全文获取类型
收费全文 | 1982篇 |
免费 | 187篇 |
国内免费 | 1篇 |
出版年
2021年 | 24篇 |
2019年 | 22篇 |
2018年 | 22篇 |
2017年 | 17篇 |
2016年 | 29篇 |
2015年 | 46篇 |
2014年 | 50篇 |
2013年 | 78篇 |
2012年 | 94篇 |
2011年 | 82篇 |
2010年 | 50篇 |
2009年 | 59篇 |
2008年 | 74篇 |
2007年 | 78篇 |
2006年 | 73篇 |
2005年 | 84篇 |
2004年 | 59篇 |
2003年 | 77篇 |
2002年 | 82篇 |
2001年 | 68篇 |
2000年 | 54篇 |
1999年 | 43篇 |
1998年 | 31篇 |
1997年 | 29篇 |
1996年 | 18篇 |
1995年 | 19篇 |
1994年 | 18篇 |
1993年 | 20篇 |
1992年 | 41篇 |
1991年 | 33篇 |
1990年 | 28篇 |
1989年 | 16篇 |
1988年 | 32篇 |
1987年 | 37篇 |
1986年 | 35篇 |
1985年 | 35篇 |
1984年 | 30篇 |
1983年 | 37篇 |
1982年 | 27篇 |
1981年 | 31篇 |
1980年 | 16篇 |
1979年 | 16篇 |
1978年 | 21篇 |
1977年 | 22篇 |
1976年 | 16篇 |
1975年 | 22篇 |
1974年 | 19篇 |
1973年 | 16篇 |
1972年 | 24篇 |
1968年 | 18篇 |
排序方式: 共有2170条查询结果,搜索用时 15 毫秒
1.
2.
Sandra L. Almeyda Zambrano Eben N. Broadbent Sam Shanee Noga Shanee Anneke Deluycker Michael Steinberg Scott A. Ford Alma Hernndez Jaramillo Robin Fernandez‐Hilario Carolina Lagos Castillo Angelica M. Almeyda Zambrano 《American journal of primatology》2019,81(8)
Habitat loss is one of the main threats to wildlife. Therefore, knowledge of habitat use and preference is essential for the design of conservation strategies and identification of priority sites for the protection of endangered species. The yellow‐tailed woolly monkey (Lagothrix flavicauda Humboldt, 1812), categorized as Critically Endangered on the IUCN Red List, is endemic to montane forests in northern Peru where its habitat is greatly threatened. We assessed how habitat use and preference in L. flavicauda are linked to forest structure and composition. The study took place near La Esperanza, in the Amazonas region, Peru. Our objective was to identify characteristics of habitat most utilized by L. flavicauda to provide information that will be useful for the selection of priority sites for conservation measures. Using presence records collected from May 2013 to February 2014 for one group of L. flavicauda, we classified the study site into three different use zones: low‐use, medium‐use, and high‐use. We assessed forest structure and composition for all use zones using 0.1 ha Gentry vegetation transects. Results show high levels of variation in plant species composition across the three use zones. Plants used as food resources had considerably greater density, dominance, and ecological importance in high‐use zones. High‐use zones presented similar structure to medium‐ and low‐use zones; thus it remains difficult to assess the influence of forest structure on habitat preference. We recommend focusing conservation efforts on areas with a similar floristic composition to the high‐use zones recorded in this study and suggest utilizing key alimentation species for reforestation efforts. 相似文献
3.
