全文获取类型
收费全文 | 4956篇 |
免费 | 591篇 |
国内免费 | 3篇 |
专业分类
5550篇 |
出版年
2022年 | 50篇 |
2021年 | 106篇 |
2020年 | 69篇 |
2019年 | 111篇 |
2018年 | 109篇 |
2017年 | 113篇 |
2016年 | 148篇 |
2015年 | 226篇 |
2014年 | 259篇 |
2013年 | 283篇 |
2012年 | 322篇 |
2011年 | 277篇 |
2010年 | 204篇 |
2009年 | 171篇 |
2008年 | 239篇 |
2007年 | 222篇 |
2006年 | 186篇 |
2005年 | 212篇 |
2004年 | 152篇 |
2003年 | 183篇 |
2002年 | 157篇 |
2001年 | 155篇 |
2000年 | 161篇 |
1999年 | 140篇 |
1998年 | 67篇 |
1997年 | 53篇 |
1996年 | 65篇 |
1995年 | 44篇 |
1994年 | 51篇 |
1993年 | 49篇 |
1992年 | 93篇 |
1991年 | 81篇 |
1990年 | 86篇 |
1989年 | 67篇 |
1988年 | 57篇 |
1987年 | 67篇 |
1986年 | 52篇 |
1985年 | 56篇 |
1984年 | 24篇 |
1983年 | 45篇 |
1982年 | 39篇 |
1981年 | 29篇 |
1980年 | 21篇 |
1979年 | 32篇 |
1978年 | 17篇 |
1977年 | 26篇 |
1976年 | 19篇 |
1975年 | 19篇 |
1974年 | 17篇 |
1973年 | 17篇 |
排序方式: 共有5550条查询结果,搜索用时 0 毫秒
1.
Fiddler crabs (Uca spp., Decapoda: Ocypodidae) are commonly found forming large aggregations in intertidal zones, where they perform rhythmic waving displays with their greatly enlarged claws. While performing these displays, fiddler crabs often synchronize their behavior with neighboring males, forming the only known synchronized visual courtship displays involving reflected light and moving body parts. Despite being one of the most conspicuous aspects of fiddler crab behavior, little is known about the mechanisms underlying synchronization of male displays. In this study we develop a spatially explicit model of fiddler crab waving displays using coupled logistic map equations. We explored two alternative models in which males either direct their attention at random angles or preferentially toward neighbors. Our results indicate that synchronization is possible over a fairly large region of parameter space. Moreover, our model was capable of generating local synchronization neighborhoods, as commonly observed in fiddler crabs under natural conditions. 相似文献
2.
Victor F. Medina Peter M. Jeffers Steven L. Larson Waleska Perez 《International journal of phytoremediation》2000,2(4):287-295
Bleach treatment of plants was studied as a simple alternative to axenic tissue cultures for demonstrating phytodegradation of aqueous and gas-phase environmental contaminants. Parrotfeather (Myriophyllum aquaticum), spinach (Spinacia oleracea), and wheat (Triticum aestivum) were exposed to 0.525% NaC10 solutions for 15 s, then rinsed in deionized water. Plate counts indicated that 97 to 100% of viable bacteria were removed from parrotfeather and spinach. Transformation rates for 2,4,6-trinitrotoluene (TNT) by bleached and untreated parrotfeather were virtually identical. Similarly, treated and untreated spinach, wheat heads, and wheat leaves removed methyl bromide (MeBr) from air at the same rates. However, wheat root with attendant adhering soil was rendered inactive by bleach treatment. Parrotfeather roots examined by dissecting microscope and by electron microscope showed no significant damage caused by bleach treatment. 相似文献
3.
4.
5.
