全文获取类型
收费全文 | 125998篇 |
免费 | 3798篇 |
国内免费 | 5671篇 |
出版年
2024年 | 24篇 |
2023年 | 451篇 |
2022年 | 639篇 |
2021年 | 1734篇 |
2020年 | 1362篇 |
2019年 | 1742篇 |
2018年 | 13099篇 |
2017年 | 11621篇 |
2016年 | 8933篇 |
2015年 | 2987篇 |
2014年 | 3180篇 |
2013年 | 3284篇 |
2012年 | 7713篇 |
2011年 | 15825篇 |
2010年 | 13875篇 |
2009年 | 9971篇 |
2008年 | 11720篇 |
2007年 | 13089篇 |
2006年 | 1896篇 |
2005年 | 1846篇 |
2004年 | 2056篇 |
2003年 | 2044篇 |
2002年 | 1639篇 |
2001年 | 803篇 |
2000年 | 631篇 |
1999年 | 456篇 |
1998年 | 284篇 |
1997年 | 275篇 |
1996年 | 228篇 |
1995年 | 182篇 |
1994年 | 184篇 |
1993年 | 152篇 |
1992年 | 152篇 |
1991年 | 141篇 |
1990年 | 85篇 |
1989年 | 83篇 |
1988年 | 78篇 |
1987年 | 53篇 |
1986年 | 51篇 |
1985年 | 60篇 |
1984年 | 30篇 |
1983年 | 46篇 |
1982年 | 31篇 |
1981年 | 23篇 |
1979年 | 20篇 |
1972年 | 252篇 |
1971年 | 282篇 |
1965年 | 13篇 |
1962年 | 24篇 |
1944年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Effect of domestic cattle stocking on the nutritional condition of white-tailed deerOdocoileus virginianus (Zimmermann, 1780) was assessed using physiological indices of collected specimens. Three study areas were delineated in McCurtain County, Oklahoma (heavy cattle stocking), and Howard (moderate to light cattle stocking) and Pike (no cattle stocking) counties, Arkansas that were similar with respect to soils and vegetation but differed with respect to cattle stocking rate. Female white-tailed deer were collected from study areas in February and August 1987–1988 to assess nutritional condition. Deer collected from study areas exposed to cattle grazing in February had lower carcass weights, fat attributes (femur marrow and kidney fat), and reproductive rates (fetuses/doe) than deer that were not exposed to cattle grazing. In August, deer collected from the moderate cattle area had heavier eviscerated carcass weights, serum glucose, albumin, and albumin/globulin ratios than deer collected from the heavy cattle area. Results suggest that if cattle are removed from managed forests in winter, nutritional condition of deer would be improved because of reduced competition for food. 相似文献
102.
Payam Pour Mohammadi Ahmad Moieni Asa Ebrahimi Farzad Javidfar 《Plant Cell, Tissue and Organ Culture》2012,108(2):251-256
An efficient method for producing doubled haploid plants of oilseed rape (Brassica
napus L.) was established using in vitro colchicine treatment of haploid embryos. Haploid embryos in the cotyledonary stage were
treated with one of four colchicine concentrations (125, 250, 500 and 1,000 mg/L); for one of three treatment durations (12,
24 and 36 h) at one of the two temperatures (8 and 25°C) and were compared to control embryos (without colchicine treatment).
The number of chromosomes, seed recovery, size and density of leaf stomata, and pollen grain size from regenerated plants
were determined. No doubled haploid plants were regenerated from control embryos; however, the doubled haploid plants were
regenerated from colchicine-treated embryos. A high doubling efficiency, 64.29 and 66.66% of regenerated plants, was obtained
from 250 mg/L colchicine treatment for 24 h and 500 mg/L colchicine treatment for 36 h, respectively, at 8°C. Following 500 mg/L
colchicine treatment for 36 h, a few plants regenerated (9 plants). At the higher colchicine concentration (1,000 mg/L), no
plant regenerated. These results indicate that the colchicine treatment of embryos derived from microspores can induce efficient
chromosome doubling for the production of doubled haploid lines of oilseed rape. 相似文献
103.
Bing Xia Haiping Ke Wei Jiang Masayori Inouye 《The Journal of biological chemistry》2002,277(8):6005-6011
The 5'-end region of cspA mRNA contains a Cold Box sequence conserved among several cold-shock mRNAs. This region forms a stable stem-loop structure followed by an AU-rich sequence. Here we show that the Cold Box region is essential for the normal scale of cspA mRNA induction after cold shock because a deletion of the stem-loop significantly destabilizes the mRNA and reduces the cold shock-induced cspA mRNA amount by approximately 50%. The AU-rich track, however, slightly destabilizes the mRNA. The integrity of the stem is essential for the stabilizing function, whereas that of the loop sequence is less important. Overexpression of a mutant cspA mRNA devoid of both the AUG initiation codon and the coding sequence results in a severe growth inhibition at low temperature along with a derepression of the chromosomal cspA expression. Furthermore, the overexpressed RNA is stably associated with the 30 S and 70 S ribosomes. Our results demonstrate that the AUG initiation codon and the coding region containing the downstream box are not required for cspA mRNA to bind ribosomes and that the 5'-untranslated region by itself has a remarkable affinity to ribosomes at low temperature. 相似文献
104.
