全文获取类型
收费全文 | 14687篇 |
免费 | 1734篇 |
国内免费 | 2949篇 |
出版年
2023年 | 340篇 |
2022年 | 390篇 |
2021年 | 532篇 |
2020年 | 690篇 |
2019年 | 825篇 |
2018年 | 655篇 |
2017年 | 720篇 |
2016年 | 730篇 |
2015年 | 645篇 |
2014年 | 759篇 |
2013年 | 988篇 |
2012年 | 719篇 |
2011年 | 702篇 |
2010年 | 570篇 |
2009年 | 748篇 |
2008年 | 770篇 |
2007年 | 785篇 |
2006年 | 723篇 |
2005年 | 657篇 |
2004年 | 568篇 |
2003年 | 569篇 |
2002年 | 512篇 |
2001年 | 424篇 |
2000年 | 403篇 |
1999年 | 359篇 |
1998年 | 276篇 |
1997年 | 240篇 |
1996年 | 281篇 |
1995年 | 263篇 |
1994年 | 234篇 |
1993年 | 221篇 |
1992年 | 171篇 |
1991年 | 199篇 |
1990年 | 146篇 |
1989年 | 140篇 |
1988年 | 128篇 |
1987年 | 126篇 |
1986年 | 122篇 |
1985年 | 131篇 |
1984年 | 144篇 |
1983年 | 81篇 |
1982年 | 126篇 |
1981年 | 75篇 |
1980年 | 95篇 |
1979年 | 81篇 |
1978年 | 57篇 |
1977年 | 54篇 |
1976年 | 44篇 |
1974年 | 33篇 |
1972年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
Tardrabassus pakneunensis, n. gen. & sp. is described and illustrated. The new genus shows morphological affinities to three leafhopper subfamilies, Tartessinae, Deltocephalinae, and Iassinae, but is tentatively placed in Iassinae based on the position of the ocelli, the reduced lateral frontal sutures, the leg chaetotaxy, and the structure of the male genitalia. 相似文献
92.
93.
94.
95.
Predictive modeling for allopatric Strix (Strigiformes: Strigidae) owls in South America: determinants of their distributions and ecological niche‐based processes 下载免费PDF全文
Strix (Strigidae) is a worldwide genus of 17 owl species typical of forested habitats, including Rusty‐barred Owls (S. hylophila), Chaco Owls (S. chacoensis), and Rufous‐legged Owls (S. rufipes) in South America. These species are distributed allopatrically, but the ecological traits that determine their distributions remain largely unknown and their phylogenetic relationships are unclear. We used species distribution models (SDMs) to identify variables explaining their distribution patterns and test hypotheses about ecological divergence and conservatism based on niche overlap analysis. For Rusty‐barred Owls and Chaco Owls, climatic factors related to temperature played a major role, whereas a rainfall variable was more important for Rufous‐legged Owls. When niche overlaps were compared, accounting for regional similarities in the habitat available to each species, an ecological niche divergence process was supported for Chaco Owl‐Rusty‐barred Owl and Chaco Owl‐Rufous‐legged Owl, whereas a niche conservatism process was supported for Rusty‐barred Owl‐Rufous‐legged Owl. Different ecological requirements support current species delimitation, but they are in disagreement with the two main hypotheses currently envisaged about their phylogenetic relationships (Chaco Owls as the sister taxa of either Rufous‐legged Owls or Rusty‐barred Owls) and support a new phylogenetic hypothesis (Rufous‐legged Owls as sister taxa of Rusty‐barred Owls). Our findings suggest that speciation of Rusty‐barred Owls and Rufous‐legged Owls was a vicariant event resulting from Atlantic marine transgressions in southern South America in the Miocene, but their niche was conserved because habitat changed little in their respective ranges. In contrast, Chaco Owls diverged ecologically from the other two species as a result of their adaptations to the habitat they currently occupy. Ecological and historical approaches in biogeography can be embedded to explain distribution patterns, and results provided by SDMs can be used to infer historical and ecological processes in an integrative way. 相似文献
96.
97.
98.
The purpose of the present study was to determine whether zinc and calcium could interact at the tissue level. In the first
part of the study, adult rats were injected with ZnCl2 dissolved in a physiological saline solution to determine the effects of Zn on Ca levels in various tissues. In the second
part of the study, weaned rats (at day 22 postnatally) were fed a diet supplemented with Zn until day 50 and were then sacrificed.
In both instances, blood, brain, heart, liver, and skeletal muscle were taken and analyzed. In the Zn-injected group, the
brain, heart, and liver showed no interaction between Zn and Ca. The skeletal muscle, in contrast, showed a decrease in Ca
in the homogenate, whereas Zn contents showed a significant increase at the sarcoplasmic reticulum (SR). Likewise, in the
Zn-supplemented group, the Zn content of the SR vesicle of the skeletal muscle showed an increase, whereas Ca content of the
pellet (14,000 g), which contains cell debris, nucleus, mitochondria, and SR vesicles of this group, showed a decrease. Current
findings suggest antagonistic effects between Zn and Ca on this tissue. Zn may play a critical role in cellular function through
the alteration of itnracellular distribution of Ca in skeletal muscle. 相似文献
99.
100.