全文获取类型
收费全文 | 980篇 |
免费 | 77篇 |
国内免费 | 42篇 |
出版年
2023年 | 11篇 |
2022年 | 13篇 |
2021年 | 35篇 |
2020年 | 33篇 |
2019年 | 34篇 |
2018年 | 19篇 |
2017年 | 33篇 |
2016年 | 33篇 |
2015年 | 30篇 |
2014年 | 42篇 |
2013年 | 75篇 |
2012年 | 37篇 |
2011年 | 35篇 |
2010年 | 28篇 |
2009年 | 36篇 |
2008年 | 49篇 |
2007年 | 43篇 |
2006年 | 38篇 |
2005年 | 28篇 |
2004年 | 38篇 |
2003年 | 35篇 |
2002年 | 21篇 |
2001年 | 28篇 |
2000年 | 28篇 |
1999年 | 28篇 |
1998年 | 21篇 |
1997年 | 20篇 |
1996年 | 21篇 |
1995年 | 17篇 |
1994年 | 12篇 |
1993年 | 19篇 |
1992年 | 20篇 |
1991年 | 9篇 |
1990年 | 13篇 |
1989年 | 8篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1985年 | 7篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 10篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1976年 | 7篇 |
1975年 | 3篇 |
1973年 | 5篇 |
1972年 | 3篇 |
排序方式: 共有1099条查询结果,搜索用时 375 毫秒
1.
C. VILLARREAL M. H. RODRIGUEZ D. N. BOWN J. I. ARREDONDO-JIMÉNEZ 《Medical and veterinary entomology》1995,9(2):187-194
Abstract. Village-scale trials were carried out in southern Mexico to compare the efficacy of indoor-spraying of the pyrethroid insecticide lambda-cyhalothrin applied either as low-volume (LV) aqueous emulsion or as wettable-powder (WP) aqueous suspension for residual control of the principal coastal malaria vector Anopheles albimanus. Three indoor spray rounds were conducted at 3-month intervals using back-pack mist-blowers to apply lambda-cyhalothrin 12.5 mg a.i./m2 by LV, whereas the WP was applied by conventional compression sprayer at a mean rate of 26.5 mg a.i./m2 .
Both treatments caused mosquito mortality indoors and outdoors (collected inside house curtains) as a result of contact with treated surfaces before and after feeding, but had no significant impact on overall population density of An. albimanus resting indoors or assessed by human bait collections. Contact bioassays showed that WP and LV treatments with lambda-cyhalothrin were effective for 12–20 weeks (>75% mortality) without causing excito-repellency.
Compared to the WP treatment (8 houses/man/day), LV treatment (25 houses/man/day) was more than 3 times quicker per house, potentially saving 68% of labour costs. This is offset, however, by the much lower unit price of a compression sprayer (e.g. Hudson 'X-pert' at US120) than a mist-blower (e.g. 'Super Jolly' at US350), and higher running costs for LV applications. It was calculated, therefore, that LV becomes more economical than WP after 18.8 treatments/100 houses/10 men at equivalent rates of application, or after 7.6 spray rounds with half-rate LV applications. 相似文献
Both treatments caused mosquito mortality indoors and outdoors (collected inside house curtains) as a result of contact with treated surfaces before and after feeding, but had no significant impact on overall population density of An. albimanus resting indoors or assessed by human bait collections. Contact bioassays showed that WP and LV treatments with lambda-cyhalothrin were effective for 12–20 weeks (>75% mortality) without causing excito-repellency.
Compared to the WP treatment (8 houses/man/day), LV treatment (25 houses/man/day) was more than 3 times quicker per house, potentially saving 68% of labour costs. This is offset, however, by the much lower unit price of a compression sprayer (e.g. Hudson 'X-pert' at US120) than a mist-blower (e.g. 'Super Jolly' at US350), and higher running costs for LV applications. It was calculated, therefore, that LV becomes more economical than WP after 18.8 treatments/100 houses/10 men at equivalent rates of application, or after 7.6 spray rounds with half-rate LV applications. 相似文献
2.
H. Daniel 《Biological trace element research》1987,13(1):301-318
Because of the large μ mass compared to the electron mass, the muonic X-rays have energies very suitable for standard γ-ray
spectroscopy (Ge detectors), so every element is easily recognized. By selecting the primary μ energies appropriately any
part of the specimen, also well inside, can be nondestructively investigated. On the other hand, surface layers may be analyzed.
Accuracies of quantitative analyses are 1% of the atomic abundance of the element in question in favorite cases. Results on
applications in nuclear medicine and surface physics are presented, and ways of improving the muon flux density are discussed. 相似文献
3.
Summary During imbibition of whole tea seeds (6 days) two purine alkaloids, caffeine and theobromine, did not decrease in the seed
coats and there was no increase in the seeds. In parallel with and after the breaking of seed coats there was a gradual release
of caffeine from coats of germinating seeds. By contrast, when the seed was freed from the outer seed coat and soaked, imbibition
of the seed required only 2 days and simultaneously caffeine was released from the inner seed coat. In such seeds, but not
in whole seeds, growth of embryonic tissues (roots and shoots) was inhibited after the breaking of the inner seed coats. Nevertheless,
caffeine increased more in such roots of the seedlings of decoated seeds than in roots of normal seedlings. 相似文献
4.
