全文获取类型
收费全文 | 398篇 |
免费 | 2篇 |
出版年
2013年 | 9篇 |
2012年 | 6篇 |
2011年 | 15篇 |
2010年 | 13篇 |
2009年 | 31篇 |
2008年 | 19篇 |
2007年 | 18篇 |
2006年 | 15篇 |
2005年 | 18篇 |
2004年 | 12篇 |
2003年 | 5篇 |
2001年 | 6篇 |
2000年 | 3篇 |
1999年 | 3篇 |
1998年 | 7篇 |
1997年 | 4篇 |
1996年 | 7篇 |
1995年 | 9篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 3篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1986年 | 7篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1974年 | 6篇 |
1973年 | 6篇 |
1972年 | 17篇 |
1971年 | 7篇 |
1970年 | 3篇 |
1969年 | 3篇 |
1968年 | 6篇 |
1960年 | 4篇 |
1959年 | 4篇 |
1958年 | 2篇 |
1956年 | 4篇 |
1955年 | 4篇 |
排序方式: 共有400条查询结果,搜索用时 15 毫秒
181.
A. CHANDLER SCHMUTZER MATTHEW J. GRAY ELIZABETH C. BURTON DEBRA L. MILLER 《Freshwater Biology》2008,53(12):2613-2625
- 1 Agricultural practices such as cattle farming may have direct or indirect negative effects on larval amphibians by decreasing water quality through deposition of nitrogenous waste, causing eutrophication, and grazing shoreline vegetation that contributes to detrital cover and food.
- 2 We sampled amphibian larvae on the Cumberland Plateau, Tennessee, U.S.A., twice per week, water quality twice per month and algal and detrital biomass once per month at seven wetlands (three cattle‐access and four non‐access) from March to August 2005 and 2006.
- 3 In general, species richness and diversity of amphibian larvae were greater in wetlands without cattle. Mean relative abundance of green frog (Rana clamitans) and American bullfrog (Rana catesbeiana) tadpoles was greater in non‐access wetlands. Body size of some ranid larvae was larger in cattle‐access wetlands but this trend did not exist for juveniles or adults. Dissolved oxygen was lower, while specific conductivity and turbidity were higher in cattle‐access wetlands. Mean biomass of detritus was lower in cattle‐access wetlands compared to non‐access wetlands; no differences were detected in algal biomass.
- 4 Given the negative impacts of cattle on water quality, detrital biomass, larval amphibian species richness and relative abundance of some amphibian species, we recommend that farmers consider excluding these livestock from aquatic environments.
182.
P. BARRY TOMLINSON FLS ELIZABETH H. ZACHARIAS 《Botanical journal of the Linnean Society. Linnean Society of London》2001,135(3):215-228
In Amentotaxus, Cephalotaxus and Torreya there is a regular seasonal alternation of foliage leaves and bud-scales, with foliage leaves largely preformed, i.e. initiated in the season before they expand. On most plagiotropic shoots phyllotaxis in the production of foliage leaves may be either bijugate ( Cephulotaxus, Torreya ) or decussate ( Amentotaxus ). In bijugate phyllotaxis successive leaf pairs originate at an angle of about 68° to each other, i.e. approximately one-half of the 'ideal' or Fibonacci angle of 137.5°. Secondary leaf orientation in Cephulotaxus and Torreya , by twisting of the leaf base, produces the dorsiventrality of plagiotropic shoots, whereas in Amentotaxus secondary orientation involves a twisting of the stemc as well as the leaf base. In Cephalotaxus cc condition is constant in the production of the numerous but imprecise number of bud-scales and in the production of foliage leaves. However, in Torreya the phyllotaxis changes from bijugate in the production of foliage leaves to decussate in the production of bud-scales, which are constant in number (about eight pairs). This allows a precise analysis of the biphasic production of leaf primordia in the seasonal cycle. The phyllotactic change in Torreya may not be the result of reported changes in shoot apex dimensions since Cephalotaxus , with its constant phyllotaxis, has a comparable seasonal change in apex dimensions. Information on architecture, chirality and cone morphology is also included. 相似文献
183.
The intertidal snail Littorina saxatilis displays a
range ofshell morphologies associated with a variety of
habitats. Sincemorphology has an environmental and genetic basis,
shell-basedtaxonomy may not accurately reflect genetic
relationships. Weexplored genetic structure among adjacent
populations of L.saxatilis (the robust open-shore type),
L. neglecta (the tinybarnacle-dwelling type) and
L. tenebrosa (the fragile brackish-watertype), at nine sites
in Britain. Using single-strand conformationalpolymorphism analysis
of a 375bp fragment of cytochrome-b wefound no evidence of species
distinction. In AMOVA tests significantvariation was contained among
populations (68%) and among individuals(32%, both P < 0.001), and
insignificant variation was foundamong ecotypes. Genetic patterns
suggested gene flow among ecotypesover small scales and a strong
random input over larger scales.
* To whom correspondence should be addressed. (Received 27 March 2000; accepted 10 July 2000) 相似文献
184.
185.
ELIZABETH L. CLARE BURTON K. LIM MARK D. ENGSTROM JUDITH L. EGER PAUL D. N. HEBERT 《Molecular ecology resources》2007,7(2):184-190
Sequence diversity in the cytochrome c oxidase subunit 1 gene has been shown to be an effective tool for species identification and discovery in various groups of animals, but has not been extensively tested in mammals. We address this gap by examining the performance of DNA barcodes in the discrimination of 87 species of bats from Guyana. Eighty‐one of these species showed both low intraspecific variation (mean = 0.60%), and clear sequence divergence from their congeners (mean = 7.80%), while the other six showed deeply divergent intraspecific lineages suggesting that they represent species complexes. Although further work is needed to examine patterns of sequence diversity at a broader geographical scale, the present study validates the effectiveness of barcoding for the identification of regional bat assemblages, even highly diverse tropical faunas. 相似文献
186.
JUDITH H. ROBINS MELANIE HINGSTON ELIZABETH MATISOO‐SMITH HOWARD A. ROSS 《Molecular ecology resources》2007,7(5):717-729
In recent years, research has shown that geographical variation in mitochondrial DNA of commensal rats provides a strong signal of human dispersal and migration. However, interpretation of genetic variation is complicated by the presence of multiple species of Rattus especially in Island Southeast Asia, by the occurrence of some of these Rattus sp. as subfossils in archaeological and natural sites, and by the difficulty of osteological identification of these remains. Amplification of DNA from ancient sources usually yields only small fragments (~200 bp). We assessed whether we could identify Rattus sp. reliably with DNA barcoding using cytochrome oxidase I (COI) sequences, or tree‐based methods using D‐loop, cytochrome b and COI sequences. Species forming well‐differentiated clades in a molecular phylogeny were accurately identified by both methods, even when we used short DNA fragments. Identification was less accurate for paraphyletic and polyphyletic species. We suggest that taxonomic revisions that recognize cryptic or polytypic species will lead to even greater accuracy of DNA‐based identification methods. 相似文献
187.
NIGEL D. SWARTS ELIZABETH A. SINCLAIR KINGSLEY W. DIXON 《Molecular ecology resources》2007,7(6):1141-1143
The orchid genus Caladenia is species rich with many threatened and endangered taxa. We report on the isolation and characterization of microsatellite loci in the rare Caladenia huegelii for molecular evaluation of this and the closely related C. thinicola as part of the development of conservation initiatives. Eight di‐ and trinucleotide loci were screened using 30 samples from each species. All loci were highly variable, with similar levels of heterozygosity and number of alleles across both species. These markers will be highly informative for population studies in both species. 相似文献
188.
189.
190.