Qingrong Sun Hongyan Sun Richard L. Bell Huifeng Li Li Xin 《Plant Cell, Tissue and Organ Culture》2011,107(1):131-140
A wide range of phenotypic variation was observed among neopolyploids obtained from the diploid pear cultivar ‘Fertility’
by in vitro colchicine treatment. The variant plantlets had alterations in leaf characteristics. Neopolyploids had significantly
different ratios of leaf length to leaf width compared to the diploid control. Shoot regeneration from leaf explants and rooting
ability from in vitro shoots of neopolyploids was examined. Regeneration frequencies of shoots from leaf explants of seven
of the nine neopolyploids were significantly decreased compared to the diploid control. The organogenic potential of neopolyploids
was highly genotype-dependent for both shoots and roots. Tetraploid clone 4x − 4 failed to regenerate shoots from leaf explants
and the pentaploid clone 5x − 2 failed to root from in vitro shoots. The results suggest that polyploidization caused the
decrease in or loss of in vitro organogenic potential. Regenerated shoots derived from neopolyploids showed different phenotypes,
depending on the ploidy of the donor plant. 相似文献
105.
Feed grain production for on-farm use is widespread in Bavarian agriculture and garants a reliable production of healthy foodstuff. Therefore the quality of feed grain is very important for farmers. In an orientating investigation of the on-farm stored feed grain quality from the harvest years 1991 to 2000 1757 samples were analysed. Based on values for guidance by the German government for DON and ZEA and Commission Regulation EC No 472/2002 for OTA the results show that only 2% of the positive samples have amounts higher then 0,05 mg ZEA/kg grain, 4% of the positive samples contain more then 1 mg DON/kg grain and 2% of the positive samples have amounts higher then 0,003 mg OTA/kg feed grain. These illustrate the good conservation and storage quality on Bavarian farms. 相似文献
106.
Inward rectifying potassium (KIR) currents in medium spiny (MS) neurons of nucleus accumbens inactivate significantly in ~40% of the neurons but not in the
rest, which may lead to differences in input processing by these two groups. Using a 189-compartment computational model of
the MS neuron, we investigate the influence of this property using injected current as well as spatiotemporally distributed
synaptic inputs. Our study demonstrates that KIR current inactivation facilitates depolarization, firing frequency and firing onset in these neurons. These effects may be
attributed to the higher input resistance of the cell as well as a more depolarized resting/down-state potential induced by
the inactivation of this current. In view of the reports that dendritic intracellular calcium levels depend closely on burst
strength and spike onset time, our findings suggest that inactivation of KIR currents may offer a means of modulating both excitability and synaptic plasticity in MS neurons. 相似文献
107.
108.
109.
Ya-Ping Xue Wei Wang Ya-Jun Wang Zhi-Qiang Liu Yu-Guo Zheng Yin-Chu Shen 《Bioprocess and biosystems engineering》2012,35(9):1515-1522
To isolate enantioselective α-hydroxyacid dehydrogenases (α-HADHs), a high-throughput screening method was established. 2,4-Dinitrophenylhydrazine solution forms a red-brown complex with ketoacid produced during the α-HADH-mediated oxidation of α-hydroxyacid. The complex can be easily quantified by spectrophotometric measurement at 458?nm. The enantioselectivity of α-HADH in each strain can be measured with this colorimetric method using (R)- and (S)-α-hydroxyacid concurrently as substrates to evaluate the apparent enantioselectivity (E app). The E app closely matches the value of true enantioselectivity (E true) determined by HPLC analysis. With this method, a total of 34 stains harboring enantioselective α-HADHs were selected from 526 potential α-HADH-producing microorganisms. Pseudomonas aeruginosa displayed the highest (S)-enantioselective α-HADH activity. This strain appears promising for potential application in industry to produce (R)-α-hydroxyacids. The method described herein represents a useful tool for the high-throughput isolation of enantioselective α-HADHs. 相似文献
110.
Food availability is important to the dynamics of animal social organizations or populations. However, the role of winter
food availability in animal population dynamics is still controversial. We carried out an experimental study to test Lack’s
hypothesis that reduced food in winter limits survival and spring numbers of breeding individuals of social groups, using
the Mongolian gerbil (Meriones unguiculatus) as model species. We established 24 gerbil social groups in 24, 10 × 10 m, pens in September 2008. We provided wheat seeds
as supplemental food in 12 enclosures from September 2008 to March 2009; the other 12 enclosures, not provided with supplemental
food, served as controls. We live-trapped gerbils at a 2-week interval from September to April. Supplemental food during winter
increased biweekly survival by 10% relative to that in control groups. Only four control social groups survived to the end
of our study whereas all 12 food-supplemented social groups survived through our study period. Supplemental food also increased
cumulative numbers of recruits and group sizes of gerbils. We conclude that winter food availability limits winter survival
and spring social groups or population sizes of Mongolian gerbils. 相似文献