全文获取类型
收费全文 | 62292篇 |
免费 | 5317篇 |
国内免费 | 17篇 |
出版年
2023年 | 259篇 |
2021年 | 512篇 |
2020年 | 393篇 |
2019年 | 429篇 |
2018年 | 1098篇 |
2017年 | 1150篇 |
2016年 | 1347篇 |
2015年 | 1463篇 |
2014年 | 1638篇 |
2013年 | 2697篇 |
2012年 | 4252篇 |
2011年 | 4605篇 |
2010年 | 2494篇 |
2009年 | 1774篇 |
2008年 | 3813篇 |
2007年 | 3934篇 |
2006年 | 3655篇 |
2005年 | 3438篇 |
2004年 | 3298篇 |
2003年 | 3086篇 |
2002年 | 3011篇 |
2001年 | 1734篇 |
2000年 | 1939篇 |
1999年 | 1150篇 |
1998年 | 651篇 |
1997年 | 490篇 |
1996年 | 555篇 |
1995年 | 507篇 |
1994年 | 518篇 |
1993年 | 456篇 |
1992年 | 550篇 |
1991年 | 471篇 |
1990年 | 415篇 |
1989年 | 410篇 |
1988年 | 398篇 |
1987年 | 386篇 |
1986年 | 346篇 |
1985年 | 453篇 |
1984年 | 538篇 |
1983年 | 479篇 |
1982年 | 477篇 |
1981年 | 455篇 |
1980年 | 397篇 |
1979年 | 370篇 |
1978年 | 383篇 |
1977年 | 351篇 |
1976年 | 367篇 |
1975年 | 407篇 |
1974年 | 340篇 |
1973年 | 325篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
161.
Summary The morphology of the tibio-tarsal antenna cleaner (strigilis) of 30 species of Formicidae, 14 species of Mutillidae and 9 species of Tiphiidae was investigated and is described comparatively. In Formicidae, there is a common type of strigilis. The spur has a posterior-dorsal comb and its anterior side is covered with squamae, which usually form a brush. Posteriorly, the basitarsal notch bears a comb and anteriorly specialized paddle-shaped hairs. These characters may be apomorphic for Formicidae. In several Ponerinae and in Myrmecia there is also a velum on the spur. In two species of ants which are strongly parasitic (Teleutomyrmex schneideri and Anergates atratulus ) there are reduced antenna cleaners. Mutillidae (except Myrmosa) have another common type of strigilis: the spur bears a velum with a smooth rim and a clear apex with two rows of teeth. The notch in the basitarsus is usually deeper than that of ants; there is a comb, but no paddle-shaped hairs. The strigilis of Myrmosa females has no velum but there are two prominent rows of teeth on the spur. In the male, the velum is reduced to a slender strip. In Tiphiidae the antenna cleaners show considerable diversity. In Methocha the spur bears a comb but no velum; the spur of Tiphia has a velum with a serrated rim; the spur of Myzinum is equipped with a velum with a smooth rim; in Thynnus the surface of the velum is wrinkled or undulating. An apex (without teeth) is present in all investigated Tiphiidae. The notch of the basitarsus bears a comb except in female Myzinum, where the teeth seem to have fused, thus forming a rim. It is suggested that a velum with a serrated or toothed edge and an apex with two rows of teeth are plesiomorphic for aculeate Hymenoptera. The antenna cleaners in Mutillidae are remarkably similar to those in some bees. This fact is interpreted as partly due to convergence and as partly symplesiomorphic. In several species of ants, there is a specialized cuticular area (bande poreuse) anterior to the comb of the notch, which is characterized by fissures or holes. These are presumably openings of an excretory gland. 相似文献
162.
Summary An analysis was made of the effects of different sampling and extraction techniques on the amounts and pattern of monoterpenoids isolated from needles of Norway spruce. The following isolation and analysis procedure was finally adopted: liquid nitrogen-cooled needles were pulverized by a microdismembrator, extracted with pentane overnight at 2°–3°C and concentrated to a volume not less than 3 ml/g fresh weight on a Vigreux column. The crude extract was injected splitless (with solvent split) onto a cold programmed temperature vaporized (PTV) precolumn of a gas chromatograph and the vaporizable compounds heated to a capillary column. This method was tested for production of artefacts and quantitative extraction and applied to needles of eleven 80-year-old spruce trees. 相似文献
163.
The amounts of free sterols, steryl esters and lipid phosphorus were determined in the sapwood and heartwood of mature, and in the outer and inner sapwood of young Pinus sylvestris trees. In the mature trees (up to 70 years old) the heartwood contains significantly higher amounts of free sterols than the sapwood. No radial gradient can be demonstrated in the amounts of steryl esters. Lipids extracted from the sapwood contain higher amounts of phosphorus than those from the heartwood. Stems of young Pinus sylvestris trees (up to 13 years old) show in the inner sapwood higher amounts of both free sterols and steryl esters than the peripheral younger wood zone. The inner sapwood of the young stems shows slightly higher amounts of lipid phosphorus than the outer sapwood. The results indicate that Pinus sylvestris accumulates both free sterols and steryl esters in the stems at a very early stage of the life cycle. Sterol accumulation in the innermost parts of the stems seems not to depend on heartwood formation. 相似文献
164.
