全文获取类型
收费全文 | 69879篇 |
免费 | 1653篇 |
国内免费 | 9871篇 |
出版年
2022年 | 325篇 |
2021年 | 479篇 |
2020年 | 510篇 |
2019年 | 1105篇 |
2018年 | 1425篇 |
2017年 | 799篇 |
2016年 | 914篇 |
2015年 | 1114篇 |
2014年 | 1861篇 |
2013年 | 2825篇 |
2012年 | 1994篇 |
2011年 | 1608篇 |
2010年 | 1308篇 |
2009年 | 1764篇 |
2008年 | 2077篇 |
2007年 | 2727篇 |
2006年 | 2896篇 |
2005年 | 2962篇 |
2004年 | 3140篇 |
2003年 | 2854篇 |
2002年 | 2804篇 |
2001年 | 2434篇 |
2000年 | 1819篇 |
1999年 | 2105篇 |
1998年 | 1852篇 |
1997年 | 1377篇 |
1996年 | 1332篇 |
1995年 | 1420篇 |
1994年 | 1499篇 |
1993年 | 1301篇 |
1992年 | 1126篇 |
1991年 | 1116篇 |
1990年 | 903篇 |
1989年 | 745篇 |
1988年 | 785篇 |
1987年 | 590篇 |
1986年 | 517篇 |
1985年 | 1973篇 |
1984年 | 3020篇 |
1983年 | 2019篇 |
1982年 | 2395篇 |
1981年 | 2043篇 |
1980年 | 1919篇 |
1979年 | 1698篇 |
1978年 | 1568篇 |
1977年 | 1426篇 |
1976年 | 1314篇 |
1975年 | 1111篇 |
1974年 | 1101篇 |
1973年 | 1028篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
Tests of associational defence provided by hairy plants for glabrous plants of Arabidopsis halleri subsp. gemmifera against insect herbivores 下载免费PDF全文
1. Trichome‐producing (hairy) and trichomeless (glabrous) plants of Arabidopsis halleri subsp. gemmifera were investigated to test whether plant resistance to herbivory depends on the plants' phenotypes and/or the phenotypes of neighbouring plants (associational effects). 2. A common garden experiment was conducted in which the relative frequency of hairy and glabrous plants was manipulated. Two species of leaf‐chewing insects (larvae of a white butterfly and a cabbage sawfly) were found less often on hairy plants than on glabrous plants. By contrast, the numbers of aphids and flea beetles did not differ significantly between hairy and glabrous plants. For none of these insects did abundance depend on the frequency of the two plant morphs. 3. A field survey was conducted in two natural populations of A. halleri. In the first population, a species of white butterfly was the dominant herbivore, and hairy plants incurred less leaf damage than glabrous plants across 2 years. By contrast, in the other population, where flea beetles were dominant, there were no consistent differences in leaf damage between the two types of plants. In neither of the two populations was any evidence found of associational effects. 4. This study did not provide any conclusive evidence of associational effects of anti‐herbivore resistance, but it was discovered that trichomes can confer resistance to certain herbivores. Given the results of previous work by the authors on associational effects against a flightless leaf beetle, such associational effects of the trichome dimorphism of A. halleri were herbivore‐specific. 相似文献
2.
3.
Takeo Horiguchi 《Phycological Research》1996,44(3):143-150
A new raphidophyte flagellate is described from the river mouth of the Daintree River, north-east Australia where there are extensive mangrove forests. The organism has two distinct phases: a club-shaped motile form, and a more or less spherical benthic non-motile form. The motile cell is metabolic and possesses 10–20 chloroplasts. The chloroplasts are imbricated, somewhat reminiscent of roofing tiles. A unique structure has been found at the posterior end of the cell. It is a tubular invagination, which consists of a single membrane supported by many small flattened vesicles. In addition, the structure is surrounded by many fibril-containing vesicles. The tubular invagination is different from both the contractile vacuoles and the pusules of dinoflagellates in both behavior and structure. Based on the possession of these unique features, it is concluded that the this mangrove flagellate should be described as a new species in a new genus and the name Haramonas dimorpha Horiguchi gen. et sp. nov. is proposed. 相似文献
4.
Ubiquity of Cysteine- and Metalloproteinase Activities in a Wide Range of Trypanosomatids 总被引:1,自引:0,他引:1
MARTA H. BRANQUINHA ALANE B. VERMELHO SAMUEL GOLDENBERG MYRNA C. BONALDO 《The Journal of eukaryotic microbiology》1996,43(2):131-135
We have analysed the proteinase profiles of 11 species from 7 different genera of trypanosomatids by in situ detection of enzyme activities on SDS-PAGE gels containing co-polymerized gelatin as substrate, and the use of specific proteinase inhibitors. Our survey indicates that while cysteine- and metalloproteinases are distributed ubiquitously among trypanosomatids, there are marked differences between the enzyme profiles from the monogenetic (Crithidia, Herpetomonas, Leptomonas) and digenetic (Trypanosoma, Endotrypanum, Phytomonas, Leishmania) species. The detected metalloproteinase activities, ranging in size from 50–100 kDa, partitioned into the detergent-phase after Triton X-114 extraction, while most of cysteine proteinases, of three distinct molecular mass ranges (30–50 kDa, 80–100 kDa and 116–205 kDa), partitioned into the aqueous phase. Thus, within this group of organisms, the metalloproteinase activities seem to be predominantly membrane-associated proteins. We also show that the plant parasites of the genus Phytomonas exhibit a distinctive cysteine proteinase profile that might be exploited further as a criterion for taxonomy of the genus. 相似文献
5.
6.
