全文获取类型
收费全文 | 1937813篇 |
免费 | 195124篇 |
国内免费 | 1914篇 |
出版年
2021年 | 18747篇 |
2018年 | 20239篇 |
2017年 | 19045篇 |
2016年 | 30222篇 |
2015年 | 44275篇 |
2014年 | 52533篇 |
2013年 | 78812篇 |
2012年 | 57870篇 |
2011年 | 50956篇 |
2010年 | 50650篇 |
2009年 | 49752篇 |
2008年 | 46478篇 |
2007年 | 46088篇 |
2006年 | 47394篇 |
2005年 | 47918篇 |
2004年 | 47101篇 |
2003年 | 44230篇 |
2002年 | 42009篇 |
2001年 | 66876篇 |
2000年 | 65264篇 |
1999年 | 56227篇 |
1998年 | 29691篇 |
1997年 | 29598篇 |
1996年 | 28458篇 |
1995年 | 26643篇 |
1994年 | 26173篇 |
1993年 | 25731篇 |
1992年 | 47734篇 |
1991年 | 45879篇 |
1990年 | 44576篇 |
1989年 | 44542篇 |
1988年 | 41125篇 |
1987年 | 39317篇 |
1986年 | 36915篇 |
1985年 | 38231篇 |
1984年 | 34387篇 |
1983年 | 29925篇 |
1982年 | 26711篇 |
1981年 | 25050篇 |
1980年 | 23431篇 |
1979年 | 31127篇 |
1978年 | 26599篇 |
1977年 | 24815篇 |
1976年 | 23439篇 |
1975年 | 24285篇 |
1974年 | 25784篇 |
1973年 | 25873篇 |
1972年 | 22675篇 |
1971年 | 20884篇 |
1970年 | 18208篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Summary A field experiment was conducted using15N-labelled urea on a Reddish Brown Lateritic (Peleustult) soil. Growing two crops on flat land and on soil ridges of 15 cm
height produced similar comparative effects from fertilizer on maize. However, fertilizer applied by broadcasting on maize
with a 50 cm effective band followed by incorporating was more useful to mungbean than that applied by banding below the cereal
seed rows when crops were grown on flat land. The reverse was observed when crops were grown on ridges. It was deduced that
the maize cultivar was not likely to affect comparative efficiencies of fertilizer.
For fertilizer application at sowing, broadcasting in 50 cm maize effective band followed by incorporating was slightly superior
to banding below maize seed rows. Side-dressing of fertilizer to maize at 4 weeks after sowing was superior to application
at sowing. Evenly-split application, at sowing and at 4 weeks after sowing, was either only slightly superior or comparable
to non-split application by banding below maize seed rows at sowing, depending on placement method of the first application.
Soil moisture status as a possible factor rendering discrepancy in the comparative efficiencies obtained by different authors
is discussed. 相似文献
992.
J H Richardson L L Steinmetz S B Deutscher W A Bookless W L Schmelzinger 《Analytical biochemistry》1979,97(1):17-23
A synchronously pumped krypton ion dye laser fluorescence system is shown to provide tunable, polarized, subnanosecond pulses at high repetition rates, modest peak powers, and low energy. Such a source is uniquely suited to fluorescence investigations of biochemical mechanisms. Applications of this fluorescence excitation source to analysis, life-time determination, and depolarization effects are discussed. 相似文献
993.
Study of the posterior abdominal epidermis in hypophysectomized/thyroidectomized male and female tokays following surgery, and subsequent androgen therapy, indicates that, contrary to a previous model, all aspects of β-gland differentiation are under direct androgenic control. On the other hand, another epidermal specialization, the digital foot-pad, shows a pattern of histogenesis directly comparable to that of β-glands, but is unaffected by androgens. These data are discussed with respect to the evolution of glandular epidermal specializations in gekkonid lizards and the possible role of androgens in modifying the control of cell differentiation in lizard epidermis. 相似文献
994.
995.
