全文获取类型
收费全文 | 2672篇 |
免费 | 234篇 |
出版年
2022年 | 18篇 |
2021年 | 38篇 |
2020年 | 32篇 |
2019年 | 46篇 |
2018年 | 44篇 |
2017年 | 43篇 |
2016年 | 75篇 |
2015年 | 119篇 |
2014年 | 135篇 |
2013年 | 135篇 |
2012年 | 166篇 |
2011年 | 172篇 |
2010年 | 127篇 |
2009年 | 115篇 |
2008年 | 143篇 |
2007年 | 132篇 |
2006年 | 120篇 |
2005年 | 109篇 |
2004年 | 102篇 |
2003年 | 105篇 |
2002年 | 86篇 |
2001年 | 64篇 |
2000年 | 57篇 |
1999年 | 48篇 |
1998年 | 25篇 |
1997年 | 19篇 |
1996年 | 24篇 |
1995年 | 14篇 |
1994年 | 20篇 |
1993年 | 15篇 |
1992年 | 40篇 |
1991年 | 38篇 |
1990年 | 24篇 |
1989年 | 31篇 |
1988年 | 38篇 |
1987年 | 26篇 |
1986年 | 21篇 |
1985年 | 31篇 |
1984年 | 34篇 |
1983年 | 22篇 |
1982年 | 16篇 |
1981年 | 21篇 |
1980年 | 15篇 |
1979年 | 19篇 |
1977年 | 12篇 |
1976年 | 16篇 |
1975年 | 14篇 |
1974年 | 18篇 |
1973年 | 12篇 |
1969年 | 12篇 |
排序方式: 共有2906条查询结果,搜索用时 11 毫秒
1.
Stephen B. Vander Wall 《Animal behaviour》1982,30(1):84-94
Five hypotheses of cache recovery behaviour in Clark's nutcracker (Nucifraga columbiana) were examined experimentally. Most caches were made in soil within 5 cm of conspicuous large objects. Both seed-caching and non-seed-caching nutcrackers were able to locate caches. Seed-caching nutcrackers relocated caches using large objects as remembered visual cues. Soil microtopography and small (<2 cm diameter) objects may be used as cues to facilitate cache recovery but are not essential. Non-seed-caching nutcrackers located caches by using soil disturbances at cache sites as visual cues and by searching preferentially near objects where caches were concentrated. Success rates of seed-caching nutcrackers ranged from 52 to 78% and those of non-seed-caching nutcrackers ranged from 8 to 12%. Nutcrackers do not use random search or olfactory cues to locate caches. 相似文献
2.
3.
4.
5.
P. Angell W.J. Machowski P.P. Paul C.M. Wall F.F. Lyle Jr. 《Journal of microbiological methods》1997,30(3):173-178
A multiple chemostat system has been developed in which metal specimens can be exposed to a consortium of bacteria. The system comprises a single test chemostat containing the test specimen operated at a high dilution rate to facilitate the wash out of planktonic bacteria, selecting for attached or biofilm growth. This chemostat is fed at a steady low rate by a number of separate chemostats each of which contains a pure axenic culture of one member of the consortium being tested. This system has the advantage of providing a continual inoculum of the test species to the test specimen allowing both aerobic and anaerobic bacteria to be grown in the same system. Constant levels of three bacterial types were maintained in the system: Pseudomonas aeruginosa, Thiobacillus ferrooxidans and Desulfovibrio vulgaris. Exposure of 316L stainless steel electrodes to this system resulted in increased corrosion of coupons exposed biotically, as compared to those exposed abiotically. A current monitoring technique and electrochemical impedance spectroscopy were used to evaluate effects of bacteria on metallic corrosion. 相似文献
6.
7.
8.
