全文获取类型
收费全文 | 217221篇 |
免费 | 9331篇 |
国内免费 | 830篇 |
专业分类
227382篇 |
出版年
2023年 | 743篇 |
2022年 | 729篇 |
2021年 | 1624篇 |
2020年 | 1445篇 |
2019年 | 1527篇 |
2018年 | 14143篇 |
2017年 | 12734篇 |
2016年 | 11148篇 |
2015年 | 6791篇 |
2014年 | 6596篇 |
2013年 | 8822篇 |
2012年 | 13957篇 |
2011年 | 21165篇 |
2010年 | 17010篇 |
2009年 | 12135篇 |
2008年 | 16440篇 |
2007年 | 17769篇 |
2006年 | 7001篇 |
2005年 | 6668篇 |
2004年 | 6762篇 |
2003年 | 6314篇 |
2002年 | 5642篇 |
2001年 | 2770篇 |
2000年 | 2612篇 |
1999年 | 2152篇 |
1998年 | 935篇 |
1997年 | 754篇 |
1996年 | 668篇 |
1995年 | 670篇 |
1994年 | 675篇 |
1993年 | 564篇 |
1992年 | 1409篇 |
1991年 | 1306篇 |
1990年 | 1122篇 |
1989年 | 1065篇 |
1988年 | 1009篇 |
1987年 | 882篇 |
1986年 | 774篇 |
1985年 | 858篇 |
1984年 | 766篇 |
1983年 | 630篇 |
1982年 | 497篇 |
1981年 | 467篇 |
1979年 | 624篇 |
1978年 | 483篇 |
1975年 | 486篇 |
1974年 | 476篇 |
1973年 | 507篇 |
1972年 | 609篇 |
1971年 | 632篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
The purpose of this review is to provide a global perspective on Oncorhynchus salmonine introductions and put-and-take fisheries based on modern stocking programs, with special emphasis on freshwater
ecosystems. We survey the global introductions of nine selected salmonines of the genus Oncorhynchus: golden trout, cutthroat trout, pink salmon, chum salmon, coho salmon, masu/cherry salmon, rainbow trout/steelhead, sockeye
salmon/kokanee, and chinook salmon. The information is organized on a geographical basis by continent, and then by species
and chronology. Two different objectives and associated definitions of ‘success’ for introductions are distinguished: (a)
seed introduction: release of individuals with the purpose of creating a wild-reproducing, self-sustaining population; and
(b) put-and-take introduction: release of individuals with the purpose of maintaining some level of wild population abundance,
regardless of wild reproduction. We identify four major phenomena regarding global salmonine introductions: (1) general inadequacy
of documentation regarding introductions; (2) a fundamental disconnect between management actions and ecological consequences
of introductions; (3) the importance of global climate change on success of previous and future introductions; and (4) the
significance of aquaculture as a key uncertainty in accidental introductions. We conclude this review with a recognition of
the need to terminate ongoing stocking programs for introduced salmonines worldwide. 相似文献
122.
123.
Giovanni Murtas 《Systems and synthetic biology》2010,4(2):85-93
One of the major properties of the semi-synthetic minimal cell, as a model for early living cells, is the ability to self-reproduce
itself, and the reproduction of the boundary layer or vesicle compartment is part of this process. A minimal bio-molecular
mechanism based on the activity of one single enzyme, the FAS-B (Fatty Acid Synthase) Type I enzyme from Brevibacterium ammoniagenes, is encapsulated in 1-palmitoyl-2oleoyl-sn-glycero-3-phosphatidylcholine (POPC) liposomes to control lipid synthesis. Consequently molecules of palmitic acid released
from the FAS catalysis, within the internal lumen, move toward the membrane compartment and become incorporated into the phospholipid
bilayer. As a result the vesicle membranes change in lipid composition and liposome growth can be monitored. Here we report
the first experiments showing vesicles growth by catalysis of one enzyme only that produces cell boundary from within. This
is the prototype of the simplest autopoietic minimal cell. 相似文献
124.