Carcinogens can induce homologous recombination between duplicated chromosomal sequences in mouse L cells. 总被引:23,自引:12,他引:11
下载免费PDF全文
![点击此处可从《Molecular and cellular biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The ability of a series of DNA-damaging agents to induce homologous intrachromosomal recombination between duplicated genes in the chromosome of mouse cells was investigated. The target cells were the thymidine kinase-deficient mouse L-cell strain 333M, which contains a single integrated copy of a plasmid with two herpes simplex virus thymidine kinase (Htk) genes, each containing an 8-base-pair XhoI linker inserted at a unique site. Expression of a functional Htk enzyme requires a productive recombinational event between the two nonfunctional genes. The spontaneous rate of recombination in this strain is 3 per 10(6) cells per generation. The agents tested represent physical carcinogens (UV and ionizing radiation), a simple alkylating agent (N-methyl-N'-nitro-N-nitrosoguanidine), an alkylating cross-linking agent (mitomycin C), and a reactive metabolite of a polycyclic aromatic hydrocarbon ((+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene [BPDE] ). The background frequency of tk+ recombinants in the untreated population averaged 18 X 10(-6) +/- 5 X 10(-6). Ionizing radiation had little or no effect on recombination; exposure to mitomycin C, N-methyl-N'-nitro-N-nitrosoguanidine, BPDE, or UV, at doses that lowered the survival to between 90 and 10% of the control, caused a dose-dependent increase in frequency of recombinants, reaching 50 X 10(-6) to 100 X 10(-6). No tk+ cells could be generated with a control cell line that contained only one mutant copy of the Htk gene. Molecular hybridization analysis showed that 85 to 90% of the tk+ recombinants retained the Htk gene duplication, consistent with nonreciprocal transfer of wild-type genetic information, gene conversion. In the rest, only a single copy of the Htk gene remained, reflecting a single reciprocal exchange within a chromatid or a single unequal exchange between sister chromatids. Each recombinant tested contained an XhoI-resistant (wild-type) Htk gene. 相似文献
996.
Iron-sulfur core extrusions from spinach [( 2Fe-2S]) and Clostridium pasteurianum (2[4Fe-4S]) ferredoxins in aqueous Triton X-100/urea containing excess benzenethiol yield quantitatively [FenSn(SPh)4]2- with n = 2 and n = 4, respectively. The iron-sulfur cluster can be separated from the corresponding apoprotein by rapid passage of the extrusion mixture over a small anaerobic column of Whatman DE-52 anion-exchange cellulose. Essentially quantitative recovery of [FenSn (SPh)4]2- is achieved in the eluate. The apoprotein remaining on the column can be eluted with 0.5 M NaCl. Most of the residual Triton X-100 and benzenethiol can be removed by passage of the apoprotein eluate over a small column of Bio-Beads SM-2, a hydrophobic polystyrene adsorbent. Apoprotein recovery is comparable to that obtained by other chromatographic methods. At least with spinach ferredoxin, the apoprotein prepared in this fashion can be reconstituted. The procedures developed in this work are potentially most applicable to selective removal of [2Fe-2S] and [4Fe-4S] centers from a multicenter enzyme without irreversible denaturation. 相似文献
997.
998.
Bruce R. Dorrbecker Susan H. Mercik Paul A. Kramer 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1984,336(1)
A high-performance liquid chromatographic procedure is reported for reproducibly and sensitively quantitating caffeine and its N-demethylated metabolite paraxanthine in micro-samples. A 5-μm reversed-phase radial compression column and 214-nm fixed wavelength ultraviolet detector were used to attain a sensitivity sufficient to quantitate these compounds at concentratios as low as 80 ng/ml using only 25 μl of sample. The assay is applicable to microliter samples of whole blood, serum, plasma, saliva, amniotic, cerebro-spinal and gastric fluids such as might be obtained in studies involving small animals or neonates. The utility of the assay is illustrated with caffeine and paraxanthine levels measured in several maternal and fetal fluids following constant-rate intravenous infusion of caffeine into a rabbit throughout pregnancy. 相似文献
999.
1000.