全文获取类型
收费全文 | 1765篇 |
免费 | 116篇 |
国内免费 | 259篇 |
出版年
2023年 | 15篇 |
2022年 | 14篇 |
2021年 | 44篇 |
2020年 | 45篇 |
2019年 | 59篇 |
2018年 | 50篇 |
2017年 | 40篇 |
2016年 | 38篇 |
2015年 | 68篇 |
2014年 | 74篇 |
2013年 | 102篇 |
2012年 | 67篇 |
2011年 | 76篇 |
2010年 | 78篇 |
2009年 | 75篇 |
2008年 | 74篇 |
2007年 | 107篇 |
2006年 | 86篇 |
2005年 | 76篇 |
2004年 | 79篇 |
2003年 | 79篇 |
2002年 | 57篇 |
2001年 | 61篇 |
2000年 | 30篇 |
1999年 | 45篇 |
1998年 | 51篇 |
1997年 | 33篇 |
1996年 | 37篇 |
1995年 | 36篇 |
1994年 | 37篇 |
1993年 | 31篇 |
1992年 | 34篇 |
1991年 | 32篇 |
1990年 | 16篇 |
1989年 | 29篇 |
1988年 | 19篇 |
1987年 | 22篇 |
1986年 | 25篇 |
1985年 | 38篇 |
1984年 | 25篇 |
1983年 | 17篇 |
1982年 | 16篇 |
1981年 | 19篇 |
1980年 | 18篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1973年 | 7篇 |
排序方式: 共有2140条查询结果,搜索用时 31 毫秒
171.
ABSTRACT. Many birds undergo bare part color changes during the breeding season. Most investigators have focused on color as a signal of individual quality. An alternative, but not exclusive, function of bare part color may be signaling readiness to breed, especially in colonial, asynchronous breeders. White Ibises ( Eudocimus albus ) are colonial waterbirds that show vivid bare part colors on their bills and legs during reproduction. We quantified bill and leg colors to describe color changes and their possible relationship to reproductive status during the breeding season of White Ibises in the Florida Everglades from 1998 to 2001. We also examined the correlation between bare part colors and circulating concentrations of sex steroids to understand the factors that regulate bare part colors. During the display stage, male and female ibises developed dark pink bills and scarlet legs. As the breeding season progressed, bills and legs faded and developed a muted pink hue. The bare part colors of female ibises were correlated with testosterone concentrations, but those of male ibises were not correlated with any hormones. A discriminant function analysis based on principal component scores (representing variation in saturation and hue) and the amount of black on the bill successfully classified ibis reproductive stage 94% of the time. The use of bare part colors to determine reproductive status may be useful for studying reproduction in colonially nesting birds, where access to breeding sites can be difficult and potential for researcher disturbance is high. 相似文献
172.
173.
CO2 exchange components of a temperate semi-desert sand grassland ecosystem in Hungary were measured 21 times in 2000–2001 using a closed IRGA system. Stand CO2 uptake and release, soil respiration rate (R
s), and micrometeorological values were determined with two types of closed system chambers to investigate the daily courses of gas exchange. The maximum CO2 uptake and release were –3.240 and 1.903 mol m–2 s–1, respectively, indicating a relatively low carbon sequestration potential. The maximum and the minimum R
s were 1.470 and 0.226 mol(CO2) m–2 s–1, respectively. Water shortage was probably more effective in decreasing photosynthetic rates than R
s, indicating water supply as the primary driving variable for the sink-source relations in this ecosystem type. 相似文献
174.
Summary. This paper reports an efficient protoplast culture technique, the “extra thin alginate film” technique. The development of
this improved method of protoplast culture was an outcome of an assessment of the efficiency and shortcomings of various protoplast
culture techniques. The efficiency of this technique was evaluated with two model plant systems, viz., Nicotiana tabacum and Lotus corniculatus, and a comparison was made with the “thin alginate layer” technique, another efficient protoplast culture system. Results
indicate that the culture technique with extra thin alginate film is as efficient as the technique with thin alginate layer,
with many additional advantages. The present innovation overcomes most of the limitations of protoplast culture techniques
described so far and can now be applied to a wide variety of crops to check its general applicability.
Correspondence and reprints: Department of Botanical and Environmental Sciences, Guru Nanak Dev University, Amritsar 143 005,
India. 相似文献
175.
