首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3314篇
  免费   289篇
  3603篇
  2021年   47篇
  2020年   31篇
  2019年   30篇
  2018年   39篇
  2017年   30篇
  2016年   55篇
  2015年   88篇
  2014年   88篇
  2013年   125篇
  2012年   135篇
  2011年   133篇
  2010年   102篇
  2009年   74篇
  2008年   132篇
  2007年   143篇
  2006年   100篇
  2005年   90篇
  2004年   96篇
  2003年   97篇
  2002年   104篇
  2001年   99篇
  2000年   102篇
  1999年   80篇
  1998年   55篇
  1997年   41篇
  1995年   31篇
  1993年   38篇
  1992年   60篇
  1991年   58篇
  1990年   56篇
  1989年   63篇
  1988年   58篇
  1987年   51篇
  1986年   52篇
  1985年   45篇
  1984年   38篇
  1983年   38篇
  1982年   36篇
  1981年   30篇
  1980年   34篇
  1979年   43篇
  1978年   38篇
  1976年   34篇
  1975年   38篇
  1974年   46篇
  1973年   42篇
  1972年   30篇
  1971年   31篇
  1969年   32篇
  1968年   38篇
排序方式: 共有3603条查询结果,搜索用时 15 毫秒
1.
Atelocauda koae, a rust of the native HawaiianAcacia koa, is considered as a demicyclic species, having spermogonial, aecial, and telial states, but is unusual in production of aeciospores simultaneously with teliospores rather than consecutively. Host inoculation with spores of each state separately confirmed that the life cycle was perpetuated by the telial state, but the aeciospores, while capable of germination and stomal penetration, did not produce detectable infection. This rust therefore behaves as a microcyclic species, and appears to be in evolutionary transition toward this reduced state. Teliospores produced vestigial, permanently attached basidiosopores which germinated to produce infective hyphae. The hyphae entered the host either through stomata or penetrated the epidermis directly, with the latter method being more common. Unusual nuclear associated with teliospore germination, in which meiosis occurs in more than one diploid nucleus was observed, in confirmation of an earlier study.  相似文献   
2.
The Lord Howe Island Stick-insect (Dryococelus australis) was formerly abundant on Lord Howe Island, Australia, but was extirpated by Black Rats (Rattus rattus) in the 1920s. The species was thought to be extinct, until freshly dead remains were found by climbers on Balls Pyramid during the 1960s. In February 2001, a survey of Balls Pyramid led to the discovery of a small population of D. australis on a precipitous terrace 65 m above sea level. Two adults and one nymph (all females) were located feeding on an endemic tea-tree (Melaleuca howeana). An accumulation of plant debris at the base of the shrub, kept moist by water seepage, provided the insects with damp hollows suitable for use as daytime refugia. All evidence indicated that the species was confined to this single small terrace. A second survey, in March 2002, located a total of 24 D. australis. Twelve individuals were in the same shrub as that occupied the previous year, and 12 were dispersed among five nearby, smaller shrubs. Ten individuals were able to be sexed – eight females and two males. A number of threats to the population of D. australis on Balls Pyramid are identified and several management actions are proposed to ensure the conservation of the species.  相似文献   
3.
    
The central Pacific Ocean with its many low lying islands and atolls is under threat from sea level rise and increased storm activity. Here, we illustrate how increasing frequency and severity of large scale storm events associated with global climate change may be particularly profound at the local scale for human populations that rely on lagoon systems for provision of a variety of goods and services. In August 2011 a storm originating in the Southern Ocean caused a large amplitude ocean swell to move northward through the Pacific Ocean. Its arrival at Palmyra Atoll coincided with transient elevated sea surface height and triggered turnover of the lagoon water column. This storm-induced change to the lagoon reflects long distance connectivity with propagated wave energy from the Southern Ocean and illustrates the increasing threats generated by climate change that are faced by human populations on most low-lying Pacific islands and atolls.  相似文献   
4.
Quinolinate phosphoribosyl transferase (QPT) activity was not affected when Escherichia coli were treated with hyperbaric oxygen. This result is not in accord with a previous report (Biochem. Biophys. Res. Comm. 91:982-990; 1979) in which the enzyme was shown to be rapidly inactivated in E. Coli exposed to 4.2 atmospheres of oxygen. Our data rule out QPT as a site of oxygen toxicity and suggest other mechanisms for the inhibitory effects of the hyperbaric oxygen on pyridine nucleotide biosynthesis.  相似文献   
5.
A membrane-bound sialidase was isolated from blood stream (BS) Trypanosoma evansi partially purified and characterized. The enzyme is a glycosyl phosphatidyl inositol (GPI) membrane anchored protein. It was solubilized from T. evansi cells recovered from infected camel blood by detergent treatment with Triton CF 54 and partially purified by a series of chromatography steps. The enzyme was optimally active at pH 5.5 and 37 degrees C. It had a KM and Vmax values of 4.8 x 10(-6) M and 3.75 x 10(-6) mol/min x mg protein with Neu5Acalpha2, 3lac as substrate respectively. The KM and Vmax values with fetuin (4-nitrophenyl-oxamic acid) as substrate were 2.9 x 10(-2) M and 4.2 x 10(-3) mol/min x mg protein in the same respect. Kinetic analysis with methly umbelliferyl sialate (MU-Neu5Ac) gave KM and Vmax values of 0.17 mM and 0.84 mmol/min x mg protein respectively. The T. evansi SD could hydrolyse internally linked sialic acid residues of the ganglioside GM2, but was inactive towards colomic acid, and NeuSAc2, 6. lac. When ghost red blood cell (RBC) was used as substrate, it desialylated the RBC in the following order of efficiency; mouse, rat, camel, goat, and dog. Similarly, cerebral cells isolated from BalbC mouse was desialylated by the T. evansi SD. Inhibition studies using 2-deoxy-2, 3 didehydro-N-acetyl neuraminic acid (NeuAc2, 3en) against MU-Neu5Ac revealed a competitive inhibition pattern with Ki of 5.8 microM. The enzyme was also inhibited non-competitively by parahydroxy oxamic acid (pHOA), and competitively by N-ethylmaleimide and N-bromosuccinate with Ki values of 25, 42, and 53 microM, respectively. It was activated by Mg2+ ion and inhibited by Cu2+ and Zn2+.  相似文献   
6.
    
