全文获取类型
收费全文 | 40240篇 |
免费 | 538篇 |
国内免费 | 236篇 |
专业分类
林业 | 1486篇 |
农学 | 373篇 |
基础科学 | 96篇 |
5867篇 | |
综合类 | 9425篇 |
农作物 | 1233篇 |
水产渔业 | 400篇 |
畜牧兽医 | 20640篇 |
园艺 | 234篇 |
植物保护 | 1260篇 |
出版年
2015年 | 68篇 |
2014年 | 136篇 |
2013年 | 105篇 |
2012年 | 3412篇 |
2011年 | 3970篇 |
2010年 | 812篇 |
2009年 | 535篇 |
2008年 | 3131篇 |
2007年 | 3306篇 |
2006年 | 2964篇 |
2005年 | 2932篇 |
2004年 | 2729篇 |
2003年 | 2624篇 |
2002年 | 2301篇 |
2001年 | 1565篇 |
2000年 | 2026篇 |
1999年 | 875篇 |
1998年 | 153篇 |
1997年 | 130篇 |
1996年 | 114篇 |
1995年 | 166篇 |
1994年 | 234篇 |
1993年 | 311篇 |
1992年 | 258篇 |
1991年 | 328篇 |
1990年 | 340篇 |
1989年 | 315篇 |
1988年 | 420篇 |
1987年 | 366篇 |
1986年 | 347篇 |
1985年 | 328篇 |
1984年 | 316篇 |
1983年 | 248篇 |
1982年 | 234篇 |
1981年 | 169篇 |
1980年 | 128篇 |
1979年 | 87篇 |
1978年 | 59篇 |
1976年 | 64篇 |
1975年 | 70篇 |
1974年 | 74篇 |
1969年 | 68篇 |
1967年 | 64篇 |
1966年 | 73篇 |
1963年 | 114篇 |
1960年 | 64篇 |
1959年 | 61篇 |
1958年 | 60篇 |
1957年 | 66篇 |
1956年 | 74篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
The entire dog population of Hanoveranian Scenthounds registered since the foundation of the kennel club Hirschmann in the year 1894 was analysed for the development of the inbreeding and relationship coefficient and the contribution of Hanoveranian Scenthound population of foreign countries. The analyses were performed using all available pedigree information. In the history, the breeding of the Hanoveranian Scenthound was difficult because of the low number of breeding dogs which was additionally reduced during World War I and II. The mean coefficient of inbreeding of the entire German population was 6.35%. From 1950 to 1990, the mean coefficient of inbreeding increased from 8.3% to 9.7%. In the birth years from 1990 to 2002, the mean coefficient of inbreeding decreased slightly to 9.2%. The rates of inbreeding decreased since 1950. For the last 8 generations the increase of the coefficients of inbreeding per generation was 0.71%. The mean degree of relationship of the entire population was 7.17%, whereby values higher than 75% were found. Gene contributions of 8 other European stud books for the Hanoveranian Scenthound could be found in the German population. Hereby, the Austrian Hanoveranian Scenthounds had the largest impact on the German population with 24.4%. The proportion of genes from the German stud book amounted to 67%. 相似文献
52.
53.
在光合作用的有机物被光线照明时,少部分所吸收的光是以叶绿素荧光的形式反复放射的.叶绿素荧光强度与光合电子传递活性有关,因而通常用叶绿素荧光作为一种光合作用灵敏度的探测方法(Papageorgiou,1975;Lavorel和Etienne,1977).当细胞处于胁迫时,活体叶绿素荧光可以提供监测叶绿体氧化还原膜系统活性的一种手段.目前,越来越多地应用活体叶绿素荧光来研究胁迫对于危害 相似文献
54.
Expression of PKC alpha, beta I, beta II, epsilon and micro has been demonstrated in the whole bovine CL with PKC epsilon being differentially expressed as a function of development. In experiment 1 we have investigated the amount of mRNA encoding PKC epsilon at different stages of luteal development (days 1, 4, 10 and 17). In experiment 2, the cellular source of luteal PKC isozymes was determined. Enriched steroidogenic (SC) and endothelial (EC) cells from day-10 CL were used to examine this question by Western blot analysis and immuno-histochemistry. In experiment 3, Western blot analysis was used to examine the ability of ET-1 to activate luteal PKC isozymes in day-10 CL. In experiment 4, the role of luteal PKC isozymes in the ET-1 mediated inhibition of P(4) accumulation in steroidogenic cell cultures from day-4 and day-10 CL was examined. Abundance of PKC epsilon mRNA gradually increased from day-1 to -10 with no further increase on day-17. In experiment 2, PKC epsilon was exclusively detected in SC (LLC and SLC). In contrast, PKC alpha, beta I and beta II were detected in both SC and EC, with EC expressing higher amounts of PKC isozymes. In day-10 CL, ET-1 induced cellular redistribution of PKC alpha, beta I, epsilon but not beta II. Inhibitors specific for conventional PKC isozymes as well as PKC epsilon were able to negate the inhibitory effects of ET-1 on P4 accumulation in the day 10 CL. In the day-4 CL, the inhibitory effect of ET-1 might be mediated via conventional PKC. Thus, an exclusive presence of PKC epsilon in luteal steroidogenic cells, its higher expression along with the ability of ET-1 to stimulate its activation in day-10 CL strongly suggests that this PKC isoform may play an important regulatory role in decreasing P(4) during luteal regression. Inhibition of P(4) by ET-1 in the early CL may be mediated via conventional PKC isozymes. 相似文献
55.