Background
Identifying essential genes in bacteria supports to identify potential drug targets and an understanding of minimal requirements for a synthetic cell. However, experimentally assaying the essentiality of their coding genes is resource intensive and not feasible for all bacterial organisms, in particular if they are infective. 相似文献125.
Following arteriolar occlusion, tissue oxygen concentration decreases and anoxic tissue eventually develops. Although anoxia
first appears in the region most distal to the capillary at the venous end, it eventually spreads throughout the entire region
of supply. In this paper the changing oxygen concentration, from the time of occlusion until the tissue is entirely anoxic,
is examined mathematically. The equations governing oxygen transport to tissue are solved by iterating a nonlinear integral
equation. This solution is valid until anoxia first appears. After anoxia develops it is necessary to solve a moving boundary
problem. This is done using the method of matched asymptotic expansions, and accurate solutions are obtained for a wide range
of physiological conditions. 相似文献
126.
Background
Phylogenies capture the evolutionary ancestry linking extant species. Correlations and similarities among a set of species are mediated by and need to be understood in terms of the phylogenic tree. In a similar way it has been argued that biological networks also induce correlations among sets of interacting genes or their protein products. 相似文献127.
128.
M. T. Vénec-Peyré 《Coral reefs (Online)》1987,5(4):205-212
The discovery of epipsammitic Foraminifera in sediments of Moorea and Scilly (French Polynesia) and the study of close relationships between embedded specimens and host-grains indicate that some species participate in the weakening and subsequent breakdown of skeletal grains and consequently contribute to the production of silt-sized particles. The study of 36 stations around the islands shows the factors that control the distribution and abundance of this incrusted microfauna. Specimens are more abundant on sand particles larger than 1,000 microns and in the high energy areas; this abundance decreases with depth. The physiological mechanism of penetrations is presumably chemical, but each species apparently has its own process: complete dissolution with removal of ions for the cytoplasm of calcareous species; partial dissolution with transport of silt-size aggregates on the test of agglutinated species. The purpose of such penetration may be to protect themselves against water turbulence and to provide material for test construction. 相似文献
129.
Ho WW Fernandes CC Alves-Gomes JA Smith GT 《Ethology : formerly Zeitschrift fur Tierpsychologie》2010,116(11):1050-1064
The South American weakly-electric knifefish (Apteronotidae) produce highly diverse and readily quantifiable electrocommunication signals. The electric organ discharge frequency (EODf), and EOD modulations (chirps and gradual frequency rises (GFRs)), vary dramatically across sexes and species, presenting an ideal opportunity to examine the proximate and ultimate bases of sexually dimorphic behavior. We complemented previous studies on the sexual dimorphism of apteronotid communication signals by investigating electric signal features and their hormonal correlates in Apteronotus bonapartii, a species which exhibits strong sexual dimorphism in snout morphology. Electrocommunication signals were evoked and recorded using a playback paradigm, and were analyzed for signal features including EOD frequency and the structure of EOD modulations. To investigate the androgenic correlates of sexually dimorphic EOD signals, we measured plasma concentrations of testosterone and 11-ketotestosterone. A. bonapartii responded robustly to stimulus playbacks. EODf was sexually monomorphic, and males and females produced chirps with similar durations and amounts of frequency modulation. However, males were more likely than females to produce chirps with multiple frequency peaks. Sexual dimorphism in apteronotid electrocommunication signals appears to be highly evolutionarily labile. Extensive interspecific variation in the magnitude and direction of sex differences in EODf and in different aspects of chirp structure suggest that chirp signals may be an important locus of evolutionary change within the clade. The weakly-electric fish represent a rich source of data for understanding the selective pressures that shape, and the neuroendocrine mechanisms that underlie, diversity in the sexual dimorphism of behavior. 相似文献
130.