Wolfgang Heinemeyer Ilka Buchmann Dave W. Tonge John D. Windass Juliane Alt-Moerbe Elmar W. Weiler Thomas Botz Joachim Schröder 《Molecular & general genetics : MGG》1987,210(1):156-164
Summary
Tzs and ipt are two Ti plasmid genes coding for proteins with isopentenyltransferase (IPT) activity in vitro. We cloned both genes for protein expression in Escherichia coli and in Agrobacterium tumefaciens, and we investigated differences between the two genes by analysing the properties of the proteins in vitro and in vivo. In vitro, extracts with tzs or ipt-coded proteins had high IPT activity, and the enzymes were identical in most properties. The most important difference was detected in vivo: the tzs-encoded protein was very active in cytokinin production, while the ipt protein required overexpression in order to obtain measurable activity in bacteria. In both cases, rans-zeatin was the major product of the gene activity. Formation of this cytokinin requires a hydroxylase function in addition to the IPT reaction. No such activity could be ascribed to tzs or ipt-encoded proteins in vitro or in vivo, but cytokinin hydroxylase activity was detected in cells and extracts of E. coli, regardless of the presence or absence of the cytokinin genes. Based on these results it is proposed that both genes code for a single enzyme activity (isopentenyltransferase), that the genes and proteins are adapted for function either in bacteria (tzs) or in transformed plant cells (ipt), and that in both prokaryotic and eukaryotic cells hydroxylation to trans-zeatin is a function contributed by host enzymes.Abbreviations DMAPP
dimethylallylpyrophosphate
- iP
isopentenyladenine
- iPA
isopentenyladenosine
- iPMP
isopentenyladenosine 5-monophosphate
- IPT
isopentenyltransferase
-
trans-Z
trans-zeatin 相似文献
9.
Preharvest aflatoxin contamination: effect of moisture and substrate variation in developing cottonseed and corn kernels.
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Variations in moisture and substrate in preharvest corn kernels and cottonseed were linked with the ability of Aspergillus parasiticus to infect the seed and produce aflatoxin. Osmotic pressures and moisture content (MC) levels of developing starch-rich corn kernels and lipid-rich cottonseed were determined. For in vivo studies, corn kernels and cottonseed were inoculated with A. parasiticus conidia and retained on plants through maturation. For in vitro studies, samples of corn kernels and cottonseed were collected at various stages, sterilized, inoculated, incubated for 2 weeks, and assayed for toxin. Aflatoxin levels were highest in corn kernels inoculated at 28 days postflowering (52% MC) in both the in vivo and in vitro tests. Toxin concentrations in cottonseed were greatest with inoculation at 35 days postflowering (70% MC) in seed retained on the plant, but toxin accumulation continued to increase with the maturity of the seed inoculated in cottonseed used in the in vitro trials. Moisture and substrate conditions in the midrange of seed development provided optimum conditions for fungal development and toxin production in seed retained on the plant. 相似文献
10.
T T Herskovits P A Blake J A Gonzalez M G Hamilton J S Wall 《Comparative biochemistry and physiology. B, Comparative biochemistry》1989,94(2):415-421
1. The hemocyanins of the Melongenidae family of marine gastropods: Melongena corona, Busycon canaliculatum, B. carica, B. contrarium, and B. spiratum exist in solution as multi-decameric aggregates characterized by sedimentation coefficients of approximately 105 S, 130 S, 150 S, 170 S, and higher values, corresponding to di-, tri-, tetra-, penta-, and larger multi-decameric particles. 2. The hemocyanins of B. contrarium and B. carica seem to form the largest decameric aggregates with the tri- to penta-decamers respresenting the major constitutents. Scanning transmission electron microscopy (STEM), both of unstained, freeze-dried and negatively-stained specimens, shows the presence of discrete aggregates consisting of up to ten decameric units. 3. The particle masses as determined by STEM mass measurements for individual molecules gave integral multiples of from 4.2 x 10(6) to 4.4 x 10(6) daltons ranging from about 8.2 x 10(6) daltons for the typical di-decamer of B. canaliculatum hemocyanin to as high as about 39 x 10(6) and 43 x 10(6) for the nano-and deca-decamers of B. contrarium hemocyanin. 4. The appearance of the higher multi-decamers in both negatively-stained and freeze-dried specimens suggest that they are formed by the addition of decameric units to a single di-decameric unit "tail-wise" in both directions. The higher aggregates formed seem to terminate with a closed head or collar at both ends of the assembly. 相似文献