver. Relatively large volumes of venous blood are associated with this region, and the corkscrew vein has been proposed to function as a bidirectional valve that impedes gravitational shifts of intravascular volume. To better understand the functional significance of the corkscrew anatomy, we investigated the histology and contractile mechanisms in isolated corkscrew segments of the hepatic portal vein of a yellow rat snake (Pantherophis obsoletus). Morphologically, the corkscrew portal vein is here shown to have two distinct layers of smooth muscle – an inner circular layer, and an outer longitudinal layer, separated by a layer of collagen, – whereas only a single circular layer of smooth muscle is present in the adjacent posterior caval vein. Low frequency (~ 0.3 cycles*min? 1) spontaneous and catecholamine-induced rhythms were observed in 11% and 89% of portal vein segments, respectively, but neither spontaneous nor agonist-induced cycling was observed in adjacent posterior (non-corkscrew) caval veins. Catecholamines, angiotensin II, or stretch increased the amplitude and/or frequency of contractile cycles. Ouabain, verapamil or indomethacin, but not tetrodotoxin, α-, or ß-adrenergic receptor antagonists, inhibited cyclical contractions indicating a dependence of these cycles on Na+/K+ ATPase, extracellular Ca2+ and prostanoid(s). These data suggest that the rhythmic contractility of the corkscrew segment of the ophidian portal vein may act in conjunction with its morphological features to improve venous return and to prevent retrograde shifts of blood that might otherwise pool in posterior veins. 相似文献
90.
Chris M Cirimotich Jaclyn C Scott Aaron T Phillips Brian J Geiss Ken E Olson 《BMC microbiology》2009,9(1):49