Seeds of the root parasitesStriga (several spp.) andBuchnera americana were examined by means of SEM. The surface patterns of the seeds in both genera resemble each other closely, especially those ofS. angustifolia andB. americana. SomeStriga spp. can be clearly distinguished by their surface characteristics, while this is quite difficult in others. The taxonomic value of the seed surface features ofStriga andBuchnera is discussed. 相似文献
5.
The seed coat surface morphology of Celtidaceae and Ulmaceae (Urticales) indicates a significant evolutionary diversity.Celtis, Chaetachme andPteroceltis (Celtidaceae) have a unique sculpturing with many crateriform holes; such holes occasionally sparsely occur in seeds ofAphananthe, Gironniera (Celtidaceae) andPlanera (Ulmaceae), but not in those of the nine remaining genera of the two families. The perforated seed coat further occurs in
at least some genera of all other urticalean families. A pattern of its occurrence in families and genera suggest that the
perforation represents a common archaic feature of all Urticales, rather than a feature derived many times independently within
the order. The seed coat of Celtidaceae and Ulmaceae seems to have lately lost the holes probably by a neotenic evolution:
one or more times within Celtidaceae, and one time in an ancestral line leading to all Ulmaceae. The derived reticulate seed
coat surface sculpturing, which is shared byGironniera (Celtidaceae) and some Ulmaceae, is probably the result of parallel evolution. On the basis of evidence from seed coat morphology
and other sources, close relationships ofLozanella, Parasponia andTrema within Celtidaceae, as well as variously distinct positions ofAmpelocera, Aphananthe andGironniera, are also discussed. 相似文献
6.
马铃薯Y病毒外壳蛋白基因的克隆及序列分析 总被引:1,自引:0,他引:1
本文报道应用聚合酶链式反应(PCR)技术,在体外扩增马铃薯 Y 病毒外壳蛋白基因及其克隆和序列分析的结果。病毒 RNA 从马铃薯 Y 病毒感染的烟草叶片中提取,用合成的PCR 3引物及 AMV 逆转录酶合成了单链的 cDNA。利用 PCR 技术,经30个循玎的扩增。得到了一特异的0.8kb 片段。克隆后对此片段进行了限制性内切酶物理图谱分析,并测定了其全序列。实验结果证明,我们克隆到的是完整的马铃薯 Y 病毒的外壳蛋白基因。与国外报道的马铃薯 Y 病毒 N 株相比,其核苷酸序列及推测的氨基酸序列的同源率分别为97.8%和97%。将该基因导入马铃薯以期获得抗 Y 病毒马铃薯的工作正在进行。本文还对 PCR 技术用于扩增植物 RNA 病毒的方法以及用基因工程方法培育抗病毒作物新品种的可行性等进行了讨论。 相似文献
7.
本研究工作中,建立了一个有效的甜菜坏死黄脉病毒的分离提纯程序,解决了该病毒粒体易于聚集难以提纯的问题,其操作要点是,(1)通过Sepharose 2B柱层析代替超离心,有效地除去一些小分子量核酸杂质;(2)经PEG再次沉淀浓缩后,调整pH至酸牲(pH3.0),使病毒充分悬浮以减少凝聚;(3)在病毒等电点(pH4.8~4.9)条件下,进一步沉淀以纯化病毒。根据病毒提取物的OD260/OD280比值,算出核酸含量约4.5%。核酸电泳出现4条带,分子量分别为:2.25×10~(?),1.8×10~(?),1.05×10~(?),0.75×10~(?)道尔顿。病毒提取物经超速离心出现4个界面,沉淀系数分别为,200.8S,165S,125.8S,100S。说明甜菜坏死黄脉病毒可能是4组分病毒粒体。病毒粒体含一蛋白亚基,分子量约为2.05±0.05×10~4道尔顿,由16种共199个氨基酸组成。 相似文献
8.
9.
The genomic sequences of several RNA plant viruses including cucumber mosaic virus, brome mosaic virus, alfalfa mosaic virus
and tobacco mosaic virus have become available recently. The former two viruses are icosahedral while the latter two are bullet
and rod shaped, respectively in particle morphology. The non-structural 3a proteins of cucumber mosaic virus and brome mosaic
virus have an amino acid sequence homology of 35% and hence are evolutionarily related. In contrast, the coat proteins exhibit
little homology, although the circular dichroism spectrum of these viruses are similar. The non-coding regions of the genome
also exhibit variable but extensive homology. Comparison of the brome mosaic virus and alfalfa mosaic virus sequences reveals
that they are probably related although with a much larger evolutionary distance. The polypeptide folds of the coat protein
of three biologically distinct isometric plant viruses, tomato Bushy stunt virus, southern bean mosaic virus and satellite
tobacco necrosis virus have been shown to display a striking resemblance. All of them consist of a topologically similar 8-standard
β-Barrel. The implications of these studies to the understanding of the evolution of plant viruses will be discussed. 相似文献
10.
George M. White 《Cell biochemistry and biophysics》1987,11(1):25-33
Computers, the human mind, and social systems have common problems of inadequate memory and insufficient data manipulation
speed. In each of these domains, information compression techniques have evolved to reduce storage and processing needs. Among
the techniques for information compression, coding of information in procedures stands out as exceptionally powerful. Procedural
information coding also gives rise to behavior that may be defined as intelligent. It is found in the human mind, in machines
and in social systems. Its use in human thought is aided by language development which promotes regular review of abstract
procedures. A practical consequence of better understanding of procedural information coding is the possibility of training
people to exhibit greater mental capacity, a controversial possibility. This paper explores the impact of data processing
resource limitations, data compression and procedural thinking in men and machines. 相似文献