全文获取类型
收费全文 | 1180篇 |
免费 | 80篇 |
专业分类
1260篇 |
出版年
2022年 | 8篇 |
2021年 | 18篇 |
2020年 | 13篇 |
2019年 | 16篇 |
2018年 | 12篇 |
2017年 | 13篇 |
2016年 | 33篇 |
2015年 | 44篇 |
2014年 | 53篇 |
2013年 | 81篇 |
2012年 | 78篇 |
2011年 | 91篇 |
2010年 | 68篇 |
2009年 | 42篇 |
2008年 | 81篇 |
2007年 | 69篇 |
2006年 | 56篇 |
2005年 | 52篇 |
2004年 | 61篇 |
2003年 | 59篇 |
2002年 | 58篇 |
2001年 | 15篇 |
2000年 | 11篇 |
1999年 | 13篇 |
1998年 | 18篇 |
1997年 | 6篇 |
1996年 | 11篇 |
1995年 | 10篇 |
1994年 | 8篇 |
1993年 | 11篇 |
1992年 | 8篇 |
1991年 | 14篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 12篇 |
1985年 | 7篇 |
1984年 | 14篇 |
1983年 | 8篇 |
1982年 | 11篇 |
1981年 | 11篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1946年 | 3篇 |
排序方式: 共有1260条查询结果,搜索用时 0 毫秒
21.
22.
Ted D. Wade 《Primates; journal of primatology》1979,20(3):355-370
In this paper I argue (a) that the study of kin selection may be facilitated by looking for influences of inbreeding, which
is an important aspect of a population's genetic structure; (b) that in nonhuman primates the level of inbreeding will be
largely a function of the rate of migration by individuals, usually only of one sex, between social units or troops; (c) that
many primate species live in relatively outbred groups, and that their social structure reflects this; and (d) that extensive
social contrasts between bonnet and pigtail macaques reflect evolution by kin selection under different levels of inbreeding. 相似文献
23.
Ted R. Angradi 《Hydrobiologia》1991,211(3):171-183
I investigated organic matter transport in the Henry's Fork of the Snake River, Idaho, USA, from August 1987 to November 1988. Mean discharge during the study was 15 m3 s–1. Screens were used to sample very coarse (> 6 mm) transported aquatic macrophyte material (VCTMM). Drift nets were used to sample coarse (1–6 mm) and fine (0.25–1 mm) transported particulate organic matter (CTOM and FTOM). Mean monthly concentration of VCTMM was 0.064 mg AFDWl–1 and was significantly higher than CTOM (0.024 mg AFDW l–1) and FTOM (0.036 mg AFDW l–1). VCTMM concentration was highest in December (0.163 mg AFDW l–1) and lowest in May (0.018 mg AFDW l–1). The sample position along a transect across the channel had a significant effect on the amount of transported organic matter collected in many months. The concentration of debris from individual species tracked the standing stock of that species during the growing season. In Fall, a dramatic increase in VCTMM corresponded to a decrease in macrophyte standing stock. FTOM and CTOM concentrations were highest in January (CTOM: 0.048; FTOM: 0.111 mg AFDW l–1), lowest in November 1988 (<0.006 mg AFDW l–1), were not correlated with discharge, and were inversely correlated with the standing stocks of macrophytes upstream, probably because macrophyte beds influenced the retentiveness of the channel. Standing stock of aquatic macrophytes was highest in September–October (5.2 kg wet weight m –2) and lowest in February (1.7 kg wet weight m–2). Annual movement of particulate organic matter past the sampling point was about 45 000 kg AFDW, of which 21 000 kg was VCTMM, 8 000 kg was CPOM, and 16 000 kg was FPOM. 相似文献
24.
William M. Scovell 《Biopolymers》1978,17(4):969-984
Raman spectra of solutions of polyriboadenylic acid have been studied in the pH range of 7.2–5.2. Bands are identified which are sensitive to the characteristics of poly(rA) in the single-and double-stranded helical forms. Thermal melting profiles were obtained as a function of pH to monitor simultaneously the changes in (1) the phosphodiester backbone, (2) the base-stacking interactions, (3) the perturbation of the PO unit, and (4) the degree of protonation at the N-1 position in the adenine base. The temperature dependence of the intensity ratio of the bands at 725 and 705 cm?1 appears to be sensitive to the noncooperative and the cooperative thermal-melting process for the single-and double-stranded forms of poly(rA), respectively. Concurrently, bands diagnostic of the degree of protonation reveal that the cooperative melting process for the “acid” poly(rA) clearly involves deprotonation. The progressive perturbation of the 1100 cm?1 band with an increasing degree of protonation of poly(rA) is consistent with earlier suggestions regarding a PO-(6)-NH2 interaction in the double-helical form of poly(rA). The stability of the double-helix parallels the degree of protonation over the pH range studied as reflected in the tm values, which increase linearly with decreasing pH. 相似文献
25.
Ted W. Keyes Baldomero M. Olivera D. J. Stewart E. W. Hanly 《Biochemical genetics》1976,14(3-4):197-207
The pyridine nucleotide metabolism of imaginal discs of Drosophila melanogaster has been studied in vitro by incubating discs with labeled nicotinic acid in the presence and absence of ecdysterone. The major labeled compounds found within the discs are NAD, NADP, and nicotinic acid. There is preferential uptake of nicotinamide over nicotinic acid, although the Priess-Handler pathway is used exclusively. The presence of ecdysterone produces a small increase in the NADP/NAD ratio, and an increase in NAD synthesis, probably to compensate for increased NAD turnover.Supported by Grant GB 43569 from the National Science Foundation. 相似文献
26.