P. Cruz C. H. Mejia‐Ruiz R. Perez‐Enriquez A. M. Ibarra 《Molecular ecology resources》2002,2(3):239-241
Five polymorphic microsatellite loci were characterized for Penaeus (Litopenaeus) vannamei. Loci were isolated using a partial Sau3A1 genomic library by the sequencing of randomly selected clones and by a biotinylated (CT)10 and (GT)10 probes screening procedure. The last strategy resulted in the most useful data. About 40% of the clones showed a previously reported satellite/microsatellite (PVS1), reducing the chance of finding new microsatellite regions. Whereas two of the microsatellite loci with more than 10 alleles will be useful for mating analysis in a breeding program, the others might prove useful for population genetic studies. 相似文献
6.
7.
M Mourelle V M Perez M Rojkind 《The journal of histochemistry and cytochemistry》1988,36(12):1471-1474
We describe a simple, sensitive, and quantitative procedure for measurement of triglycerides and protein contents in formalin-fixed liver sections. The method can detect as little as 0.27 microgram of triglycerides per mg of protein. It is based on selective binding of Sudan IV and Fast Green FCF to fat and total proteins, respectively, and their sequential elution with solvents. Sudan IV is eluted readily with acetone and Fast Green with NaOH-methanol, and the absorbances obtained at 500 and 610 nm can be used to determine the amount of triglycerides and total protein. The color equivalence for Fast Green was obtained after destaining the sections and measuring the protein contents by micro-Kjeldahl analysis. The color equivalence of Sudan IV was estimated by determining the triglyceride content in liver homogenates by an enzymatic procedure and then measuring the amount of dye bound to multiple fixed sections. There was a strong linear correlation between the triglyceride content as determined chemically and that obtained using the equivalence colors (r2 = 0.98). This method is useful to measure fat content in tissue samples and could be applied to evaluate the progression of liver disease. 相似文献
8.
Kathryn S. Evans Janneke Wit Lewis Stevens Steffen R. Hahnel Briana Rodriguez Grace Park Mostafa Zamanian Shannon C. Brady Ellen Chao Katherine Introcaso Robyn E. Tanny Erik C. Andersen 《PLoS pathogens》2021,17(3)
Parasitic nematodes cause a massive worldwide burden on human health along with a loss of livestock and agriculture productivity. Anthelmintics have been widely successful in treating parasitic nematodes. However, resistance is increasing, and little is known about the molecular and genetic causes of resistance for most of these drugs. The free-living roundworm Caenorhabditis elegans provides a tractable model to identify genes that underlie resistance. Unlike parasitic nematodes, C. elegans is easy to maintain in the laboratory, has a complete and well annotated genome, and has many genetic tools. Using a combination of wild isolates and a panel of recombinant inbred lines constructed from crosses of two genetically and phenotypically divergent strains, we identified three genomic regions on chromosome V that underlie natural differences in response to the macrocyclic lactone (ML) abamectin. One locus was identified previously and encodes an alpha subunit of a glutamate-gated chloride channel (glc-1). Here, we validate and narrow two novel loci using near-isogenic lines. Additionally, we generate a list of prioritized candidate genes identified in C. elegans and in the parasite Haemonchus contortus by comparison of ML resistance loci. These genes could represent previously unidentified resistance genes shared across nematode species and should be evaluated in the future. Our work highlights the advantages of using C. elegans as a model to better understand ML resistance in parasitic nematodes. 相似文献
9.
The intensity of sperm competition is often measured using the gonadosomatic index (testes/body weight). But sperm competition could be mediated more by size of the epididymis than by size of the testicles, and little information is available on the relationship between testicular and epididymal size. We found that both organs were positively correlated in size among male rhesus macaques. Body weight accounted for over 70% of the variance in testicle size and volumetric estimates of testicle size accurately reflected testicle weight. We conclude that methods for ascertaining testicle size are accurate, but the covariation in size between testicles and epididymis will hamper understanding of the physiological mechanisms involved in sperm competition in primates. © 1993 Wiley-Liss, Inc. 相似文献
10.
Summary The xylanolytic and cellulolytic activity fromCellulomanas were reduced by high O2 concentrations in continous culture as well as by an air flow passed trough the samples, suggesting an inhibition or inactivation of enzymes in such conditions. 相似文献