Summary Traditional protocols for the immunohistochemical localization of peptides and tyrosine hydroxylase (TH) in nerve fibers in cryostat sections require the tissue to be thoroughly fixed and rinsed and to be processed for the cryostat sectioning and the immunohistochemical staining more or less directly after freezing. In the present study it was tested whether also unfixed, rapidly frozen tissue, conforming to guinea pig and bovine heart specimens, can be used for the visualization of neuropeptides [neuropeptide Y (NPY) and substance P (S P)] and TH in cryostat sections. The following observations were made: 1) NPY-immunoreactive (IR) and S P-IR nerve fibers could be clearly identified in both fixed and unfixed sections of this type of tissue. 2) TH-IR nerve fibers could be detected in unfixed tissue if the sections were post-fixed with aldehydes by the use of a two-step fixation process related to a sudden change of pH. However, the outlines of the nerve fibers were sometimes diffuse. 3) Storage of unfixed tissue for periods of up to 2.5 yeart at –80° C did not lead to a decrease in immunoreactivity. 4) Somewhat higher concentrations of primary antibodies had to be used for sections of unfixed tissue than for sections of fixed tissue when the FITC method was used. This waste of antibodies was partly overcome by use of the biotin-streptavidin method. The glyoxylic acid induced catecholamine(CA)-fluorescence method for demonstration of sympathetic nerve fibers was also applied and was found to give optimal results after storage of tissue for up to 2.5 years. The study shows that the use of unfixed rapidly frozen tissue represents a fast and realistic method for the demonstration of neuropeptide immunoreactivity, that it to some extent can be used for the visualization of TH-containing nerve fibers and that it is a suitable method to maintain longterm neuropeptide and TH immunoreactivity as well as long-term CA-fluorescence reaction. 相似文献
165.
Characterization of cuticle-degrading proteases produced by the entomopathogen Metarhizium anisopliae 总被引:6,自引:0,他引:6
Two chymoelastases and three trypsinlike proteases were separated from culture filtrates of the entomopathogen Metarhizium anisopliae. A chymoelastase (Pr1) (pI 10.3 Mr 25,000) and trypsin (Pr2) (pI 4.42, Mr 28,500) were purified to homogeneity by ammonium sulphate precipitation, isoelectric focusing, and affinity chromatography. Inhibition studies showed that both enzymes possessed essential serine and histidine residues in the active site. Pr1 shows greater activity than Pr2 or mammalian enzymes against locust cuticle and also possesses activity vs elastin. Pr1 shows a broad primary specificity toward amino acids with hydrophobic side groups in synthetic ester and amide substrates. The kinetic properties of Pr1 demonstrate a preference for extended peptide chains with the active site recognising at least five substrate residues. The S5 and S4 subsites show a preference for negatively charged succinyl and hydrophobic acetyl groups, respectively. The S3 and S2 subsites both discriminated in favor of alanine and against proline. Pr2 rapidly hydrolyzed casein and synthetic substrates containing arginine or lysine. It possessed little or no activity vs cuticle, elastin, or synthetic substrates for chymotrypsin and elastase. Specific active site inhibitors confirmed the similarities between Pr2 and trypsin. 相似文献
166.
Håkan Tegelström 《Biochemical genetics》1987,25(1-2):95-110
Genetic variation of mitochondrial DNA (mtDNA) in 18 great tits (Parus major) from three neighboring localities in Sweden was investigated with eight tetranucleotide restriction endonucleases. The 18 individuals could be separated into 13 different maternal lineages. The high number of female lineages present in this regional population contrasts with a low level of sequence divergence between the different mtDNA clones, with a mean of 0.19% sequence divergence between all individuals. There was no obvious spatial structuring of mtDNA clones among the three localities. The presence of a high number of different clones with a low degree of sequence divergence could be explained by the effects of a large long-term effective population size, with the mtDNA clones having diverged about 25,000–200,000 years ago.This study was supported by the Swedish Natural Science Research Council, the Erik Philip-Sörensen Foundation, and the Nilsson-Ehle Foundation. 相似文献
167.
