全文获取类型
收费全文 | 1576744篇 |
免费 | 151950篇 |
国内免费 | 1524篇 |
出版年
2021年 | 16321篇 |
2019年 | 16302篇 |
2018年 | 18973篇 |
2017年 | 17658篇 |
2016年 | 29010篇 |
2015年 | 43209篇 |
2014年 | 51292篇 |
2013年 | 77748篇 |
2012年 | 43672篇 |
2011年 | 34005篇 |
2010年 | 46333篇 |
2009年 | 47020篇 |
2008年 | 31418篇 |
2007年 | 30167篇 |
2006年 | 33992篇 |
2005年 | 34987篇 |
2004年 | 33965篇 |
2003年 | 31532篇 |
2002年 | 29561篇 |
2001年 | 46492篇 |
2000年 | 44235篇 |
1999年 | 40331篇 |
1998年 | 25797篇 |
1997年 | 25801篇 |
1996年 | 24973篇 |
1995年 | 24511篇 |
1994年 | 24229篇 |
1993年 | 23576篇 |
1992年 | 35847篇 |
1991年 | 34526篇 |
1990年 | 33041篇 |
1989年 | 33580篇 |
1988年 | 31057篇 |
1987年 | 29483篇 |
1986年 | 27917篇 |
1985年 | 29878篇 |
1984年 | 28024篇 |
1983年 | 24684篇 |
1982年 | 23521篇 |
1981年 | 22196篇 |
1980年 | 20818篇 |
1979年 | 24461篇 |
1978年 | 21709篇 |
1977年 | 20681篇 |
1976年 | 19776篇 |
1975年 | 19748篇 |
1974年 | 20701篇 |
1973年 | 20876篇 |
1972年 | 18137篇 |
1971年 | 16632篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
The Consequences of Precipitation Seasonality for Mediterranean-Ecosystem Vegetation of South Africa
Globally, mediterranean-climate ecosystem vegetation has converged on an evergreen, sclerophyllous and shrubby growth form. The particular aspects of mediterranean-climate regions that contribute to this convergence include summer droughts and relatively nutrient-poor soils. We hypothesised that winter-precipitation implies stressful summer droughts and leaches soils due to greater water availability (i.e. balance between precipitation and potential evapotranspiration; P–PET) during cold periods. We conducted a comparative analysis of normalised difference vegetation indices (NDVI) and edaphic and climate properties across the biomes of South Africa. NDVI was strongly correlated with both precipitation and P–PET (r2 = 0.8). There was no evidence, however, that winter-precipitation reduces NDVI in comparison to similar amounts of summer-precipitation. Base saturation (BS), a measure of soil leaching was, however, negatively related to P–PET (r2 = 0.64). This led to an interaction between P–PET and BS in determining NDVI, indicating the existence of a trade-off between water availability and soil nutrients that enables NDVI to increase with precipitation, despite negative consequences for soil nutrient availability. The mechanism of this trade-off is suggested to be that water increases nutrient accessibility. This implies that along with nutrient-depauperate geologies and long periods of time since glaciation, the winter-precipitation may have contributed to the highly leached status of the soils. Since many of the ecophysiological characteristics of mediterranean-ecosystem flora are associated with low nutrient availabilities (e.g. evergreen foliage, sclerophylly, cluster roots), we conclude that mediterranean-climates promote convergence of growth-forms in these regions through high leaching capacity. 相似文献
82.
Vanesa L. De Pietri Gerald Mayr 《Journal of Zoological Systematics and Evolutionary Research》2014,52(4):312-322
In Europe, Miocene rails (Aves, Rallidae) are quite abundant, but their phylogenetic placement in the context of recent forms has remained elusive. Rails from the early Miocene of the Saint‐Gérand‐le‐Puy area in central France were first described in the 19th century, and currently, only two species are recognized, namely Palaeoaramides christyi and Paraortygometra porzanoides. Our examination of the material however suggests the presence of four, likely coeval, species of rail from these deposits. Palaeoaramides eximius, previously synonymized with Palaeoaramides christyi, is here shown to probably be a distinct species, and a previously unrecognized rail, Baselrallus intermedius gen. et sp. nov., is described. To find out how these fossil rails are related to modern Rallidae, we compared them with an extensive sample of extant rails and identified plesiomorphic and derived features for crown group Rallidae. Our assessment does not support a particularly close relationship of either Palaeoaramides to Aramides or Paraortygometra to Crex (Ortygometra), and overall, these fossil rails are more primitive than previously assumed. Based on our observations of the morphology of the previously undescribed humerus of Palaeoaramides, we show this taxon to be outside crown group Rallidae, and perhaps closely related to the early Oligocene taxon Belgirallus. On the other hand, Paraortygometra porzanoides bears a resemblance to recent flufftails (Sarothrura spp.) in some elements, but whether it can be included in a clade together with flufftails is uncertain. 相似文献
83.
