全文获取类型
收费全文 | 714篇 |
免费 | 80篇 |
专业分类
林业 | 32篇 |
农学 | 12篇 |
基础科学 | 2篇 |
97篇 | |
综合类 | 87篇 |
农作物 | 37篇 |
水产渔业 | 65篇 |
畜牧兽医 | 385篇 |
园艺 | 26篇 |
植物保护 | 51篇 |
出版年
2023年 | 18篇 |
2022年 | 9篇 |
2021年 | 37篇 |
2020年 | 37篇 |
2019年 | 56篇 |
2018年 | 33篇 |
2017年 | 42篇 |
2016年 | 35篇 |
2015年 | 23篇 |
2014年 | 33篇 |
2013年 | 20篇 |
2012年 | 55篇 |
2011年 | 62篇 |
2010年 | 30篇 |
2009年 | 31篇 |
2008年 | 39篇 |
2007年 | 45篇 |
2006年 | 38篇 |
2005年 | 34篇 |
2004年 | 14篇 |
2003年 | 17篇 |
2002年 | 17篇 |
2001年 | 4篇 |
2000年 | 8篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1996年 | 2篇 |
1994年 | 2篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1979年 | 2篇 |
1976年 | 1篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1969年 | 3篇 |
1967年 | 1篇 |
1966年 | 2篇 |
1965年 | 1篇 |
1960年 | 1篇 |
1928年 | 1篇 |
1915年 | 1篇 |
排序方式: 共有794条查询结果,搜索用时 93 毫秒
41.
Stewart A.W. Diemont Jessica L. BohnDonald D. Rayome Sarah J. KelsenKaity Cheng 《Forest Ecology and Management》2011,261(10):1696-1705
Numerous communities associated with at least five distinct ethnic Mayan groups in southern Mexico and Central America continue to rely upon forested areas as integral components of their agricultural systems. They carefully manage these areas so that forests provide food, raw materials, and animals. Management practices include removing and planting of woody and herbaceous species, apiculture, and seed harvest. Mayan agroforestry systems in geographically and ecologically distinct areas of Mesoamerica were evaluated to better understand traditional agroforestry system components and how indigenous Mayan agroforestry could be a part of regional forest conservation and restoration. Systems were within Mexican land grant areas (ejidos) or on contested land. Although these systems rely upon different woody species and management techniques, common among them are: (1) the use of multi-stage and successional pathways with forest as a part of the larger system, (2) species that are believed by traditional ecological knowledge (TEK) to accelerate forest regeneration - more than 30 tree species are recognized and managed as potential facilitators of forest regeneration and (3) direct human consumption of forest products at all stages of regeneration. 相似文献
42.
Fry FH Okarter N Baynton-Smith C Kershaw MJ Talbot NJ Jacob C 《Journal of agricultural and food chemistry》2005,53(3):574-580
Allicin, an active ingredient of garlic, possesses a range of antimicrobial properties. Unfortunately, certain properties of the compound, such as chemical instability and low miscibility with water, have hampered its practical use in the past. Here, we show that it is possible to use a binary system consisting of the plant enzyme alliinase and its substrate alliin to generate allicin, and hence antifungal activity, in situ. During application, the two inactive components generate compounds that inhibit growth and infection-related development of the rice blast fungus Magnaporthe grisea. It is therefore possible to "trigger" biological activity in a controlled, yet effective manner. Apart from circumventing many of the drawbacks of allicin, this binary system has additional important advantages, such as low toxicity of its individual components and selective activation. Importantly, alliinase is also able to use different substrates, therefore paving the way to a range of novel, binary antimicrobial systems with custom-made chemical and biochemical properties. 相似文献
43.
