全文获取类型
收费全文 | 11462篇 |
免费 | 881篇 |
国内免费 | 4篇 |
专业分类
12347篇 |
出版年
2023年 | 77篇 |
2022年 | 151篇 |
2021年 | 256篇 |
2020年 | 204篇 |
2019年 | 274篇 |
2018年 | 335篇 |
2017年 | 290篇 |
2016年 | 391篇 |
2015年 | 675篇 |
2014年 | 658篇 |
2013年 | 798篇 |
2012年 | 1016篇 |
2011年 | 910篇 |
2010年 | 612篇 |
2009年 | 538篇 |
2008年 | 651篇 |
2007年 | 608篇 |
2006年 | 590篇 |
2005年 | 546篇 |
2004年 | 514篇 |
2003年 | 415篇 |
2002年 | 407篇 |
2001年 | 108篇 |
2000年 | 78篇 |
1999年 | 94篇 |
1998年 | 126篇 |
1997年 | 98篇 |
1996年 | 72篇 |
1995年 | 66篇 |
1994年 | 68篇 |
1993年 | 75篇 |
1992年 | 48篇 |
1991年 | 48篇 |
1990年 | 49篇 |
1989年 | 50篇 |
1988年 | 37篇 |
1987年 | 30篇 |
1986年 | 26篇 |
1985年 | 35篇 |
1984年 | 43篇 |
1983年 | 26篇 |
1982年 | 32篇 |
1981年 | 27篇 |
1980年 | 18篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 15篇 |
1976年 | 13篇 |
1975年 | 21篇 |
1973年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
43.
We examine how the distribution of a leafgalling aphid (Pemphigus betae) affects other species associated with natural stands of hybrid cottonwoods (Populus angustifolia x P. fremontii). Aphid transfers on common-garden clones and RFLP analysis show that resistance to aphids in cottonwoods is affected by plant genotype. Because susceptible trees typically support thousands of galls, while adjacent resistant trees have few or none, plant resistance traits that affect the distribution of this abundant herbivore may directly and/or indirectly affect other species. We found that the arthropod community of aphid-susceptible trees had 31% greater species richness and 26% greater relative abundance than aphid-resistant trees. To examine direct and indirect effects of plant resistance traits on other organisms, we experimentally excluded aphids and found that abundances and/or foraging behavior of arthropods, fungi, and birds were altered. First, exclusion of gall aphids on susceptible trees resulted in a 24% decrease in species richness and a 28% decrease in relative abundance of the arthropod community. Second, exclusion of aphids also caused a 2- to 3-fold decrease in foraging and/or presence of three taxa of aphid enemies: birds, fungi, and insects. Lastly, aphidexclussion resulted in a 2-fold increase in inquilines (animals who live in abodes properly belonging to another). We also found that fungi and birds responded to variation in gall density at the branch level. We conclude plant resistance traits affect diverse species from three trophic levels supporting a bottom-up influence of plants on community structure. 相似文献
44.
Settler agriculture and the dynamics of resource allocation in frontier environments 总被引:1,自引:0,他引:1
Francisco J. Pichón 《Human ecology: an interdisciplinary journal》1996,24(3):341-371
This article attempts to conceptualize the dynamics of resource allocation by colonist farmers under the unique conditions of land abundance and labor scarcity which characterize frontier environments, such as the smallholder agricultural settlement areas in the Amazon basin. In contrast, most previous theoretical literature on household agricultural decision making and land-use change in rural areas considers conditions of high population density and land scarcity, and is not, therefore, adequate for understanding critical land-use changes which may be occurring in frontier regions. This article first discusses the appropriateness and inadequacies of the analytical frameworks commonly used to explain the expansion of settler agriculture into remote forest regions and the unsustainable land-use practices observed in these areas. This review serves as the basis for characterizing resource allocation under the particular conditions of frontier environments. A conceptual advance in the analysis is its consideration of the way institutional/policy factors and farm-level characteristics can interact to produce land-use outcomes. This knowledge is essential to understand not only the social and economic factors affecting present land use and choice of technology, but also those factors influencing farmers' demand for more optimal systems of land use which are consistent with varying agro-ecological potentials, demographic situations, and their own management capacity. 相似文献
45.
Graciela C. Theiler Yanina C. Marcos Edgardo Kolkowski Nancy Lindel Mónica Capucchio Paula Barrionuevo Francisco R. Carnese M. Leonardo Satz 《Immunogenetics》1996,43(6):398-399
The nucleotide sequence data reported in this paper have been submitted to the GenBank nucleotide sequence database and have been assigned the accession number U17107. The nameB*3509 was officially assigned by the WHO Nomenclature Committee in December 1994 相似文献
46.
Francisco M. Pinto Yves Chupeau Vicente M. Cabrera 《In vitro cellular & developmental biology. Plant》1995,31(2):96-100
An efficient and easy method for genetic characterization of plant somatic hybrids is proposed. In a first qualitative approach,
four somatic hybrids and their parental species (Nicotiana tabacum andN. plumbaginifolia) were characterized by DNA fingerprinting and Random Amplification of Polymorphic DNA (RAPD). After this, a quantitative
estimation of the degree of parental contribution to the hybrids was carried out by means of a slot-blot analysis. Both qualitative
methods, showed one hybrid identical toN. tabacum, two almost identical toN. plumbaginifolia, and a fourth similar to this parental species, but with someN. tabacum admixture. The quantitative method, for the same hybrids, gave 83%, 7%, 7%, and 37%N. tabacum DNA contribution, respectively. 相似文献
47.