Up to 2006, there have been 82 cases of BSE in cattle born in the Netherlands. This article reviews the current situation regarding BSE in the Netherlands and summarizes the clinical symptoms of the disease. Data from the Netherlands show that a passive surveillance system, by which farmers and veterinarians have to report suspect clinical cases, has a low sensitivity. The epidemiology of, and risk factors for, BSE are discussed. All the Dutch cases of BSE can be attributed to cross-contamination of feed with meat-and-bone meal. On the basis of information about the epidemic and the cases reported to date, it is anticipated that the number of cases of BSE will continue to decline in the Netherlands and Europe. The European Commission has presented a road map that describes how the European BSE policy can be changed in the short and long term if the current favourable trend in BSE cases continues. It is time for a new phase in the management of BSE but with continued protection of the public's health and eradication of BSE. 相似文献
56.
Buchanan MF Carter WC Cowgill LM Hurley DJ Lewis SJ MacLeod JN Melton TR Moore JN Pessah I Roberson M Robertson TP Smith ML Vandenplas ML 《Journal of veterinary medical education》2005,32(1):72-78
Traditional methods of teaching intracellular biological processes and pathways use figures or flowcharts with the names of molecules linked with arrows. Many veterinary students, presented with such material, simply memorize the names or chemical structures of the molecules and are then likely to forget the material once the examination is completed. To address this problem, the authors designed, created, and field-tested new teaching media that incorporate realistic three-dimensional (3D) animations depicting the dynamic changes that occur in intracellular molecules during cellular activation. Testing found that veterinary students taught using traditional teaching media (e.g., lectures, handouts, textbooks) are proficient in memorizing the names and order of intracellular molecules but unable to appreciate the interactions between these elements or their spatial relationships within cells. In contrast, more than 90% of veterinary students taught using 3D animations not only recall the facts about the intracellular elements but also develop accurate mental images of the interactions among these molecules and their spatial relationships. These findings strongly suggest that the comprehension of complex biological processes by veterinary students can be enhanced by the use of dynamic 3D depictions of these processes in the classroom. 相似文献
57.
<正> 许多研究工作者,已观察到鱼接触各类应激因子(包括驯养,物理干扰、饥饿和缺氧)时对碳水化合物体内代谢的改变。因应激反应使血萘酚胺和皮质激素类分泌增加,引起鱼体能量的储备产生明显改变。用黄鳝作为实验动物,其平均体重60克。试验开始前鱼在适宜的条件下,置于水簇箱一周。 相似文献
58.
59.
ABSTRACT Thirteen aphid species were tested for their ability to transmit Pennsylvania isolates of Plum pox virus (PPV) collected in Columbia (PENN-3), Franklin (PENN-4), and York (PENN-7) Counties, PA. Four species, Aphis fabae, A. spiraecola, Brachycaudus persicae, and Myzus persicae, consistently transmitted PPV in preliminary transmission tests. Two species, Metopolophium dirhodum and Rhopalosiphum padi, were occasional inefficient vectors. Toxoptera citricida, from Florida, also was an effective vector but it does not occur in major stone-fruit-growing states. Species not transmitting PPV in parallel tests included Acyrthosiphon pisum, Aphis glycines, Aulacorthum solani, Macrosiphum euphorbiae, Rhopalosiphum maidis, and Sitobion avenae. When given a 3-day probing access period simultaneously on PPV-infected peach seedlings and healthy peach seedlings, Myzus persicae, Aphis spiraecola, A. fabae, and B. persicae transmitted PPV to 63, 31, 38, and 32% of the healthy peach seedlings, respectively. When given a similar probing period on PPV-infected peach fruit and healthy peach seedlings, the same aphid species transmitted PPV to 50, 35, 0, and 0% of seedlings, respectively. Results support the hypothesis of secondary PPV spread by indigenous aphids in Pennsylvania, and suggest that PPV-infected fruit has the potential to function as a virus source for long-distance dispersal. 相似文献
60.
The IgG index measures the intrathecal immunoglobulin production and it is a useful tool for diagnosis of inflammatory diseases involving the central nervous system. This index is based on the precise quantification of albumin and IgG in canine cerebrospinal fluid and serum. Here, we report the development of an indirect competitive ELISAs for the detection of both antigens. Thirty-two dogs were included in this study, divided into three experimental groups. Group A was composed of 22 healthy animals, as determined by standard clinical examination. In group B, six animals, presented neurological pathologies associated with endogenous IgG production and, in group C four animals presented neurological diseases or symptoms not associated with intrathecal IgG production. Cerebrospinal fluid and serum samples were obtained from these animals. As expected, by using the indirect ELISAs proposed here, the IgG indexes obtained in healthy animals (A) were 0.371+/-0.252 (SD). In B and C, the values (3.002+/-1.897; 0.36+/-0.306, respectively), were in agreement with the pathologic conditions of the individuals in each group. Thus, the immunometric competition ELISA methods proposed here allow the discrimination of abnormal intrathecal IgG production, in a variety of inflammatory pathologic conditions of the central nervous system. 相似文献