全文获取类型
收费全文 | 8857篇 |
免费 | 899篇 |
国内免费 | 3篇 |
出版年
2023年 | 41篇 |
2022年 | 111篇 |
2021年 | 216篇 |
2020年 | 119篇 |
2019年 | 153篇 |
2018年 | 182篇 |
2017年 | 170篇 |
2016年 | 253篇 |
2015年 | 452篇 |
2014年 | 471篇 |
2013年 | 570篇 |
2012年 | 753篇 |
2011年 | 698篇 |
2010年 | 448篇 |
2009年 | 455篇 |
2008年 | 547篇 |
2007年 | 572篇 |
2006年 | 531篇 |
2005年 | 457篇 |
2004年 | 490篇 |
2003年 | 385篇 |
2002年 | 359篇 |
2001年 | 101篇 |
2000年 | 87篇 |
1999年 | 97篇 |
1998年 | 90篇 |
1997年 | 56篇 |
1996年 | 56篇 |
1995年 | 53篇 |
1994年 | 44篇 |
1993年 | 49篇 |
1992年 | 51篇 |
1991年 | 52篇 |
1990年 | 54篇 |
1989年 | 49篇 |
1988年 | 31篇 |
1987年 | 38篇 |
1986年 | 40篇 |
1985年 | 46篇 |
1984年 | 39篇 |
1983年 | 32篇 |
1982年 | 26篇 |
1981年 | 25篇 |
1980年 | 15篇 |
1979年 | 16篇 |
1974年 | 21篇 |
1973年 | 18篇 |
1972年 | 15篇 |
1969年 | 13篇 |
1967年 | 12篇 |
排序方式: 共有9759条查询结果,搜索用时 654 毫秒
81.
To characterize the deep subsurface environment of Rainier Mesa, Nevada Test Site, rock samples were taken from tunnels U 12b, U12g, U12p, and U 12n, which varied in depth from 50 m to 450 m and in gravimetric moisture content from 4% to 27%. Values for total count, viable count, biomass, Simpson diversity, equitability, similarity coefficient, and number of distinct colony types indicated microbiological variability between samples. Viable counts ranged from less than 1 × 101 to 2.4 × 105 CFU g dry wt–1 of rock. Direct counts and enumeration based on phospholipid determination indicated larger numbers of cells g dry wt-1 of rock than viable counts. Simpson diversity indices, equitability, and numbers of distinct colony types varied from 3.00 to 8.05, 0.21 to 0.89, and 7 to 19, respectively, and indicated heterogeneity between samples. Each distinct morphotype was purified and characterized. Gram reaction, morphology, metal and antibiotic resistances, and metabolic activities of each isolate confirmed spatial variability among microbiota isolated from different locations. Most probable numbers of nitrifying, sulfur oxidizing, and sulfur-reducing bacteria were below the limit of detection in all samples, while the numbers of nitrogen fixing bacteria ranged from below the level of detection to 7.8 × 102 cells g dry wt–1 of rock sample, and the numbers of dentrifying bacteria ranged from below the level of detection to greater than 1.6 × 103 cells g dry wt–1 of rock sample.
Offprint requests to: P. S. Amy. 相似文献
82.
David M. Spooner Gregory J. Anderson Robert K. Jansen 《American journal of botany》1993,80(6):676-688
We used chloroplast DNA restriction site analysis to test hypotheses of relationships of Solarium subgenus Potatoe (including potatoes and pepinos), two other Solanum, Cyphomandra (the tree tomatoes), and Lycopersicon (the tomatoes). Capsicum and Datura were used as outgroups. The results support two main clades among the taxa we studied: 1) Solanum subgenus Potatoe and Lycopersicon; and 2) other Solanum and Cyphomandra. Within the first clade, the following groups were supported: a) sect. Basarthrum and sect. Anarrhichomenum; b) sect. Etuberosum; c) sect. Petota; d) sect. Juglandifolium, including subsect. Lycopersicoides; and e) the genus Lycopersicon. These results, in combination with an analysis of morphological data, advocate the controversial, but previously suggested, treatment of Lycopersicon as congeneric with Solanum in subgenus Potatoe. Thus, the cultivated tomato will be recognized as Solanum lycopersicum L. Solanum chmielewskii and Solanum lycopersicum var. cerasiforme are proposed as new combinations; Solanum neorickii is proposed as a new name for Lycopersicon parviflorum. Our data also suggest that Cyphomandra should be included within Solanum. 相似文献
83.