Type 1 protein phosphatase acts in opposition to IpL1 protein kinase in regulating yeast chromosome segregation. 总被引:19,自引:6,他引:13 下载免费PDF全文
The IPL1 gene is required for high-fidelity chromosome segregation in the budding yeast Saccharomyces cerevisiae. Conditional ipl1ts mutants missegregate chromosomes severely at 37 degrees C. Here, we report that IPL1 encodes an essential putative protein kinase whose function is required during the later part of each cell cycle. At 26 degrees C, the permissive growth temperature, ipl1 mutant cells are defective in the recovery from a transient G2/M-phase arrest caused by the antimicrotubule drug nocodazole. In an effort to identify additional gene products that participate with the Ipl1 protein kinase in regulating chromosome segregation in yeast, a truncated version of the previously identified DIS2S1/GLC7 gene was isolated as a dosage-dependent suppressor of ipl1ts mutations. DIS2S1/GLC7 is predicted to encode a catalytic subunit (PP1C) of type 1 protein phosphatase. Overexpression of the full-length DIS2S1/GLC7 gene results in chromosome missegregation in wild-type cells and exacerbates the mutant phenotype in ipl1 cells. In addition, the glc7-1 mutation can partially suppress the ipl1-1 mutation. These results suggest that type 1 protein phosphatase acts in opposition to the Ipl1 protein kinase in vivo to ensure the high fidelity of chromosome segregation. 相似文献
48.
Jan Kwiatowski Douglas Skarecky Kevin Bailey Francisco J. Ayala 《Journal of molecular evolution》1994,38(5):443-454
The phylogeny and taxonomy of the drosophilids have been the subject of extensive investigations. Recently, Grimaldi (1990)
has challenged some common conceptions, and several sets of molecular data have provided information not always compatible
with other taxonomic knowledge or consistent with each other. We present the coding nucleotide sequence of the Cu,Zn superoxide
dismutase gene (Sod) for 15 species, which include the medfly Ceratitis capitata (family Tephritidae), the genera Chymomyza and Zaprionus, and representatives of the subgenera Dorsilopha, Drosophila, Hirtodrosophila, Scaptodrosophila, and Sophophora. Phylogenetic analysis of the Sod sequences indicates that Scaptodrosophila and Chymomyza branched off the main lineage before the major Drosophila radiations. The presence of a second intron in Chymomyza and Scaptodrosophila (as well as in the medfly) confirms the early divergence of these two taxa. This second intron became deleted from the main
lineage before the major Drosophila radiations. According to the Sod sequences, Sophophora (including the melanogaster, obscura, saltans, and willistoni species groups) is older than the subgenus Drosophila; a deep branch splits the willistoni and saltans groups from the melanogaster and obscura groups. The genus Zaprionus and the subgenera Dorsilopha and Hirtodrosophila appear as branches of a prolific “bush” that also embraces the numerous species of the subgenus Drosophila. The Sod results corroborate in many, but not all, respects Throckmorton's (King, R.C. (ed) Handbook of Genetics. Plenum Press, New York, pp. 421–469, 1975) phylogeny; are inconsistent in some important ways with Grimaldi's (Bull. Am. Museum Nat. Hist.
197:1–139, 1990) cladistic analysis; and also are inconsistent with some inferences based on mitochondrial DNA data. The Sod results manifest how, in addition to the information derived from nucleotide sequences, structural features (i.e., the deletion
of an intron) can help resolve phylogenetic issues.
Correspondence requests to: F. J. Ayala 相似文献
49.
Gabriela Sánchez-Mejorada Horacio Merchant-Larios Francisco Alonso-deFlorida Luis B. Morales 《Bulletin of mathematical biology》1994,56(1):147-160
Are there enough mast cells in denervated skeletal muscle to account for autopharmacological mediation of the antigen potentials (APs) elicited by microtaps? Through rough qualitative estimations, some authors have suggested a positive answer to this question. However, in view of measurements performed in this investigation of both the density of mast cells and the diffusion coefficient of antigens, the probability of such mediated effects was found to be relatively low:P=0.016 for egg albumin andP=0.004 for ferritin. Therefore, most APs induced by microtaps should be attributed to the direct effect of antigen over the sensitized muscle fibers. Yet, both the density of mast cells found in this work and the known amount of histamine they are capable of releasing when challenged with antigen, support the hypothesis regarding the involvement of these cells when antigen is massively superfused so as to induce Schultz-Dale reactions in muscle strips. Under this circumstance, the direct and mediated mechanisms may coexist. 相似文献
50.
Guerrero Francisco; Blanco Jose M; Rodriguez Valeriano 《Journal of plankton research》1994,16(1):95-103
A comparative analysis was carried out on the several equationsmost commonly used to describe the dependence of the developmentof organisms on temperature. Goodness of fit, number of parameters,ease of fitting data and biological significance were compared. 相似文献