全文获取类型
收费全文 | 15390篇 |
免费 | 1078篇 |
国内免费 | 7篇 |
出版年
2023年 | 144篇 |
2022年 | 163篇 |
2021年 | 528篇 |
2020年 | 391篇 |
2019年 | 511篇 |
2018年 | 623篇 |
2017年 | 535篇 |
2016年 | 720篇 |
2015年 | 912篇 |
2014年 | 981篇 |
2013年 | 1111篇 |
2012年 | 1215篇 |
2011年 | 1103篇 |
2010年 | 668篇 |
2009年 | 603篇 |
2008年 | 709篇 |
2007年 | 707篇 |
2006年 | 608篇 |
2005年 | 555篇 |
2004年 | 504篇 |
2003年 | 444篇 |
2002年 | 375篇 |
2001年 | 273篇 |
2000年 | 245篇 |
1999年 | 211篇 |
1998年 | 72篇 |
1997年 | 73篇 |
1996年 | 62篇 |
1995年 | 61篇 |
1994年 | 61篇 |
1993年 | 55篇 |
1992年 | 119篇 |
1991年 | 89篇 |
1990年 | 64篇 |
1989年 | 78篇 |
1988年 | 72篇 |
1987年 | 87篇 |
1986年 | 74篇 |
1985年 | 59篇 |
1984年 | 60篇 |
1983年 | 39篇 |
1982年 | 35篇 |
1981年 | 34篇 |
1980年 | 26篇 |
1979年 | 52篇 |
1978年 | 31篇 |
1976年 | 29篇 |
1975年 | 33篇 |
1973年 | 42篇 |
1972年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Pedro J. N. Silva Richard K. Koehn Walter J. Diehl III Robin P. Ertl Elaine B. Winshell Mauro Santos 《Biochemical genetics》1989,27(7-8):451-467
Four samples of the musselMytilus edulis were taken between 1984 and 1987 from Stony Brook, New York, and used to study the glucose-6-phosphate isomerase (GPI) polymorphism
in this species.In vitro specific activity andin vivo flux measured in the same animals were found to be significantly correlated. A significant effect of GPI genotype on flux
was observed in one of the samples; overall, significant evidence of effect of genotype on enzyme activity was also obtained.
GPI activities of common genotypes tend to deviate less from the population mean than those of rare (frequency less than 5%)
genotypes. This suggests the possibility that rare GPI genotypes are rare as a consequence of having biochemical properties
that deviate from an optimum level and, therefore, having a lower fitness. In support of this hypothesis, we found in one
of our samples that shell length is a concave function of GPI activity with an intermediate optimum activity level.
The financial support provided to P.J.N.S. by the Luso-American Educational Commission (Fulbright Program), the Instituto
Nacional de Investigacao Científica (Portugal), and the Faculdade de Ciências da Universidade de Lisboa during several stages
of this research is gratefully acknowledged. Financial support from the Ministerio de Educatión y Ciencia (Spain) in the form
of a postdoctoral Fulbright/MEC fellowship to M.S. is also gratefully acknowledged. Research was supported by National Science
Foundation Grant BSR-8415060 to R.K.K. This is contribution No. 736 from the Program in Ecology and Evolution, State University
of New York at Stony Brook.
On leave from Departamento de Biologia Vegetal, Faculdade de Ciências, Universidade de Lisboa, Campo Grande C2, Lisboa, Portugal. 相似文献
52.
The major type-1 protein phosphatase catalytic subunits are the same gene products in rabbit skeletal muscle and rabbit liver 总被引:3,自引:0,他引:3
P T Cohen D L Schelling O B da Cruz e Silva H M Barker P Cohen 《Biochimica et biophysica acta》1989,1008(1):125-128
The catalytic subunit of protein phosphatase-1 (PP1) isolated from rabbit liver had the same electrophoretic mobility as, and yielded peptide maps identical to those of the 33 kDa form of rabbit skeletal muscle PP1. The predicted amino-acid sequences of PP1 obtained from three rabbit liver cDNA clones were identical to that of PP1 alpha from rabbit skeletal muscle. These findings suggest that the distinctive substrate specificities and regulatory properties of hepatic and skeletal muscle type-1 protein phosphatases are not conferred by the catalytic subunits themselves, but by regulatory subunits that are complexed to the catalytic subunits in vivo. 相似文献
53.
