全文获取类型
收费全文 | 21269篇 |
免费 | 1859篇 |
国内免费 | 9篇 |
专业分类
23137篇 |
出版年
2023年 | 77篇 |
2022年 | 175篇 |
2021年 | 342篇 |
2020年 | 213篇 |
2019年 | 276篇 |
2018年 | 317篇 |
2017年 | 318篇 |
2016年 | 525篇 |
2015年 | 842篇 |
2014年 | 941篇 |
2013年 | 1172篇 |
2012年 | 1616篇 |
2011年 | 1663篇 |
2010年 | 1018篇 |
2009年 | 891篇 |
2008年 | 1308篇 |
2007年 | 1375篇 |
2006年 | 1265篇 |
2005年 | 1249篇 |
2004年 | 1226篇 |
2003年 | 1157篇 |
2002年 | 1074篇 |
2001年 | 248篇 |
2000年 | 206篇 |
1999年 | 251篇 |
1998年 | 285篇 |
1997年 | 196篇 |
1996年 | 166篇 |
1995年 | 192篇 |
1994年 | 190篇 |
1993年 | 164篇 |
1992年 | 139篇 |
1991年 | 145篇 |
1990年 | 147篇 |
1989年 | 127篇 |
1988年 | 120篇 |
1987年 | 138篇 |
1986年 | 120篇 |
1985年 | 119篇 |
1984年 | 119篇 |
1983年 | 131篇 |
1982年 | 122篇 |
1981年 | 123篇 |
1980年 | 95篇 |
1979年 | 66篇 |
1978年 | 67篇 |
1977年 | 71篇 |
1976年 | 63篇 |
1975年 | 50篇 |
1973年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
We determined that leaner gene (la) is located in the linkage group XVIII and closely linked to Es-1, which is known to be located closely to tottering gene (tg). Double heterozygote (la/tg) produced by mating between la heterozygote and tg heterozygote showed an intermediate syndrome between those seen in tottering (tg/tg) and leaner (la/la) mice. Both leaner and tottering mice showed neuromuscular disorders, but their clinical and pathological characteristics were different. Leaner mice were found to represent a so-called cerebellar mutant having the reduced size of cerebellum and severe cytoarchitectonic abnormalities with focal losses of Purkinje and granular layer cells. Tottering was, however, another mutation having epileptiform seizures, and it was characterized pathologically by cellular losses and shrinkage as well as vesiculations of cytoplasmic membranous structures in the cerebellum. The double heterozygote was shown to have both pathologic characteristics seen in each homozygote, and also showed shrinkage of Purkinje cells and vesiculation of the endoplasmic reticulum and Golgi apparatus. These clinical and pathological findings supported the genetic data suggesting that la and tg constitute an allele. 相似文献
52.
53.
54.
Cross-comparisons of nectar production data are complicated because different workers use bags made of various materials to exclude animal visitors. Using clonal populations of Asclepias syriaca and A. exaltata in northern Virginia, we carefully measured the effects of four bagging treatments (bridal veil, pellon, paper, plastic) on microenvironment (temperature, relative humidity) and nectar production (volume, concentration, sucrose amount) over the course of a day. In general, bridal veil bags changed the microenvironment least relative to unbagged controls. Plastic bags resulted in higher temperatures and constantly higher relative humidities. Temperature and relative humidity were also elevated, though less dramatically, in paper and pellon bags. Under more humid conditions, flowers contained larger volumes of more dilute nectar. Therefore, researchers who wish to obtain nectar production data that reflect natural field conditions should use bridal veil, or a material with similar properties, to bag inflorescences. We also performed a watering experiment, involving the addition of the equivalent of a 10-cm rain to the A. syriaca plot. After watering, nectar volumes and sucrose amounts were increased approximately twofold. 相似文献
55.
Hanafi H. Russell Richard J. Jackson David P. Spath Steven A. Book 《The Western journal of medicine》1987,147(5):615-622
Drinking water contamination by toxic chemicals has become widely recognized as a public health concern since the discovery of 1,2-dibromo-3-chloropropane in California''s Central Valley in 1979. Increased monitoring since then has shown that other pesticides and industrial chemicals are present in drinking water. Contaminants of drinking water also include naturally occurring substances such as asbestos and even the by-products of water chlorination. Public water systems, commercially bottled and vended water and mineral water are regulated, and California is also taking measures to prevent water pollution by chemicals through various new laws and programs. 相似文献
56.
