全文获取类型
收费全文 | 13342篇 |
免费 | 1364篇 |
国内免费 | 2篇 |
出版年
2022年 | 152篇 |
2021年 | 273篇 |
2020年 | 145篇 |
2019年 | 159篇 |
2018年 | 217篇 |
2017年 | 177篇 |
2016年 | 352篇 |
2015年 | 602篇 |
2014年 | 652篇 |
2013年 | 686篇 |
2012年 | 927篇 |
2011年 | 915篇 |
2010年 | 548篇 |
2009年 | 480篇 |
2008年 | 696篇 |
2007年 | 704篇 |
2006年 | 616篇 |
2005年 | 586篇 |
2004年 | 532篇 |
2003年 | 557篇 |
2002年 | 495篇 |
2001年 | 267篇 |
2000年 | 237篇 |
1999年 | 217篇 |
1998年 | 135篇 |
1997年 | 115篇 |
1996年 | 112篇 |
1995年 | 106篇 |
1994年 | 92篇 |
1993年 | 80篇 |
1992年 | 159篇 |
1991年 | 139篇 |
1990年 | 134篇 |
1989年 | 138篇 |
1988年 | 131篇 |
1987年 | 114篇 |
1986年 | 92篇 |
1985年 | 97篇 |
1984年 | 103篇 |
1983年 | 80篇 |
1982年 | 87篇 |
1981年 | 66篇 |
1980年 | 76篇 |
1979年 | 98篇 |
1978年 | 75篇 |
1977年 | 74篇 |
1976年 | 80篇 |
1975年 | 78篇 |
1974年 | 72篇 |
1973年 | 70篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
1.
2.
3.
4.
5.
Michelle van der Bank F. H. van der Bank B. -E. van Wyk 《Plant Systematics and Evolution》1999,219(1-2):27-38
We have tested the hypothesis that reseeding is a plesiomorphic character state and that sprouting is a derived state inAspalathus linearis, and that the latter is an adaptation to ensure fire-survival in a fireprone environment. Samples of five seeder and four sprouter populations of A.linearis were examined by horizontal starch gel electrophoresis to assess the amount of genetic differentiation within and between sprouting and seeding populations, and to determine the extent of gene flow between the populations. Leaf extracts were surveyed for ten enzymes and gene products revealed genetic variation at 13 (76%) of 17 protein coding loci. Allele frequency differences were found between sprouting and seeding populations and genetic distance values show that the sprouters are grouped separate from the seeders, thus providing support for the morphological data on which the above mentioned hypothesis is based. It is evident that evolution operates at the population level inA. linearis. 相似文献
6.
7.
8.
Susan Morrison Grace John-Stewart John J. Egessa Sezi Mubezi Sylvia Kusemererwa Dennis K. Bii Nulu Bulya Francis Mugume James D. Campbell Jonathan Wangisi Elizabeth A. Bukusi Connie Celum Jared M. Baeten Partners PrEP Study Team 《PloS one》2015,10(10)
During an HIV-1 prevention clinical trial in East Africa, we observed 16 cases of primary HIV-1 infection in women coincident with pregnancy or breastfeeding. Nine of eleven pregnant women initiated rapid combination antiretroviral therapy (ART), despite having CD4 counts exceeding national criteria for ART initiation; breastfeeding women initiated ART or replacement feeding. Rapid ART initiation during primary HIV-1 infection during pregnancy and breastfeeding is feasible in this setting. 相似文献
9.
Brief admission of the new diabetic child and of a parent to an enlightened hospital for stabilisation, preliminary education, and familiarisation with hospital and community staff is well worth while. The greater the demand for constant control of the highest quality, the greater the need for a close understanding of the psychosocial factors concerned and for clinical skill. The nature of the home and the family relationships should in theory be available from the child''s general practitioner at the time of the first referral since he has so much information about the whole family. With the virtual disappearance, however, of mutual consultation in the patient''s home in many places, the opportunity for oral communication has declined, and availability on the telephone is not always easy. The busy general practitioner (far less an unknown physician from a deputising service without access to the records) has little time to write a comprehensive letter. In practice a relatively small hospital-based mobile team of specially experienced sisters who are keen to communicate in the home, the GP''s surgery, and the school makes a major contribution to the diabetic care of a young population vulnerable to major handicap in what should be the prime of life. Their cost effectiveness may be difficult to prove but it is not at all in doubt--especially when the sisters as in this area deal in the community with a wider range of chronic illnesses and handicaps in children. 相似文献
10.
Stefan Schoombie Janine Schoombie Christiaan W. Brink Kim L. Stevens Christopher W. Jones Michelle M. Risi Peter G. Ryan 《Journal of Field Ornithology》2019,90(4):361-372
The use of miniaturized video cameras to study the at‐sea behavior of flying seabirds has increased in recent years. These cameras allow researchers to record several behaviors that were not previously possible to observe. However, video recorders produce large amounts of data and videos can often be time‐consuming to analyze. We present a new technique using open‐source software to extract bank angles from bird‐borne video footage. Bank angle is a key facet of dynamic soaring, which allows albatrosses and petrels to efficiently search vast areas of ocean for food. Miniaturized video cameras were deployed on 28 Wandering Albatrosses (Diomedea exulans) on Marion Island (one of the two Prince Edward Islands) from 2016 to 2018. The OpenCV library for the Python programming language was used to extract the angle of the horizon relative to the bird’s body (= bank angle) from footage when the birds were flying using a series of steps focused on edge detection. The extracted angles were not significantly different from angles measured manually by three independent observers, thus being a valid method to measure bank angles. Image quality, high wind speeds, and sunlight all influenced the accuracy of angle estimates, but post‐processing eliminated most of these errors. Birds flew most often with cross‐winds (58%) and tailwinds (39%), resulting in skewed distributions of bank angles when birds turned into the wind more often. Higher wind speeds resulted in extreme bank angles (maximum observed was 94°). We present a novel method for measuring postural data from seabirds that can be used to describe the fine‐scale movements of the dynamic‐soaring cycle. Birds appeared to alter their bank angle in response to varying wind conditions to counter wind drift associated with the prevailing westerly winds in the Southern Ocean. These data, in combination with fine‐scale positional data, may lead to new insights into dynamic‐soaring flight. 相似文献