全文获取类型
收费全文 | 95722篇 |
免费 | 334篇 |
国内免费 | 897篇 |
专业分类
96953篇 |
出版年
2018年 | 11847篇 |
2017年 | 10680篇 |
2016年 | 7457篇 |
2015年 | 591篇 |
2014年 | 286篇 |
2013年 | 350篇 |
2012年 | 4257篇 |
2011年 | 12858篇 |
2010年 | 12081篇 |
2009年 | 8354篇 |
2008年 | 9867篇 |
2007年 | 11444篇 |
2006年 | 321篇 |
2005年 | 566篇 |
2004年 | 1007篇 |
2003年 | 1043篇 |
2002年 | 790篇 |
2001年 | 276篇 |
2000年 | 179篇 |
1999年 | 53篇 |
1998年 | 52篇 |
1997年 | 78篇 |
1996年 | 50篇 |
1995年 | 34篇 |
1994年 | 33篇 |
1993年 | 53篇 |
1992年 | 58篇 |
1991年 | 59篇 |
1990年 | 30篇 |
1989年 | 31篇 |
1988年 | 37篇 |
1987年 | 40篇 |
1984年 | 25篇 |
1983年 | 35篇 |
1981年 | 24篇 |
1979年 | 23篇 |
1972年 | 256篇 |
1971年 | 287篇 |
1962年 | 25篇 |
1959年 | 53篇 |
1958年 | 125篇 |
1957年 | 139篇 |
1956年 | 140篇 |
1955年 | 149篇 |
1954年 | 152篇 |
1953年 | 120篇 |
1952年 | 104篇 |
1951年 | 100篇 |
1950年 | 59篇 |
1948年 | 23篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
Many antagonistic species attack plants and consume specific plant parts. Understanding how these antagonists affect plant
fitness individually and in combination is an important research focus in ecology and evolution. We examined the individual
and combined effects of herbivory, nectar robbing, and pre-dispersal seed predation on male and female estimates of fitness
in the host plant Ipomopsis aggregata. By examining the effects of antagonists on plant traits, we were able to tease apart the direct consumptive effects of antagonists
versus the indirect effects mediated through changes in traits important to pollination. In a three-way factorial field experiment,
we manipulated herbivory, nectar robbing, and seed predation. Herbivory and seed predation reduced some male and female fitness
estimates, whereas plants tolerated the effects of robbing. The effects of herbivory, robbing, and seed predation were primarily
additive, and we found little evidence for non-additive effects of multiple antagonists on plant reproduction. Herbivory affected
plant reproduction through both direct consumptive effects and indirectly through changes in traits important to pollination
(i.e., nectar and phenological traits). Conversely, seed predators primarily had direct consumptive effects on plants. Our
results suggest that the effects of multiple antagonists on estimates of plant fitness can be additive, and investigating
which traits respond to damage can provide insight into how antagonists shape plant performance. 相似文献
962.
R. Kanchana Usha Devi Muraleedharan Seshagiri Raghukumar 《World journal of microbiology & biotechnology》2011,27(9):2125-2131
Two thraustochytrid protists of the genus Thraustochytrium isolated from coastal and mangrove habitats of Goa, India were studied for extracellular alkaline lipase production. Maximum lipase production was supported by a combination of peptone and yeast extract in the growth medium while strong inhibition of enzyme production was observed in presence of glucose. The inducible nature of the enzyme production was evidenced by the requirement of olive oil in the medium. Lipase production was salt-dependent and optimum production required 3.4% (w/v) crude sea salt. Ideal conditions for maximum production of lipases were therefore adopted as incubation at 30 ± 2°C for 168 h at an initial pH of 6.0 in a medium consisting of 0.5% peptone, 0.01% yeast extract, 0.5% olive oil and 3.4% crude salt. Extracellular lipase production by the two thraustochytrid isolates [designated TZ (ATCC #PRA-295) and AH-2 (ATCC #PRA-296)] was increased threefold under these optimized culture conditions. This appears to be the first report on optimization of cultivation conditions for the production of alkaline lipases by thraustochytrids. 相似文献
963.
Azam Sadat Hosseini Alhashemi Abdolreza R. Karbassi Bahram Hassanzadeh Kiabi Seyed Masoud Monavari Seyed Mohammad Bagher Nabavi Mohammad Sadegh Sekhavatjou 《Biological trace element research》2011,142(3):500-516
Present study investigates relationships between total and bioaccessibility of trace elements (Cd, Co, Cr, Cu, Mn, NI, Pb,
V, and Zn) concentrations in sediment and their bioaccumulation in species in Shadegan wetland in southwest of Iran. Bioavailability
factor (BAF) and translocation factor (TF) were calculated in plants and trophic transfer factor (TTF) was determined in bird
species. For this purpose, sampling of sediments, aquatic plants including Phragmites australis, Typha australis, Scripus maritimus and two bird species encircling Porphyrio porphyrio and globally threatened Marmaronetta angustirostris were carried out during winter 2009. Result of chemical analysis show that bioaccessibility concentrations of Mn (8.31 mg/kg),
V (1.33 mg/kg), and Pb (1.03 mg/kg) are higher than other metals. The uptake trend of trace elements in plant decreases as
root > stem > leaf. Accumulation levels of trace elements in different tissues of P. porphyrio and M. angustirostris are almost identical and considerable. Accumulation and toxicity of Cd in birds is more than plants. In addition, BAF of
V, Pb, and Cr indicates high accumulation by plants and great pollution rate in the area of study. In S. maritimus TF for Mn, Cu, Pb, and V are high whereas in T. australis, Cu and Pb posses the highest TF. Also Cr, Co, Mn, Ni, and Zn have higher TF from stem to leaf than root to stem in P. australis. Finally, TTFs were compared in various bird species. 相似文献
964.