Paul D. Pratt Paul T. Madeira Gevork Arakelian Matthew Purcell Min B. Rayamajhi Ted D. Center 《Biocontrol Science and Technology》2013,23(5):602-606
The Australian psyllid Boreioglycaspis melaleucae is a biological control agent of Melaleuca quinquenervia in Florida (USA) but was observed attacking M. quinquenervia trees in southern California (USA). Genotyping revealed the California population matched three of eight Australian haplotypes and all three Florida haplotypes. It remains unclear if the California psyllid population arrived directly from Australia or via Florida. 相似文献
27.
The pecan weevil, Curculio caryae (Horn), is a key pest of pecan [Carya illinoinensis (Wangenh.) K. Koch]. Current control recommendations are based on chemical insecticide applications. Microbial control agents such as the entomopathogenic nematode, Steinernema carpocapsae (Weiser) and the fungus Beauveria bassiana (Balsamo) Vuillemin occur naturally in southeastern U.S. pecan orchards and have shown promise as alternative control agents for C. caryjae. Conceivably, the chemical and microbial agents occur simultaneously within pecan orchards or might be applied concurrently. The objective of this study was to determine the interactions between two chemical insecticides that are used in commercial C. caryae control (i.e., carbaryl and cypermethrin applied below field rates) and the microbial agents B. bassiana and S. carpocapsae. In laboratory experiments, pecan weevil larval or adult mortality was assessed after application of microbial or chemical treatments applied singly or in combination (microbial + chemical agent). The nature of interactions (antagonism, additivity, or synergy) in terms of weevil mortality was evaluated over 9 d (larvae) or 5 d (adults). Results for B. bassiana indicated synergistic activity with carbaryl and antagonism with cypermethrin in C. caryae larvae and adults. For S. carpocapsae, synergy was detected with both chemicals in C. caryae larvae, but only additive effects were detected in adult weevils. Our results indicate that the chemical-microbial combinations tested are compatible with the exception of B. bassiana and cypermethrin. In addition, combinations that exhibited synergistic interactions may provide enhanced C. caryae control in commercial field applications; thus, their potential merits further exploration. 相似文献
28.
Murray E McKenna EO Burch LR Dillon J Langridge-Smith P Kolch W Pitt A Hupp TR 《Biochemistry》2007,46(48):13742-13751
Anterior gradient-2 protein was identified using proteomic technologies as a p53 inhibitor which is overexpressed in human cancers, and this protein presents a novel pro-oncogenic target with which to develop diagnostic assays for biomarker detection in clinical tissue. Combinatorial phage-peptide libraries were used to select 12 amino acid polypeptide aptamers toward anterior gradient-2 to determine whether methods can be developed to affinity purify the protein from clinical biopsies. Selecting phage aptamers through four rounds of screening on recombinant human anterior gradient-2 protein identified two classes of peptide ligand that bind to distinct epitopes on anterior gradient-2 protein in an immunoblot. Synthetic biotinylated peptide aptamers bound in an ELISA format to anterior gradient-2, and substitution mutagenesis further minimized one polypeptide aptamer to a hexapeptide core. Aptamers containing this latter consensus sequence could be used to affinity purify to homogeneity human anterior gradient-2 protein from a single clinical biopsy. The spotting of a panel of peptide aptamers onto a protein microarray matrix could be used to quantify anterior gradient-2 protein from crude clinical biopsy lysates, providing a format for quantitative screening. These data highlight the utility of peptide combinatorial libraries to acquire rapidly a high-affinity ligand that can selectively bind a target protein from a clinical biopsy and provide a technological approach for clinical biomarker assay development in an aptamer microarray format. 相似文献
29.
30.
William T. Evanochko Ted T. Sakai Thian C. Ng N.Rama Krishna Hyun Dju Kim Robert B. Zeidler Vithal K. Ghanta R.Wallace Brockman Lewis M. Schiffer Paul.G. Braunschweiger Jerry D. Glickson 《Biochimica et Biophysica Acta (BBA)/Molecular Cell Research》1984,805(1):104-116
Perchloric acid extracts of radiation-induced fibrosarcoma (RIF-1) tumors grown in mice have been analyzed by multinuclear NMR spectroscopy and by various chromatographic methods. This analysis has permitted the unambiguous assignment of the 31P resonances observed in vivo to specific phosphorus-containing metabolites. The region of the in vivo spectra generally assigned to sugar phosphates has been found in RIF-1 tumors to contain primarily phosphorylethanolamine and phosphorylcholine rather than glycolytic intermediates. Phosphocreatine was observed in extracts of these tumor cells grown in culture as well as in the in vivo spectra, indicating that at least some of the phosphocreatine observed in vivo arises from the tumor itself and not from normal tissues. In the 31P-NMR spectra of the perchloric acid extract, resonances originating from purine and pyrimidine nucleoside di- and triphosphate were resolved. HPLC analyses of the nucleotide pool indicate that adenine derivatives were the most abundant components, but other nucleotides were present in significant amounts. The 1H and 13C resonance assignments of the majority of metabolites present in RIF-1 extracts have also been made. Of particular importance is the ability to observe lactate, the levels of which may provide a noninvasive measure of glycolysis in these cells in both the in vivo and in vitro states. In addition, the aminosulfonic acid, taurine, was found in high levels in the tumor extracts. 相似文献