Mark D. Rausher David Augustine Amy VanderKooi 《Evolution; international journal of organic evolution》1993,47(6):1688-1695
Throughout southeastern North America, the annual morning glory Ipomoea purpurea exhibits a polymorphism at a locus that influences the intensity of floral pigmentation. Previous studies have shown that when rare, the homozygous white genotype has a greater selfing rate than the homozygous dark genotype. In the absence of pollen discounting (a reduction in transmission of pollen to other plants by genotypes that exhibit increased selfing) and inbreeding depression, this increased selfing rate should favor the white allele. Experiments reported here confirm that the white genotype has elevated selfing rates when rare but indicate pollen discounting is not associated with elevated selfing. Rather, white genotypes contribute more pollen to the outcross pollen pool. The disparity between genotypes in both selfing rates and success at pollen contribution to other plants disappears at intermediate to high frequencies of the white allele. Pollinator movements are consistent with the pattern of selfing. These results suggest that elevated selfing and enhanced success at pollen donation contribute to maintenance of the white allele in natural populations of morning glories. 相似文献
84.
Identifying the drivers of community structure and dynamics is a major pursuit in ecology. Emphasis is typically placed on the importance of local scale interactions when attempting to explain these fundamental ecological patterns. However, regional scale phenomena are also important predictors. The importance of regional scale context should be more evident in assemblages where multiple species are close to their range margins. Here, we test the importance of regional scale context using data from a temperate forest plot that contains two species groups – one near its northern range limit and one near its southern range limit. We show the proximity of species to their southern or northern range margins is linked to local scale co-occurrence, similarity in gene expression responses to a key environmental driver, demographic performance and inter-specific variation in conspecific negative density dependence. In sum, many of the key local scale patterns and processes of interest to community ecologists are linked to biogeographic context that is frequently ignored. 相似文献
85.
Restriction site mapping of chloroplast DNA from 31 species representing 26 genera of theRanunculaceae was performed using eleven restriction endonucleases. The chloroplast genome varies in length from approximately 152 to 160 kb. Length variants are frequent in theRanunculaceae and range from usually less than 300 bp to rarely 1.5 kb. The inverted repeat is extended into the large single copy (LSC) region by 4–4.5 kb inAnemone, Clematis, Clematopsis, Hepatica, Knowltonia, andPulsatilla. Several inversions are present in the LSC-region of the cpDNA in all these genera and inAdonis. The frequency of restriction site mutations varies within the chloroplast genome in theRanunculaceae between 4 and 32 mutations per kilobase, and is lowest in the inverted repeat and the regions containing the ATPase-genes and the genespsaA, psaB, psbA, rpoB, andrbcL. A total of 547 phylogenetically informative restriction sites was utilized in cladistic analyses of the family using Wagner, Dollo, and weighted parsimony. These three parsimony analyses result in different tree topologies. Four, six, and one equally most parsimonious trees were obtained with Wagner, Dollo, and weighted parsimony, respectively. The amount of support for the monophyletic groups was evaluated using bootstrapping and decay analysis. All three parsimony methods suggest thatHydrastis is the sister group to the remainder of theRanunculaceae, and that theAnemone-Clematis group, which shares several derived cpDNA rearrangements, is monophyletic. Only a few of the traditional groups in theRanunculaceae are supported by cpDNA restriction side data. Only Dollo parsimony provides support for the hypothesis thatThalictroideae andRanunculoideae are monophyletic. 相似文献
86.