54.
I. B. Kerr A. M. Mendes da Silva E. Drouhet P. de Oliveira S. C. G. da Costa 《Mycopathologia》1988,101(1):3-11
Congenitally athymic nude mice (nu/nu) and their phenotypically normal littermates (nu/+) were intraperitoneally infected with yeast cells of a strain of Paracoccidioides brasiliensis. The nude mice developed a severe and generalized infection with an intense parasitism of several organs, accompanied by a low-grade of tissue reaction. The lesions were characterized by abundant yeast-like cells of the fungus, and in some animals, numerous hyphal forms could be well visualized. In control animals, infection was moderate, almost exclusively restricted to the area of inoculation, and the lesions presented few parasites surrounded by an inflammatory response. Filamentous forms of the fungus were never encountered in these animals. 相似文献
55.
56.
β-thalassemia mutations in the Portuguese population 总被引:2,自引:0,他引:2
M. P. Coutinho Gomes M. G. Gomes da Costa L. B. Braga N. T. Cordeiro-Ferreira A. Loi M. Pirastu A. Cao 《Human genetics》1988,78(1):13-15
Summary In this study we have carried out haplotype analysis on the -globin gene cluster and characterized the -thalassemia mutation by oligonucleotide hybridization in 14 patients with thalassemia major and 5 with sickle cell/-thalassemia originating from southern Portugal. We found that three mutations, namely the °-39, ° IVS-1 nt 1 and + IVS-1 nt 110 are prevalent accounting for 53%, 32% and 10% of the -thalassemia chromosomes respectively. In general each mutation was associated with a specific chromosomal haplotype; the ° mutation, however, was linked to three different haplotypes. These results indicate that three oligo-probes complementary to the most common mutations allow prenatal diagnosis by oligonucleotide analysis in 96% of the couples at risk of having offspring with thalassemia major in southern Portugal. 相似文献
57.
The interaction of Trichomonas vaginalis and Tritrichomonas foetus with epithelial cells in vitro 总被引:3,自引:0,他引:3
The Madin-Darby canine kidney epithelial cell line (MDCK) was used as a model for trichomonad-host cell interaction. Two laboratory strains of the human parasite Trichomonas vaginalis and the cattle's parasite Tritrichomonas foetus or their supernatants from axenic cultures were allowed to interact with confluent epithelial cultures. The interaction process studied by transmission and scanning electron microscopy revealed that both parasites adhere to monolayers through flagella, cell body and particularly for T. foetus, through the posterior projection of the axostyle. A close contact region between the trichomonad's surface and MDCK cells was observed. A study of the involvement of trichomonad surface component in the interaction process indicated that cytochalasin B treated-parasites adhere much less to epithelial monolayers than untreated parasites. Colchicine treatment did not affect such adhesion. Treatment of the parasites with trypsin reduced the adhesion of trichomonads to monolayers but did not interfere with the cytopathic effect. In contrast, treatment of the parasites with neuraminidase did not interfere with their adhesion to epithelial cells and the monolayer destruction was further increased. 相似文献
58.
Ola Palm W. Lionel Weerakoon M. Ananda P. de Silva Thomas Rosswall 《Plant and Soil》1988,108(2):201-209
Sesbania sesban was evaluated as green manure crop for lowland rice in the Dry Zone of Sri Lanka. The legume was grown during a fallow period
before lowland rice (Oryza sativa) and ploughed under just before transplanting. Weight loss and nitrogen content in litterbags containing leaves, stems and
roots of the legume were monitored. Comparisons were made between rice yields from 20 m2 plots after green manuring in combination with different nitrogen fertilizer levels (0, 2.4, 4.8 and 7.2 gm−2) and nitrogen fertilizer (9.6 gm−2) alone. Above-ground biomass ofS. sesban was 440 gm−2 (dry wt) when ploughed under after 84 days growth. N-content in leaves, stems and roots was 3.76%, 0.41% and 0.73%, respectively.