The availability of large amounts of genomic DNA is of critical importance for many of the molecular biology assays used in the analysis of human disease. However, since the amount of patient tissue available is often limited and as particular foci of interest may consist of only a few hundred cells, the yield of DNA is often insufficient for extensive analysis. To address this problem, several whole genome amplification (WGA) methodologies have been developed. Initial WGA approaches were based on the polymerase chain reaction (PCR). However, recent reports have described the use of non-PCR-based linear amplification protocols for WGA. Using these methods, it is possible to generate microgram quantities of DNA starting with as little as 1mg of genomic DNA. This review will provide an overview of WGA approaches and summarize some of the uses for amplified DNA in various high-throughput genetic applications. 相似文献
176.
Flash photolysis has become an essential technique for dynamic investigations of living cells and tissues. This approach offers several advantages for instantly changing the concentration of bioactive compounds outside and inside living cells with high spatial resolution. Light sources for photolysis need to deliver pulses of high intensity light in the near UV range (300-380 nm), to photoactivate a sufficient amount of molecules in a short time. UV lasers are often required as the light source, making flash photolysis a costly approach. Here we describe the use of a high power 365 nm light emitting diode (UV LED) coupled to an optical fiber to precisely deliver the light to the sample. The ability of the UV LED light source to photoactivate several caged compounds (CMNB-fluorescein, MNI-glutamate, NP-EGTA, DMNPE-ATP) as well as to evoke the associated cellular Ca(2+) responses is demonstrated in both neurons and astrocytes. This report shows that UV LEDs are an efficient light source for flash photolysis and represent an alternative to UV lasers for many applications. A compact, powerful, and low-cost system is described in detail. 相似文献
177.
A practical and convenient method of rearing Eucyclops serrulatus in a microculture environment is described. A complete life cycle of E. serrulatus was maintained in a narrow space on a microscope slide glass on which a cover glass of 22 x 40 mm in size was mounted at a height of 0.8 mm. The culture medium was constituted by bottled mineral water boiled with grains of Glycine max (soybean). Chilomonas paramecium, a free-living protozoan organism, was provided as live food. Growth of nauplii hatched from eggs to the first stage of copepodite took an average of 7.7 days, and the growth of copepodite 1 to the egg-bearing adult female took an average of 20.1 days in the microculture cell with an average life time of 44.7 days. Continuous passage of copepods was successfully maintained as long as sufficient medium and food were provided. The microculture method enables an in situ microscopic observation on the growth and developmental process of helminth larvae experimentally infected to copepods as well as of copepod itself. Furthermore, it does not require anesthetization and, therefore, minimize the amount of stress exposed to copepods during the handling process. 相似文献
178.
Cátia Ornelas José Mesquita M. Helena Garcia Nelson Lopes Kalle Nättinen 《Inorganica chimica acta》2005,358(8):2482-2488
The bimetallic cyano-bridged [(η5-C5H5)(PPh3)2Ru(μ-CN)Ru(PPh3)2(η5-C5H5)][PF6] (1) was prepared by reaction of [(η5-C5H5)(PPh3)2RuCl] with N,N′-bis(cyanomethyl)ethylenediamine. The single crystal structure determined by X-ray diffraction showed crystallization on the triclinic P1 space group with a perfect alignment of the cyanide bridges. This accentric crystallization was explored having in view the NLO properties at the macroscopic level, determined by the Kurtz Powder technique. Besides the very low efficiency values for the second harmonic generation, the value obtained for the bimetallic complex 1 showed to be higher than one of the parent complex [(η5-C5H5)(PPh3)2RuCN] (2). 相似文献
179.
Microinjection of the Minos transposon is the only reported technique for generating stable transgenic lines in the cosmopolitan ascidian, Ciona intestinalis. To establish a more amenable method for generating stable transgenic Ciona, we examined the possibility of using electroporation of DNA into eggs. From 0-44.4% of electroporated individuals transmitted transgenes to the next generation. The transgene was integrated into one chromosome and multiple copies of the transgene were inserted into one site of the chromosome, indicating that electroporation is an easy and powerful technique for achieving stable transgenesis in C. intestinalis. Together with possible inland culture of this ascidian, this technique will be useful for generating stable lines which have reporter gene expression in a specific tissue or organ and the generation of transposase-expressing stable transgenic (jump-starter) lines and mutator lines which contain a lot of Minos transposons in an insertion position. 相似文献
180.
The tandem affinity purification (TAP) tag technique has been used with success to identify under nondenaturing conditions protein complexes in yeast. The technique can be used in mammalian cells, but we found that the original technique does not yield enough recovery for the identification of proteins when mammalian cells growing in monolayer have to be used. We present here a modified TAP tag technique that allows sufficient recovery of proteins from mouse fibroblasts growing in monolayer cultures. The recovery allows protein identification by mass spectrometry. 相似文献