全文获取类型
收费全文 | 754篇 |
免费 | 77篇 |
专业分类
831篇 |
出版年
2023年 | 2篇 |
2022年 | 6篇 |
2021年 | 13篇 |
2020年 | 3篇 |
2019年 | 8篇 |
2018年 | 13篇 |
2017年 | 13篇 |
2016年 | 21篇 |
2015年 | 32篇 |
2014年 | 38篇 |
2013年 | 41篇 |
2012年 | 55篇 |
2011年 | 53篇 |
2010年 | 30篇 |
2009年 | 38篇 |
2008年 | 42篇 |
2007年 | 31篇 |
2006年 | 34篇 |
2005年 | 30篇 |
2004年 | 29篇 |
2003年 | 23篇 |
2002年 | 42篇 |
2001年 | 23篇 |
2000年 | 18篇 |
1999年 | 13篇 |
1998年 | 10篇 |
1997年 | 12篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 3篇 |
1993年 | 7篇 |
1992年 | 12篇 |
1991年 | 13篇 |
1990年 | 8篇 |
1989年 | 14篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 4篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1967年 | 2篇 |
1958年 | 1篇 |
排序方式: 共有831条查询结果,搜索用时 15 毫秒
71.
Goidts V Armengol L Schempp W Conroy J Nowak N Müller S Cooper DN Estivill X Enard W Szamalek JM Hameister H Kehrer-Sawatzki H 《Human genetics》2006,119(1-2):185-198
Copy number differences (CNDs), and the concomitant differences in gene number, have contributed significantly to the genomic
divergence between humans and other primates. To assess its relative importance, the genomes of human, common chimpanzee,
bonobo, gorilla, orangutan and macaque were compared by comparative genomic hybridization using a high-resolution human BAC
array (aCGH). In an attempt to avoid potential interference from frequent intra-species polymorphism, pooled DNA samples were
used from each species. A total of 322 sites of large-scale inter-species CND were identified. Most CNDs were lineage-specific
but frequencies differed considerably between the lineages; the highest CND frequency among hominoids was observed in gorilla.
The conserved nature of the orangutan genome has already been noted by karyotypic studies and our findings suggest that this
degree of conservation may extend to the sub-microscopic level. Of the 322 CND sites identified, 14 human lineage-specific
gains were observed. Most of these human-specific copy number gains span regions previously identified as segmental duplications
(SDs) and our study demonstrates that SDs are major sites of CND between the genomes of humans and other primates. Four of
the human-specific CNDs detected by aCGH map close to the breakpoints of human-specific karyotypic changes [e.g., the human-specific
inversion of chromosome 1 and the polymorphic inversion inv(2)(p11.2q13)], suggesting that human-specific duplications may
have predisposed to chromosomal rearrangement. The association of human-specific copy number gains with chromosomal breakpoints
emphasizes their potential importance in mediating karyotypic evolution as well as in promoting human genomic diversity.
Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
72.
In abandoned or extensively managed grasslands, the mechanisms involved in pioneer tree species success are not fully explained. Resource competition among plants and microclimate modifications have been emphasised as possible mechanisms to explain variation of survivorship and growth. In this study, we evaluated a number of mechanisms that may lead to successful survival and growth of seedlings of a pioneer tree species (Pinus sylvestris) in a grass-dominated grassland. Three-year-old Scots pines were planted in an extensively managed grassland of the French Massif Central and for 2 years were either maintained in bare soil or subjected to aerial and below-ground interactions induced by grass vegetation. Soil temperatures were slightly higher in bare soil than under the grass vegetation, but not to an extent explaining pine growth differences. The tall grass canopy reduced light transmission by 77% at ground level and by 20% in the upper part of Scots pine seedlings. Grass vegetation presence also significantly decreased soil volumetric water content (Hv) and soil nitrate in spring and in summer. In these conditions, the average tree height was reduced by 5% compared to trees grown in bare soil, and plant biomass was reduced by 85%. Scots pine intrinsic water-use efficiency (A/g), measured by leaf gas-exchange, increased when Hv decreased owing to a rapid decline of stomatal conductance (g). This result was also confirmed by δ
13C analyses of needles. A summer 15N labelling of seedlings and grass vegetation confirmed the higher NO3 capture capacity of grass vegetation in comparison with Scots pine seedlings. Our results provide evidence that the seedlings' success was linked to tolerance of below-ground resource depletion (particularly water) induced by grass vegetation based on morphological and physiological plasticity as well as to resource conservation. 相似文献
73.
Various physical and chemical properties of host plants influence insect larval performance and subsequent adult fitness. Tomato plants are relatively new hosts to the potato tuber moth, Phthorimaea operculella (Zeller), with the fruit being its preferred feeding site. However, it is unclear how the biochemical and physical properties of tomato fruits relate to potato tuber moth performance. Significant amounts of alpha-tomatine were detected in maturing green and ripening fruits of cherry (cv. Ceres) and processing (cv. Serio) types of tomatoes whereas none was detected in a fresh market variety (cv. Marglobe), at comparable stages. alpha-Tomatine is negatively and significantly correlated with development rate (head capsule size) of larvae reared in the fruits of the cherry and processing type tomatoes. Generally, survival, growth and development were significantly superior for larvae reared in the ripening fruits of the fresh market cultivar. At this stage, the fruits of this cultivar are also the largest. Based on these results it is concluded that fruit alpha-tomatine content, as well as fruit size and maturity, all affect performance of P. operculella larvae in the fruits of cultivated tomatoes. 相似文献
74.