T. A. Jaarsma E. Lohmanns H. Hendriks T. W. J. Gadella T. M. Malingré 《Plant Systematics and Evolution》1990,169(1-2):31-39
InS. tuberosum subspp.tuberosum andnodosum, S. grandiflorum andS. ibericum the presence of the pyrrolizidine alkaloids lycopsamine, echimidine and symphytine could be demonstrated. The taxonS. tuberosum contains an unknown compound that seems to be specific for this taxon. This compound is not the pyrrolizidine alkaloid anadoline which has previously been reported for this species. It is possibly represented by a peak on GC/MS with a molecular ion peak at m/z 623 (as TMS derivative) and can be used as a chemotaxonomic marker for the speciesS. tuberosum. The pyrrolizidine alkaloid pattern of the two subspecies ofS. tuberosum reinforces the close relationship. Fresh material ofS. tuberosum contained the triterpene isobauerenol, but in herbarium material isobauerenol was lacking. InS. grandiflorum, neither fresh nor dried material contains isobauerenol. In herbarium material ofS. ibericum also no isobauerenol could be found. More extensive chemotaxonomical research is necessary to support the view thatS. abchasicum is more closely related toS. ibericum than toS. grandiflorum. 相似文献
7.
Growth form and distribution of introduced plants in their native and non-native ranges in Eastern Asia and North America 总被引:1,自引:1,他引:0
There is a growing interest in understanding the influence of plant traits on their ability to spread in non-native regions. Many studies addressing this issue have been based on relatively small areas or restricted taxonomic groups. Here, we analyse a large data base involving 1567 plant species introduced between Eastern Asia and North America or from elsewhere to both regions. We related the extent of species distributions in each region to growth form and the distinction between upland and wetland habitats. We identified significant relationships between geographical distribution and plant traits in both native and exotic ranges as well as regional differences in the relationships. Range size was larger for herbaceous graminoids and forbs, especially annuals compared to perennials, than for woody species, and range size also was larger for plants of wetland compared to upland habitats. Distributions were more extensive in North America than in Eastern Asia, although native plants from both regions had broader distributions than non-natives, with exotics from elsewhere intermediate. Growth form and environment explained more of the variance in distribution of plants in North America than in Eastern Asia. The influence of growth form and habitat on distribution suggests that these traits might be related to tolerance of ecological conditions. In addition, the smaller extents of species in non-native compared to native areas suggest roles for dispersal limitation and adaptation to region-specific ecological conditions in determining distribution. 相似文献
8.
Ralf Uptmoor Tobias Schrag Hartmut Stützel Elisabeth Esch 《Molecular breeding : new strategies in plant improvement》2008,21(2):205-216
Studying quantitative traits is complicated due to genotype by environment interactions. One strategy to overcome these difficulties
is to combine quantitative trait loci (QTL) and ecophysiological models, e.g. by identifying QTLs for the response curves
of adaptive traits to influential environmental factors. A B. oleracea DH-population segregating for time to flowering was cultivated at different temperature regimes. Composite interval mapping
was carried out on the three parameters of a model describing time to flowering as a function of temperature, i.e. on the
intercept and slope of the response of time to floral induction to temperature and on the duration from transition to flowering.
The additive effects of QTLs detected for the parameters have been used to estimate time to floral induction and flowering
in the B. oleracea DH-population. The combined QTL and crop model explained 66% of the phenotypic variation for time to floral induction and
56% of the phenotypic variation for time to flowering. Estimation of time to floral induction and flowering based on environment
specific QTLs explained 61 and 41% of the phenotypic variation. Results suggest that flowering time can be predicted effectively
by coupling QTL and crop models and that using crop modelling tools for QTL analysis increases the power of QTL detection. 相似文献
9.
Tetsuro Ohba Kunihiko Uemura Hiroshi Nabetani 《Bioscience, biotechnology, and biochemistry》2017,81(4):724-734
Metabolome analysis and physicochemical analyses were executed with cell extracts of a Lactococcus lactis subspecies cremoris strain treated by moderate pulsed electric field (PEF) to elucidate the mechanism of enhanced production of exopolysaccharide (EPS) by the treatment. Metabolome analysis by capillary electrophoresis time of flight mass spectrometry annotated 224 metabolites from the cytoplasmic extract of the strain, which, however, showed no significant changes in metabolites related to the EPS production. Electron microscopic observation and chemical analysis of undecaprenoids as carrier of EPS biosynthetic intermediates suggested that PEF treatment dissociated immature EPSs from the intermediates due to the focal electro-condensation of hydrogen ions at the cell surface. Thus, liberated undecaprenyl phosphates were recycled efficiently, which resulted in mass increase of EPS with smaller molecular weight. The study suggested the feasibility of moderate PEF treatment as a food processing technique and revealed the mechanism of enhanced production of EPS by the treatment. 相似文献
10.
Ante Vujić Rüstem Hayat Ozan Demirözer Ximo Mengual Farzaneh Kazerani 《Zoology in the Middle East.》2017,63(3):250-268
New data and records of the genus Chrysotoxum Meigen, 1803 are reported, arising from taxonomic and faunistic examination of adult specimens collected from 1920 to 2011 from four northeastern provinces of Turkey (Erzurum, Bayburt, Kars, and Artvin), and from the neighboring countries of Armenia, Azerbaijan and Iran. Three new species are described: Chrysotoxum antennalis Vuji?, Nedeljkovi? &; Hayat sp. n., C. clausseni Vuji?, Nedeljkovi? &; Hayat sp. n. and C. persicum Vuji?, Nedeljkovi? &; Hayat sp. n. The first two are known only from northeastern Turkey, and the third also occurs in Armenia, Azerbaijan and Iran. These new species have in common an antenna with the basoflagellomere being shorter than the scape and pedicel together.http://www.zoobank.org/urn:lsid:zoobank.org:pub:4C264678-8E47-4DE0-AC7D-91DABC597BCD 相似文献