BACKGROUND: Although many birth defects surveillance programs incorporate georeferenced records into their databases, practical methods for routine spatial surveillance are lacking. We present a macroprogram written for the software package R designed for routine exploratory spatial analysis of birth defects data, the Automated Spatial Surveillance Program (ASSP), and present an application of this program using spina bifida prevalence data for metropolitan Atlanta. METHODS: Birth defects surveillance data were collected by the Metropolitan Atlanta Congenital Defects Program. We generated ASSP maps for two groups of years that correspond roughly to the periods before (1994-1998) and after (1999-2002) folic acid fortification of flour. ASSP maps display census tract-specific spina bifida prevalence, smoothed prevalence contours, and locations of statistically elevated prevalence. We used these maps to identify areas of elevated prevalence for spina bifida. RESULTS: We identified a large area of potential concern in the years following fortification of grains and cereals with folic acid. This area overlapped census tracts containing large numbers of Hispanic residents. CONCLUSIONS: The potential utility of ASSP for spatial disease monitoring was demonstrated by the identification of areas of high prevalence of spina bifida and may warrant further study and monitoring. We intend to further develop ASSP so that it becomes practical for routine spatial monitoring of birth defects.  相似文献   
7.
The predator avoidance hypothesis suggests that the failure of subordinate birds to provision nestlings in communally breeding species is a consequence of increased predation risk. Parents exclude subordinates from the nest area and thus reduce the frequency of predator-attracting visits when the nest is most vulnerable, leading to increased reproductive success. I evaluated this hypothesis for the speckled warbler Chthonicola sagittata , a group-living member of the Pardalotidae in which subordinate males never feed nestlings or fledglings even though they are unrelated to the primary pair, compete for copulations and sometimes sire young in the brood. Parents did not modify provisioning behaviour relative to the risk of nest predation; provisioning rates to 10 d-old nestlings were similar on high and low risk territories. Furthermore, there was no evidence that parents modified the timing of deliveries or adjusted the relative size of deliveries in relation to predation risk. The condition (residual mass) of nestlings differed between high and low risk territories because nestlings on high risk territories had smaller tarsi but similar body mass to those at low risk. Tarsus length was the result of parental phenotype, not modified provisioning behaviour. Given that parents were unresponsive to predation risk, it seems unlikely that predation can account for the failure of subordinates to provision at the nest.  相似文献   
8.
Nitrogen removal from wastewater by algae provides the potential benefit of producing lipids for biodiesel and biomass for anaerobic digestion. Further, ammonium is the renewable form of nitrogen produced during anaerobic digestion and one of the main nitrogen sources associated with wastewater. The wastewater isolates Scenedesmus sp. 131 and Monoraphidium sp. 92 were grown with ammonium, nitrate, or urea in the presence of 5 % CO2, and ammonium and nitrate in the presence of air to optimize the growth and biofuel production of these chlorophytes. Results showed that growth on ammonium, in both 5 % CO2 and air, caused a significant decrease in pH during the exponential phase causing growth inhibition due to the low buffering capacity of the medium. Therefore, biological buffers and pH controllers were utilized to prevent a decrease in pH. Growth on ammonium with pH control (synthetic buffers or KOH dosing) demonstrated that growth (rate and yield), biodiesel production, and ammonium utilization, similar to nitrate- and urea-amended treatments, can be achieved if sufficient CO2 is available. Since the use of buffers is economically limited to laboratory-scale experiments, chemical pH control could bridge the gap encountered in the scale-up to industrial processes.  相似文献   
9.
    
  相似文献   
10.
Robust growth in many bacteria is dependent upon proper regulation of the adaptive response to phosphate (Pi) limitation. This response enables cells to acquire Pi with high affinity and utilize alternate phosphorous sources. The molecular mechanisms of Pi signal transduction are not completely understood. PhoU, along with the high-affinity, Pi-specific ATP-binding cassette transporter PstSCAB and the two-component proteins PhoR and PhoB, is absolutely required for Pi signaling in Escherichia coli. Little is known about the role of PhoU and its function in regulation. We have demonstrated using bacterial two-hybrid analysis and confirmatory coelution experiments that PhoU interacts with PhoR through its PAS (Per-ARNT-Sim) domain and that it also interacts with PstB, the cytoplasmic component of the transporter. We have also shown that the soluble form of PhoU is a dimer that binds manganese and magnesium. Alteration of highly conserved residues in PhoU by site-directed mutagenesis shows that these sites play a role in binding metals. Analysis of these phoU mutants suggests that metal binding may be important for PhoU membrane interactions. Taken together, these results support the hypothesis that PhoU is involved in the formation of a signaling complex at the cytoplasmic membrane that responds to environmental Pi levels.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号