The Israeli population of the cotton bollworm, Helicoverpa armigera (Hübner), undergoes a short-day, low-temperature pupal diapause and is also suspected of being a seasonal migrant in the eastern Mediterranean region. H. armigera were reared in the laboratory under several constant temperature and photoperiodic combinations which simulate average conditions encountered in the spring, summer, early-autumn and late-autumn in Israel. Juvenile hormone (JH) biosynthesis, the onset of calling behavior, sex pheromone production and ovarian development were examined in virgin female moths subsequent to eclosion. Allatal maturation, defined as acquisition of competence to synthesize JH, was significantly delayed in moths reared under simulated spring conditions. This was probably the cause for the observed delay in ovarian development and the onset of calling behavior, and to the reduction in sex pheromone biosynthesis. The delay in female sexual maturation, commonly associated with migratory flight, is consistent with presumptive pre-reproductive migration in H. armigera. 相似文献
75.
Martinez-Navio JM Climent N Gallart T Lluis C Franco R 《Immunology and cell biology》2012,90(6):594-600
After nearly three decades of searching for a vaccine against HIV, a cure for this pandemic disease still remains elusive. The low immunogenicity of the surface proteins and the huge variability of the virus, together with the immunocompromised status of the host, have made developing an HIV vaccine an uphill battle. Over the past few years, both immunogen design and immunization strategies have improved, providing hope for future, although the anti-HIV responses achieved still remain modest. As developing a prophylactic vaccine seems unlikely nowadays, efforts have focused on alternative therapeutic immunization approaches, although these still need to be further optimized. Using an immunomodulator capable of restoring immune function in the context of infection, thereby boosting cell-mediated and humoral responses, could be critical in effectively improving current therapeutic approaches. Adenosine deaminase, a protein with a pivotal role in T-cell co-stimulation, has been shown to robustly enhance specific T-cell responses against HIV in vitro. Although its role in humoral responses has not yet been assessed, genetic defects in this enzyme are associated with impaired cellular and humoral responses. Importantly, this molecule is already commercially available pharmaceutically and, therefore, it fulfils all the requirements to be assayed as an anti-HIV vaccine adjuvant. 相似文献
76.
77.
78.
Marta Coll Chiara Piroddi Camille Albouy Frida Ben Rais Lasram William W. L. Cheung Villy Christensen Vasiliki S. Karpouzi François Guilhaumon David Mouillot Michelle Paleczny Maria Lourdes Palomares Jeroen Steenbeek Pablo Trujillo Reg Watson Daniel Pauly 《Global Ecology and Biogeography》2012,21(4):465-480
Aim A large body of knowledge exists on individual anthropogenic threats that have an impact on marine biodiversity in the Mediterranean Sea, although we know little about how these threats accumulate and interact to affect marine species and ecosystems. In this context, we aimed to identify the main areas where the interaction between marine biodiversity and threats is more pronounced and to assess their spatial overlap with current marine protected areas in the Mediterranean. Location Mediterranean Sea. Methods We first identified areas of high biodiversity of marine mammals, marine turtles, seabirds, fishes and commercial or well‐documented invertebrates. We mapped potential areas of high threat where multiple threats are occurring simultaneously. Finally we quantified the areas of conservation concern for biodiversity by looking at the spatial overlap between high biodiversity and high cumulative threats, and we assessed the overlap with protected areas. Results Our results show that areas with high marine biodiversity in the Mediterranean Sea are mainly located along the central and north shores, with lower values in the south‐eastern regions. Areas of potential high cumulative threats are widespread in both the western and eastern basins, with fewer areas located in the south‐eastern region. The interaction between areas of high biodiversity and threats for invertebrates, fishes and large animals in general (including large fishes, marine mammals, marine turtles and seabirds) is concentrated in the coastal areas of Spain, Gulf of Lions, north‐eastern Ligurian Sea, Adriatic Sea, Aegean Sea, south‐eastern Turkey and regions surrounding the Nile Delta and north‐west African coasts. Areas of concern are larger for marine mammal and seabird species. Main conclusions These areas may represent good candidates for further research, management and protection activities, since there is only a maximum 2% overlap between existing marine protected areas (which cover 5% of the Mediterranean Sea) and our predicted areas of conservation concern for biodiversity. 相似文献
79.
I Le Viol F Jiguet L Brotons S Herrando A Lindström JW Pearce-Higgins J Reif C Van Turnhout V Devictor 《Biology letters》2012,8(5):780-782
Biotic homogenization (BH) is a process whereby some species (losers) are systematically replaced by others (winners). While this process has been related to the effects of anthropogenic activities, whether and how BH is occurring across regions and the role of native species as a driver of BH has hardly been investigated. Here, we examine the trend in the community specialization index (CSI) for 234 native species of breeding birds at 10 111 sites in six European countries from 1990 to 2008. Unlike many BH studies, CSI uses abundance information to estimate the balance between generalist and specialist species in local assemblages. We show that bird communities are more and more composed of native generalist species across regions, revealing a strong, ongoing BH process. Our result suggests a rapid and non-random change in community composition at a continental scale is occurring, most likely driven by anthropogenic activities. 相似文献
80.