This gave a N-input fromS. sesban of 9.2 gm−2 (8.3 g from above-ground parts and 0.9 g from roots). The corresponding K and P inputs were 7.3 and 0.6 gm−2 respectively. The nitrogen rich leaves, which contained 88% of the nitrogen in the above-ground parts, decomposed and released
its nitrogen much more rapidly than the stems and roots. After only four days the leaves had released 5.3 g Nm−2 and after 14 days they had released 6.4 g Nm−2. The highest rice yield (505 gm−2) was obtained usingS. sesban and 4.8 gm−2 of N-fertilizer. The yields with only N-fertilizer or onlyS. sesban were 442 gm−2 and 396 gm−2, respectively. Due to the rapid decomposition of the nitrogen rich leaves,S. sesban did not behave as a slow release fertilizer. Thus, it is not necessary to apply nitrogen fertilizers as a basal dose. 相似文献
59.
V de Silva S Thapa L R Wilkens M G Farr K Jayasinghe J E McMahan 《Journal of biosocial science》1988,20(2):143-156
The effects of different levels of compensatory payment for vasectomy on sterilization acceptance were examined in 496 vasectomized men in urban Sri Lanka. The results indicate that compensatory payments significantly enhanced the participation of economically poor men in vasectomy programs, especially those who had already achieved a large family size. The proportion of poor acceptors (those with a monthly income of Rs 1000 or less) increased with higher levels of payment; the acceptor's level of education was negatively correlated with the compensation amount, and the mean age of the youngest child was higher among those who received higher payments. 60% of vasectomy acceptors reported using contraception immediately before the vasectomy, suggesting a high level of motivation not to have another child. Respondents cited high effectiveness, no extra expense, and no side effects for their wives as the main reasons for selecting vasectomy over other means of contraception, regardless of the amount of payment received. Only 5% said cash payment was an important reason for choosing vasectomy, and this response did not vary significantly by level of payment. There was no influence of payment level on postoperative complications or satisfaction. While compensatory payments significantly enhanced the participation of poor men in vasectomy programs, they were not effective in attracting poorer men with few children or those whose last child was relatively young. Overall, this study's findings suggest that the decision to provide compensatory payments and how much to provide should be based on economic and political factors, not on the grounds that higher compensatory payments have led to the recruitment of ineligible men or that the promotion of vasectomy has been at the expense of a loss in the quality of services provided. 相似文献
60.
M. M. Antunes P. L. da Cunha A. P. Duarte E. P. Mendonça 《Journal of fish biology》1988,33(SA):185-190
Ria de Alvor is a small estuary between Lagos and Portimão on the south coast of Portugal, 37°7'–37°10'N, 8°35'–8°38'W.
Samples were collected monthly in the period April 1985–March 1986 in an ichthyoplankton and ichthyofauna survey of the estuary. A WP-2 plankton net was used for catching ichthyoplankton, and a beam trawl and a beach seine were used for catching ichthyofauna.
Scaldfish, Arnoglossus laterna (Walbaum, 1792), even though the most abundant species in ichthyoplankton samples, was never found in ichthyofauna samples.
From the results obtained, it is concluded that a great part of the fish species are temporary marine visitors using the Ria as a nursery ground, although this system appears to provide a spawning area for species with benthic eggs. No evidence was found of pelagic spawning. 相似文献
Samples were collected monthly in the period April 1985–March 1986 in an ichthyoplankton and ichthyofauna survey of the estuary. A WP-2 plankton net was used for catching ichthyoplankton, and a beam trawl and a beach seine were used for catching ichthyofauna.
Scaldfish, Arnoglossus laterna (Walbaum, 1792), even though the most abundant species in ichthyoplankton samples, was never found in ichthyofauna samples.
From the results obtained, it is concluded that a great part of the fish species are temporary marine visitors using the Ria as a nursery ground, although this system appears to provide a spawning area for species with benthic eggs. No evidence was found of pelagic spawning. 相似文献