S L Rakestraw W E Ford R G Tompkins M A Rodgers W P Thorpe M L Yarmush 《Biotechnology progress》1992,8(1):30-39
A set of anti-melanoma immunoconjugates were prepared which contained chlorin e6: antibody molar ratios of 18.9:1, 11.2:1, 6.8:1, and 1.7:1. All immunoconjugates retained antigen binding activity regardless of the chromophore:antibody substitution ratio that was attained. In contrast, the ground-state absorption spectra of the immunoconjugates showed features which appeared to be dependent on the chromophore:antibody molar ratio. In addition, the quantum yield of singlet oxygen generated by the conjugated chromophores was observed to be significantly less than that observed with the unbound dye. Time-resolved absorbance spectroscopy of the chromophore excited triplet state indicated that the loss of singlet oxygen quantum yield resulted from diminished chromophore triplet yield. Analysis of data obtained from in vitro photolysis of target melanoma cells, in combination with that obtained from the immunochemical and photochemical studies, indicates that the observed immunoconjugate phototoxicity can be reasonably quantitatively represented by (1) the ability of the immunoconjugate to bind SK-MEL-2 cell surface antigen, (2) the amount of chromophore localized at the target cells by immunoconjugate binding, (3) the delivered dose of light at 634 nm, and (4) the singlet oxygen quantum yield of the antibody-bound photosensitizer. Though these data argue strongly for photolysis by the cumulative dosage of singlet oxygen at the cell membrane, nonetheless, the concurrent photoinduced release of other cytotoxic agents should not be ruled out. 相似文献
4.
R. Munro Ford 《BMJ (Clinical research ed.)》1958,1(5075):887-888
5.
6.
7.
Indirect evidence suggests that legumes can adjust rapidly theresistance of their root nodules to O2 diffusion. Here we describeexperiments using O2 specific micro-electrodes and dark fieldmicroscopy to study directly the operation of this diffusionbarrier. The O2 concentration sensed by the electrode decreasedsharply in the region of the inner cortex and was less than1.0 mmol m3 throughout the infected tissue in nodulesof both pea (Pisum sativum) and french bean (Phaseolus vulgaris).In a number of experiments the ambient O2 concentration wasincreased to 40% while the electrode tip was just inside theinner cortex. In 13 out of 21 cases the O2 concentration atthis position either remained low and unchanged or increasedirreversibly to near ambient values. In the remaining casesthe O2 concentration increased after 1 to 2.5 min and then decreasedto its former value. These results are ascribed to an increasein resistance of the barrier in response to increased O2 fluxinto the nodule. It was shown microscopically that air spacesboth at the boundary between the infected zone and the innercortex, and within the infected zone started to disappear 3min after nodules were exposed to high ambient O2 concentrationsand had disappeared completely after 8 min. These spaces werenot changed by exposure of the nodule for 10 min to either N2or air. Key words: Oxygen, root nodules, air spaces 相似文献
8.
Evolution of the 28S ribosomal RNA gene in anurans: regions of variability and their phylogenetic implications 总被引:1,自引:0,他引:1
Fifteen restriction sites were mapped to the 28S ribosomal RNA gene of
individuals representing 54 species of frogs, two species of salamanders, a
caecilian, and a lungfish. Eight of these sites were present in all species
examined, and two were found in all but one species. Alignment of these
conserved restriction sites revealed, among anuran 28S rRNA genes, five
regions of major length variation that correspond to four of 12 previously
identified divergent domains of this gene. One of the divergent domains
(DD8) consists of two regions of length variation separated by a short
segment that is conserved at least throughout tetrapods. Most of the
insertions, deletions, and restriction-site variations identified in the
28S gene will require sequence-level analysis for a detailed reconstruction
of their history. However, an insertion in DD9 that is coextensive with
frogs in the suborder Neobatrachia, a BstEII site that is limited to
representatives of two leptodactylid subfamilies, and a deletion in DD10
that is found only in three ranoid genera are probably synapomorphies.
相似文献
9.
Molecular modelling of the interactions of tetra-(4-N-methylpyridyl) porphin with TA and CG sites on DNA. 总被引:1,自引:1,他引:0
The molecular structure of the DNA-intercalating ligand tetra-(4-N-methylpyridyl) porphin has been determined by X-ray crystallography. The porphyrin has a precise centre of symmetry; the central core is planar, with the N-methylpyridyl groups inclined to it at angles of 66-72 degrees. Molecular modelling of this structure into TpA and CpG sites of intercalated DNA, has been performed, and approximate energetics calculated. It has been shown that only the CpG site can have full ligand intercalation, since the thymine methyl group sterically hinders such geometry at TpA sites. Modelling indicates the importance of electrostatic effects in the low-energy forms of intercalated and part-intercalated complexes at both sequences. 相似文献
10.