Vasil Pirgozliev Stephen C. Mansbridge Izzy M. Whiting Stephen P. Rose Kristina Kljak Amy E. Johnson Falko Drijfhout Sylwia Orczewska-Dudek Atanas G. Atanasov Teodora Mihova 《The Annals of applied biology》2023,183(1):7-14
The aim of this study was to assess the effects of graded levels (0, 3, 6, 9 and 12 g/kg) of dry Sea buckthorn (SB) berries on growth performance, gastrointestinal tract (GIT) development, jejunal histomorphology, bird antioxidant status and caecal short-chain fatty acid concentration when fed to female Ross 308 broiler chickens. In addition, expression of cytokine biomarker genes in the jejunum was evaluated. The five experimental diets were fed from 7 to 21 days age to 8 pens (two birds in each) following randomisation. Feeding SB did not influence bird growth performance (p > .05). There was a linear decrease in butyric, acetic and valeric acid concentrations in caecal digesta (p < .05) and a decrease (p < .05) in crypt depth. The expression of IFNG and CD40LG responded quadratically (p < .05), peaking at 6–9 g/kg dietary inclusion of SB, respectively. Other studied variables were not affected by dietary SB inclusion (p > .05). Feeding dry SB berries up to 12 g/kg of diet did not improve the zootechnical variables of healthy commercial-strain broilers in this study. 相似文献
87.
Melinda A. Novak Amy Musante Heather Munroe Peggy L. O'Neill Cristofer Price Stephen J. Suomi 《Zoo biology》1993,12(3):285-298
Recent research has indicated that old, individually housed monkeys show little interest in novel objects. Yet unanswered is whether this effect is caused primarily by age or housing condition. The purpose of this study was to assess the role of social living in promoting responsiveness to objects. We measured the rates of object manipulation in older animals, assessed responsiveness over time to particular objects as a measure of habituation, and examined social influences on object use. Several social groups of rhesus monkeys that contained older adults were studied. These groups were housed in indoor pens or in an outdoor enclosure, and all monkeys had continuous access to a variety of objects in their home environment. In contrast to previous studies of individually housed monkeys, our group-housed monkeys showed sustained interest in objects. Old monkeys manipulated objects extensively, and this response was all the more significant, given that the objects were not novel. Monkeys housed in an outdoor enclosure showed object manipulation patterns that were not different from monkeys housed in indoor pens. However, females exhibited much higher object-related responses than males. Social facilitation played a role in the reactions of some monkeys to objects. Patterns of social facilitation as well as avoidance were present in two of the three indoor groups that were observed. Failure to manipulate objects in rhesus macaques appears to be more a function of individual housing than of old age. Factors such as environmental complexity, social needs, and early experience should be considered in order to understand why individually housed rhesus monkeys are unresponsive to objects. © 1993 Wiley-Liss, Inc. 相似文献
88.
Dirk T. Witte Frank J. Bruggeman Jan Piet Franke Swier Copinga Johanna M. Jansen Rokus A. De Zeeuw 《Chirality》1993,5(7):545-553
Direct enantiomeric separations of 17 chiral amidotetralins by means of high performance liquid chromatography were performed on stationary phases composed of tris(3,5-dimethylphenylcarbamate) derivatives of cellulose and amylose, coated on silica gel. The enantiomers of 15 out of 17 amidotetralins were resolved with a resolution of more than 1.5 by at least one of the chiral stationary phases. The stationary phases showed complementary results with regard to the separation of the amidotetralins, that is, pairs that did not separate on the cellulose-type column were well separated on the amylose-type column, and vice versa. There was no significant correlation between the chromatographic properties of the chiral stationary phases. © 1993 Wiley-Liss, Inc. 相似文献
89.
Subcellular fractions of three human brain specimens were found to contain esterase activities which hydrolyzed racemic oxazepam 3-acetate (rac-OXA). All three human brain preparations were highly selective toward the S-enantiomer of rac-OXA. © 1993 Wiley-Liss, Inc. 1 This article is a US Government work and, as such, is in the public domain in the United States of America. 相似文献
90.