Drosophila metamorphosis is characterized by the histolysis of larval structures by programmed cell death, which paves the way for the establishment of adult-specific structures under the influence of the steroid hormone ecdysone. Malpighian tubules function as an excretory system and are one of the larval structures that are not destroyed during metamorphosis and are carried over to adulthood. The pupal Malpighian tubules evade destruction in spite of expressing apoptotic proteins, Reaper, Hid, Grim, Dronc and Drice. Here we show that in the Malpighian tubules expression of apoptotic proteins commences right from embryonic development and continues throughout the larval stages. Overexpression of these proteins in the Malpighian tubules causes larval lethality resulting in malformed tubules. The number and regular organization of principal and stellate cells of Malpighian tubules is disturbed, in turn disrupting the physiological functioning of the tubules as well. Strikingly, the localization of beta-tubulin, F-actin and Disclarge (Dlg) is also disrupted. These results suggest that the apoptotic proteins could be having non-apoptotic function in the development of Malpighian tubules. 相似文献
965.
Li Shen Evert Nieuwlaar Ernst Worrell Martin K. Patel 《The International Journal of Life Cycle Assessment》2011,16(6):522-536
Purpose
The demand of PET bottles has increased rapidly in the past decades. The purpose of this study is to understand the environmental impact of PET recycling system, in which used bottles are recycled into both fibre and bottles, and to compare the recycling system with single-use PET. 相似文献966.
ERNESTO FUENTES BRITO TIMOTHY P. MOULTON MARCELO L. DE SOUZA STUART E. BUNN 《Austral ecology》2006,31(5):623-633
Abstract: Stable isotope studies of food webs in floodplains, large rivers, mangroves, and seagrasses have shown that, although a large proportion of the biomass may come from higher plants, microalgae provide a disproportionate amount of carbon assimilated by metazoan consumers. Evidence is building that this may also be the case for streams, especially those in the tropics. At the level of individual consumer species we also see that the apparent diet may not be reflected in the carbon assimilated. Tropical streams commonly have omnivore‐detritivore species that potentially show this phenomenon. We tested these concepts in four moderately shaded sites in a stream in well‐preserved Atlantic rainforest at Ilha Grande, Rio de Janeiro. We sampled aquatic insects, shrimps and fish as well as potential terrestrial and aquatic primary food sources. Carbon stocks from terrestrial sources predominated over carbon of algal origin (>99% of total). The primary sources of carbon showed distinctly different isotopic signatures: terrestrial sources had δ13C values close to ?30‰, microalgae were ?20‰ and macroalgae were ?25‰. All fauna had δ13C values consistent with a carbon source derived from microalgae. Baetid mayflies and atyid shrimps exert a strong grazing pressure on periphyton and organic sediments but appear to assimilate predominantly microalgae. The palaemonid shrimp Macrobrachium olfersi also ingests large amounts of detritus of terrestrial origin, but apparently assimilates animal prey with algal δ13C signatures. These results support the growing view that tropical stream food chains are primarily algal based. 相似文献
967.
The field of structural biology is becoming increasingly important as new technological developments facilitate the collection
of data on the atomic structures of proteins and nucleic acids. The solid-state NMR method is a relatively new biophysical
technique that holds particular promise for determining the structures of peptides and proteins that are located within the
cell membrane. This method provides information on the orientation of the peptide planes relative to an external magnetic
field. In this article, we discuss some of the mathematical methods and tools that are useful in deriving the atomic structure
from these orientational data. We first discuss how the data are viewed as tensors, and how these tensors can be used to construct
an initial atomic model, assuming ideal stereochemistry. We then discuss methods for refining the models using global optimization,
with stereochemistry constraints treated as penalty functions. These two processes, initial model building followed by refinement,
are the two crucial steps between data collection and the final atomic model. 相似文献
968.
969.
Paula N. Friedman Edith H. Wang Karen Meerovitch Nahum Sonenberg Carol Prives 《Chromosoma》1992,102(1):S60-S66
We have characterized the effects of p53 on several biochemical activities of simian virus 40 (SV40) large tumor (T) antigen. While p53 induced a strong inhibition of the T antigen DNA helicase activity, surprisingly, its RNA helicase activity was stimulated. This supports the liklihood that the DNA and RNA helicase activities of T antigen reflect discrete functions. p53 did not significantly affect the ATP-dependent conversion of T antigen monomers to hexamers. However, the ability of these hexamers to assemble on a DNA fragment containing the viral origin was impaired by p53. Thus, these results suggest that p53 inhibits the function but not the formation of T antigen multimers. This conclusion was further supported by the observation that the addition of a purified p53:T antigen complex was as inhihitory as free p53 to the DNA helicase activity of free T antigen. Thus our data indicates that the targets of p53 inhibition are the functional units of T antigen, namely the hexamers. 相似文献
970.