Ronald J. Prokopy Daniel R. Papaj Susan B. Opp Tim T. Y. Wong 《Entomologia Experimentalis et Applicata》1987,45(3):251-258
We examined the intra-tree foraging behavior of individually-released, wild-population Mediterranean fruit flies (medflies), Ceratitis capitata (Wiedemann), on field-caged host trees bearing each of three different densities (0, 3, or 12 per tree) of non-infested host fruit (kumquat) or each of two levels of fruit quality (12 non-infested fruit or 12 fruit infested with eggs and covered with host marking pheromone). With increasing density of non-infested fruit, medflies tended to remain longer in trees, visit more fruit before leaving, oviposit more often, accept a proportionately smaller number of fruit visited, and emigrate sooner after the last egg was laid (i.e. have a shorter Giving-Up-Time). Medflies spent much less time, oviposited much less often, and exhibited a longer Giving-Up-Time on trees harboring pheromone-marked fruit than non-infested fruit. Variation in temperature within the range at which experiments were conducted (25–36°C) had little detectable influence on foraging behavior. We compare our findings with published findings on the intra-tree foraging behavior of another tephritid fly, Rhagoletis pomonella (Walsh), and with current foraging behavior theory. We discuss implications of our findings with respect to medfly management strategies, particularly fruit stripping in eradication programs and use of synthetic marking pheromone for control.
Résumé Nous avons étudié le comportement de prospection dans un arbre, de femelles d'une population sauvage de C. capitata, libérées individuellement à l'intérieur de cages contenant des Eriobotrya japonica (kumquat), portant chacun 3 densités différentes de fruits no contaminés (0, 3, 12 par arbre) et chacun 2 niveaux de qualité de fruits: 12 fruits non infestés ou 12 fruits contaminés par des oeufs et recouverts de phéromone de marquage de l'hôte. C. capitata avait terndance à rester plus longtemps dans les arbres, à visiter plus de fruits avant le quitter, à pondre plus souvent, à accepter proportionnellement un nombre plus réduit de fruits déjà visités, à émigrer plus tôt après la ponte du dernier oeuf (c'est-à-dire à présenter un temps d'abandon plus bref), quand la densité des fruits non contaminés augmentait. C. capitata a dépensé beaucoup moins de temps, pondu beaucoup moins souvent, et présenté un temps d'abandon plus long sur les arbres portant des fruits marqués par la phéromone que sur ceux ayant des fruits non contaminés. Les variations de température dans la gamme de cells où les observations ont eu lieu (23–36°C) n'ont eu qu'une faible influence décelable sur le comportement de prospection. Nous avons comparé nos résultats avec ceux publiés sur la prospection à l'intérieur de l'arbre par une autre téphritide (Rhagoletis pomonella) et avec la théorie dominante sur le comportement de prospection. Nous discutons les conséquences de nos résultats sur les stratégies de lutte contre C. capitata, en particulier l'élimination des fruits dans les plans d'erradication et l'utilisation de phéromone synthétique de marquage.相似文献
168.
Enzyme loci, visible marker genes and -cloned DNA-sequences from a D. miranda library were mapped cytologically on the chromosome elements C and E of D. pseudoobscura and D. subobscura. New data are incorporated into the linkage maps of the two species. Homologous segments can now be localized in the polytene chromosomes with these markers. A comparison of the chromosome elements E of D. melanogaster and D. subobscura shows 12 conserved subsections which have been rearranged by paracentric inversions in the evolution of the two lineages. 相似文献
169.
The title disaccharide glycoside was synthesized by halide ion-promoted glycosidation, using methanol and the disaccharide bromide derived from methyl 2-azido-3-O-(2,3,4,6-tetra-O-benzoyl--d-galactopyranosyl)-4,6-O-benzylidene-2-deoxy-1-thio--d-galactopyranoside. This derivative in turn was prepared by silver triflate-promoted condensation of monosaccharide derivatives. 相似文献
170.
Göran Ekborg Martin Klinger Lennart Rodén John Wjesen John S Schutzbach Dee H Huang N Rama Krishna Gattadalli M Anantharamaiah 《Glycoconjugate journal》1987,4(3):255-266
3-O-β-d-Xylopyranosyl-l-serine (xylosylserine) was synthesized by the following three-step procedure: 1) 2,3,4-tri-O-benzoyl-α-d-xylopyranosyl bromide (benzobromoxylose) was condensed withN-carbobenzoxy-l-serine benzyl ester using the silver triflate-collidine complex as promoter; 2) theN-carbobenzoxy and benzyl ester groups in the resultant glycoside were cleaved by transfer hydrogenation with palladium black as catalyst and ammonium formate as hydrogen donor; and 3) the benzoyl groups were removed with methanolic ammonia. Xylosylserine was obtained in an overall yield of 70%. O-β-d-Galactopyranosyl-(1-4)-O-β-d-xylopyranosyl-(1-3)-l-serine (galactosylxylosylserine) was also synthesized by this methodology and was characterized by 2-dimensional (2D) NMR spectroscopy techniques. The two serine glycosides (xylosylserine and galactosylxylosylserine) were used in detection and partial purification of galactosyltransferase I (UDP-d-galactose:d-xylose galactosyltransferase) from adult rat liver. 相似文献