Induction of tumors in mice by genomic hypomethylation 总被引:1,自引:0,他引:1
Gaudet F Hodgson JG Eden A Jackson-Grusby L Dausman J Gray JW Leonhardt H Jaenisch R 《Science (New York, N.Y.)》2003,300(5618):489-492
Genome-wide DNA hypomethylation occurs in many human cancers, but whether this epigenetic change is a cause or consequence of tumorigenesis has been unclear. To explore this phenomenon, we generated mice carrying a hypomorphic DNA methyltransferase 1 (Dnmt1) allele, which reduces Dnmt1 expression to 10% of wild-type levels and results in substantial genome-wide hypomethylation in all tissues. The mutant mice were runted at birth, and at 4 to 8 months of age they developed aggressive T cell lymphomas that displayed a high frequency of chromosome 15 trisomy. These results indicate that DNA hypomethylation plays a causal role in tumor formation, possibly by promoting chromosomal instability. 相似文献
44.
Isolation of a cDNA from the virus responsible for enterically transmitted non-A, non-B hepatitis 总被引:59,自引:0,他引:59
G R Reyes M A Purdy J P Kim K C Luk L M Young K E Fry D W Bradley 《Science (New York, N.Y.)》1990,247(4948):1335-1339
Major epidemic outbreaks of viral hepatitis in underdeveloped countries result from a type of non-A, non-B hepatitis distinct from the parenterally transmitted form. The viral agent responsible for this form of epidemic, or enterically transmitted non-A, non-B hepatitis (ET-NANBH), has been serially transmitted in cynomolgus macaques (cynos) and has resulted in typical elevation in liver enzymes and the detection of characteristic virus-like particles (VLPs) in both feces and bile. Infectious bile was used for the construction of recombinant complementary DNA libraries. One clone, ET1.1, was exogenous to uninfected human and cyno genomic liver DNA, as well as to genomic DNA from infected cyno liver. ET1.1 did however, hybridize to an approximately 7.6-kilobase RNA species present only in infected cyno liver. The translated nucleic acid sequence of a portion of ET1.1 had a consensus amino acid motif consistent with an RNA-directed RNA polymerase; this enzyme is present in all positive strand RNA viruses. Furthermore, ET1.1 specifically identified similar sequences in complementary DNA prepared from infected human fecal samples collected from five geographically distinct ET-NANBH outbreaks. Therefore, ET1.1 represents a portion of the genome of the principal viral agent, to be named hepatitis E virus, which is responsible for epidemic outbreaks of ET-NANBH. 相似文献
45.
Thymic requirement for clonal deletion during T cell development 总被引:14,自引:0,他引:14
During T cell differentiation, self tolerance is established in part by the deletion of self-reactive T cells within the thymus (negative selection). The presence of T cell receptor (TCR)-alpha beta + T cells in older athymic (nu/nu) mice indicates that some T cells can also mature without thymic influence. Therefore, to determine whether the thymus is required for negative selection, TCR V beta expression was compared in athymic nu/nu mice and their congenic normal littermates. T cells expressing V beta 3 proteins are specific for minor lymphocyte stimulatory (Mlsc) determinants and are deleted intrathymically due to self tolerance in Mlsc+ mouse strains. Here it is shown that V beta 3+ T cells are deleted in Mlsc+ BALB/c nu/+ mice, but not in their BALB/c nu/nu littermates. Thus, the thymus is required for clonal deletion during T cell development. 相似文献
46.
47.
Three crystalline forms of calcium carbonate were identified in washings of the hamster vagina. Spherical concretions of vaterite and hexagonal concretions of calcite predominate on days 3 and 4 of the 4-day estrous cycle. Dumbbell-like concretions of aragonite predominate during pregnancy and pseudopregnancy. Each polymorph is associated with an acid-insoluble matrix. Concretions disappear after ovariectomy and reappear during daily injections of estrogen and progesterone. 相似文献
48.
Three phenotypically distinct processes-somatic hypermutation, gene conversion, and switch recombination-remodel the functionally rearranged immunoglobulin (Ig) loci in B cells. Somatic hypermutation and switch recombination have recently been shown to depend on the activation-induced deaminase (AID) gene product. Here, we show that the disruption of the AID gene in the chicken B cell line DT40 completely blocks Ig gene conversion and that this block can be complemented by reintroduction of the AID complementary DNA. This demonstrates that the AID master gene controls all B cell-specific modifications of vertebrate Ig genes. 相似文献
49.
Nutrition and learning 总被引:2,自引:0,他引:2
50.