In this article, we discuss molecular mechanisms involved in the evolution of amygdala kindling and the episodic loss of response
to pharmacological treatments during tolerance development. These phenomena allow us to consider how similar principles (in
different neurochemical systems) could account for illness progression, cyclicity, and drug tolerance in affective disorders.
We describe the phenomenon of amygdala-kindled seizures episodically breaking through effective daily pharmacotherapy with
carbamazepine and valproate, suggesting that these observations could reflect the balance of pathological vs compensatory
illness-induced changes in gene expression. Under certain circumstances, amygdala-kindled animals that were initially drug
responsive can develop highly individualized patterns of seizure breakthroughs progressing toward a complete loss of drug
efficacy. This initial drug efficacy may reflect the combination of drug-related exogenous neurochemical mechanisms and illness-induced
endogenous compensatory mechanisms. However, we postulate that when seizures are inhibited, the endogenous illness-induced
adaptations dissipate (the “time-off seizure” effect), leading to the re-emergence of seizures, a re-induction of a new, but
diminished, set of endogenous compensatory mechanisms, and a temporary period of renewed drug efficacy. As this pattern repeats,
an intermittent or cyclic response to the anticonvulsant treatment emerges, leading toward complete drug tolerance.
We also postulate that the cyclic pattern accelerates over time because of both the failure of robust illness-induced endogenous
adaptations to emerge and the progression in pathophysiological mechanisms (mediated by long-lasting changes in gene expression
and their downstream consequences) as a result of repeated occurrences of seizures. In this seizure model, this pattern can
be inhibited and drug responsivity can be temporarily reinstated by several manipulations, including lowering illness drive
(decreasing the stimulation current.), increasing drug dosage, switching to a new drug that does not show crosstolerance to
the original medication, or temporarily discontinuing treatment, allowing the illness to re-emerge in an unmedicated animal.
Each of these variables is discussed in relation to the potential relevance to the emergence, progression, and suppression
of individual patterns of episodic cyclicity in the recurrent affective disorders. A variety of clinical studies are outlined
that specifically test the hypotheses derived from this formulation. Data from animal studies suggest that illness cyclicity
can develop from the relative ratio between primary pathological processes and secondary endogenous adaptations (assisted
by exogenous medications). If this proposition is verified, it further suggests that illness cyclicity is inherent to the
neurobiological processes of episode emergence and amelioration, and one does not need to postulate a separate defect in the
biological clock. The formulation predicts that early and aggressive long-term interventions may be optimal in order to prevent
illness emergence and progression and its associated accumulating neurobiological, vulnerability factors. 相似文献
84.
A highly abundant satellite DNA comprising 17% of the Tribolium castaneum
(Insecta, Coleoptera) genome was cloned and sequenced. The satellite
monomer is 360 bp long, has a high A+T content of 73%, and lacks
significant internal substructures. The sequence variability is 3.6%,
essentially due to random distribution of single-point mutations. The
satellite is evenly distributed in the regions of centromeric
heterochromatin of all 20 chromosomes, as shown by fluorescent in situ
hybridization. Comparison of T. castaneum satellite with those from three
different but congeneric species reveals the highest sequence similarity of
47.1% with the satellite from the sibling species Tribolium freemani. The
phylogenetic relationships among Tribolium species deduced from satellite
sequence agree with those based on karyological, chemotaxonomic, and
hybridization data. This indicates a parallel in the divergence of
satellites and some genetic and cytogenetic characters. Despite low mutual
sequence similarity, which makes them species-specific, Tribolium
satellites have a common structural characteristic: a block of about 95%
A+T content, 20 to 42 bp long, flanked at one side by an inverted repeat
which can potentially form a thermodynamically stable dyad structure. Since
similar structural features are found in centromeric DNA of Saccharomyces
cerevisiae and Chironomus pallidivittatus, their possible importance in
centromere function may be inferred.
相似文献
85.
86.
87.
88.
89.
H. James Spooner 《CMAJ》1996,154(12):1875-1876
90.