全文获取类型
收费全文 | 1257篇 |
免费 | 38篇 |
专业分类
林业 | 32篇 |
农学 | 22篇 |
基础科学 | 9篇 |
109篇 | |
综合类 | 300篇 |
农作物 | 37篇 |
水产渔业 | 99篇 |
畜牧兽医 | 577篇 |
园艺 | 16篇 |
植物保护 | 94篇 |
出版年
2020年 | 18篇 |
2019年 | 8篇 |
2018年 | 18篇 |
2016年 | 13篇 |
2015年 | 9篇 |
2014年 | 22篇 |
2013年 | 46篇 |
2012年 | 38篇 |
2011年 | 50篇 |
2010年 | 30篇 |
2009年 | 26篇 |
2008年 | 54篇 |
2007年 | 55篇 |
2006年 | 58篇 |
2005年 | 43篇 |
2004年 | 48篇 |
2003年 | 35篇 |
2002年 | 21篇 |
2001年 | 29篇 |
2000年 | 30篇 |
1999年 | 19篇 |
1997年 | 8篇 |
1996年 | 9篇 |
1994年 | 10篇 |
1993年 | 13篇 |
1992年 | 20篇 |
1991年 | 38篇 |
1990年 | 39篇 |
1989年 | 37篇 |
1988年 | 38篇 |
1987年 | 24篇 |
1986年 | 20篇 |
1985年 | 15篇 |
1984年 | 16篇 |
1983年 | 15篇 |
1982年 | 10篇 |
1980年 | 13篇 |
1979年 | 19篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1975年 | 8篇 |
1974年 | 22篇 |
1973年 | 20篇 |
1972年 | 15篇 |
1971年 | 13篇 |
1970年 | 14篇 |
1969年 | 18篇 |
1968年 | 10篇 |
1967年 | 12篇 |
1966年 | 10篇 |
排序方式: 共有1295条查询结果,搜索用时 15 毫秒
91.
Roberts HF 《Science (New York, N.Y.)》1919,49(1274):516-517
92.
L Roberts 《Science (New York, N.Y.)》1989,246(4927):204-205
93.
94.
95.
Cystic fibrosis pilot projects go begging 总被引:2,自引:0,他引:2
L Roberts 《Science (New York, N.Y.)》1990,250(4984):1076-1077
96.
Memory in mice analyzed with antibiotics. Antibiotics are useful to study stages of memory and to indicate molecular events which sustain memory 总被引:11,自引:0,他引:11
It is apparent that antibiotics are useful in differentiating different stages in the formation of memory. Puromycin gave the first indication that very early memory can be established and survive, for a short period at least, in spite of inhibition of protein synthesis (12). Injection of actinomycin D indicates that RNA synthesis is not essential during this early stage (13). The duration of this early period seems to vary with the inhibiting agent; with puromycin memory was notably degraded in less than an hour, but with actinomycin D or with acetoxycycloheximide it persisted for several hours or more. The fixation or consolidation of memory involves whatever processes give permanence to memory. These processes are disrupted when electroconvulsive shock is administered shortly after a learning experience, presumably because of the interference with organized patterns of neuronal electrical activity. Memory acquired in the presence of antibiotics appears to proceed to a stage beyond that based purely on electrical activity because the memory persists beyond the period usually reported as sensitive to electroconvulsive shock. Further work should show whether this stage is truly insensitive to electroconvulsive shock. Memory acquired in the presence of puromycin does not seem to achieve any durable consolidation. In contrast, memory acquired in the presence of or immediately before injection of acetoxycycloheximide does appear to initiate the later stages of consolidation, as permanent memory. reappears some days after the initial stages have become ineffective in controlling performance. Finally, puromycin has provided evidence of the enlarged area of the neocortex which participates as memory matures. Puromycin also indicates the time required for this maturation process. Since antibiotics have also been useful in studying learning and memory in goldfish (14), this approach seems to have general applicability in defining various stages in the process of memory formation. The initial purpose of these investigations was to determine the molecular basis of the "memory trace" This goal still remains distant, although there are some indications that protein synthesizing systems are involved. This objective, though of enormous interest, is to be regarded as only a necessary first step. Whether new proteins or some other molecules cause the changes in synapses thought to underlie memory, this knowledge of itself will contribute only a beginning to our understanding of the events which account for the functioning of the brain. A determination of the composition of computer components would provide very little information towards unraveling their function. As the experiments proceeded, however, information of a more general nature was being obtained. The identification of different stages of consolidation show how injections of antibiotics can supplement electroconvulsive shock as a way of disrupting the establishment of memory and how it can supplement ablation in destroying memory already laid down in a permanent mode. Applied to larger animals the localization of various regions sensitive or insensitive to the action of the drugs should become more definitive. We hope that such experiments will contribute increasingly to the general problem of brain function. 相似文献
97.
Steps toward computer analysis of nucleotide sequences 总被引:23,自引:0,他引:23
Advances in recombinant DNA technology have allowed the isolation of large numbers of biologically interesting fragments of DNA. Concomitant improvements in methods for nucleic acid sequencing have led many investigators to characterize their clones by sequencing them. This has resulted in the accumulation of such large amounts of sequence data that computer-assisted methods, with programs directed toward the manipulation of nucleic acid sequences, have become indispensable during the collection and analysis of that data. 相似文献
98.
Middle Paleolithic assemblages from the Indian subcontinent before and after the Toba super-eruption
Petraglia M Korisettar R Boivin N Clarkson C Ditchfield P Jones S Koshy J Lahr MM Oppenheimer C Pyle D Roberts R Schwenninger JL Arnold L White K 《Science (New York, N.Y.)》2007,317(5834):114-116
The Youngest Toba Tuff (YTT) eruption, which occurred in Indonesia 74,000 years ago, is one of Earth's largest known volcanic events. The effect of the YTT eruption on existing populations of humans, and accordingly on the course of human evolution, is debated. Here we associate the YTT with archaeological assemblages at Jwalapuram, in the Jurreru River valley of southern India. Broad continuity of Middle Paleolithic technology across the YTT event suggests that hominins persisted regionally across this major eruptive event. 相似文献
99.
Rational design of peptide-based HIV proteinase inhibitors 总被引:48,自引:0,他引:48
N A Roberts J A Martin D Kinchington A V Broadhurst J C Craig I B Duncan S A Galpin B K Handa J Kay A Kr?hn 《Science (New York, N.Y.)》1990,248(4953):358-361
A series of peptide derivatives based on the transition-state mimetic concept has been designed that inhibit the proteinase from the human immunodeficiency virus (HIV). The more active compounds inhibit both HIV-1 and HIV-2 proteinases in the nanomolar range with little effect at 10 micromolar against the structurally related human aspartic proteinases. Proteolytic cleavage of the HIV-1 gag polyprotein (p55) to the viral structural protein p24 was inhibited in chronically infected CEM cells. Antiviral activity was observed in the nanomolar range (with one compound active below 10 nanomolar) in three different cell systems, as assessed by p24 antigen and syncytium formation. Cytotoxicity was not detected at 10 and 5 micromolar in C8166 and JM cells, respectively, indicating a high therapeutic index for this new class of HIV proteinase inhibitors. 相似文献
100.