Big Moose Basin: simulation of response to acidic deposition 总被引:2,自引:1,他引:1
George F. Davis John J. Whipple Steven A. Gherini Carl W. Chen Robert A. Goldstein Arland H. Johannes Peter W.H Chan Ronald K. Munson 《Biogeochemistry》1987,3(1-3):141-161
The ILWAS model has been enhanced for application to multiple-lake hydrologic basins. This version of the model has been applied to the Big Moose basin, which includes Big Moose Lake and its tributary streams, lakes, and watersheds. The basin, as defined, includes an area of 96 km2, with over 20 lakes and ponds, and 70 km of streams. Hydrologic and chemical calibrations have been made using data from seven sampling stations. When total atmospheric sulfur loading to the basin is halved, the model predicts, after four years of simulation, a decreasing sulfate concentration and to a lesser extent a rising alkalinity at Big Moose Lake outlet. At the end of four years, the results show an increase in pH of 0.1 to 0.5 pH units depending upon season. 相似文献
57.
We have cloned and characterized three distinct alpha-globin haplotypes obtained from inbred strains of the mouse, Mus domesticus. We report here the complete nucleotide sequence of the six alpha-globin genes that the haplotypes contain. Our analysis of these genes and those from one other previously described haplotype indicates that recurrent gene conversion events have played a major role in their history. The pattern of nucleotide substitutions suggests that conversions have occurred both within and between haplotypes. Limited segments of coding and noncoding DNA have been involved in these gene conversion events. In two of the haplotypes, the nonallelic genes of each maintain DNA sequence identity over discrete intervals and encode the same alpha-globin polypeptide. On the other hand, the coding regions of some genes have accumulated replacement changes that result in distinct alpha-globins. In one instance, these changes appear to reflect positive selection of advantageous mutations. 相似文献
58.
Populations of the vernal cladoceranDaphniopsis ephemeralis are found in woodland ponds throughout southern Ontario. The species reproduces by cyclic parthenogenesis, and genotype frequencies
at allozyme loci are ordinarily in good agreement with Hardy-Weinberg expectations. Occasional heterozygote deficiencies are
apparently the consequence of admixture of ephippial hatchlings produced in temporally separated bouts of sexual reproduction.
Considerable heterogeneity in genotypic frequencies exists among local populations in southwestern Ontario, indicating that
gene flow among populations is restricted. Inbreeding coefficients suggest that populations receive an average of 0.3 migrants
per generation. The completion of a sexual life cycle is made possible despite the brief persistence of populations by the
emergence of males from ephippial eggs and by the production of equal numbers of male and female progeny in the first parthenogenetic
brood. 相似文献
59.
60.
Steven Branstetter 《Environmental Biology of Fishes》1987,19(3):161-173
Synopsis The silky shark, Carcharhinus falciformis, and scalloped hammerhead, Sphyrna lewini, represent >80% of the shark by-catch of the winter swordfish/tuna longline fishery of the northwestern Gulf of Mexico. This
catch represents a potential supplemental fishery, yet little is known of the life histories of the two species. This report
relates reproductive biology data to age and growth estimates for 135 C. falciformis and 78 S. lewini. Unlike other regional populations, C. falciformis in the Gulf of Mexico may have a seasonal 12 month gestation period. Males mature at 210–220 cm TL (6–7 yr); females at >225
cm TL (7–9 yr). Application of age at length data for combined sexes produced von Bertalanffy growth model parameter estimates
of L∞ = 291 cm TL, K = 0.153, t0 = −2.2 yr. Adult male S. lewini outnumbered adult females in catches because of differences in the distributions of the sexually segregated population. Males
mature at 180 cm TL (10 yr); females at 250 cm TL (15 yr). von Bertalanffy parameter estimates for combined sexes of this
species were L∞ = 329 cm TL, K = 0.073, to = −2.2 yr. 相似文献