全文获取类型
收费全文 | 1241929篇 |
免费 | 133235篇 |
国内免费 | 546篇 |
专业分类
1375710篇 |
出版年
2018年 | 11241篇 |
2016年 | 15270篇 |
2015年 | 20031篇 |
2014年 | 23745篇 |
2013年 | 34070篇 |
2012年 | 38098篇 |
2011年 | 39162篇 |
2010年 | 26825篇 |
2009年 | 25068篇 |
2008年 | 35365篇 |
2007年 | 36761篇 |
2006年 | 34632篇 |
2005年 | 33280篇 |
2004年 | 32971篇 |
2003年 | 31694篇 |
2002年 | 31088篇 |
2001年 | 53181篇 |
2000年 | 53090篇 |
1999年 | 42490篇 |
1998年 | 15648篇 |
1997年 | 15996篇 |
1996年 | 15029篇 |
1995年 | 13978篇 |
1994年 | 13547篇 |
1993年 | 13788篇 |
1992年 | 35256篇 |
1991年 | 34767篇 |
1990年 | 33922篇 |
1989年 | 33204篇 |
1988年 | 30947篇 |
1987年 | 29598篇 |
1986年 | 27545篇 |
1985年 | 27438篇 |
1984年 | 22653篇 |
1983年 | 19780篇 |
1982年 | 15101篇 |
1981年 | 13636篇 |
1980年 | 12833篇 |
1979年 | 21418篇 |
1978年 | 16950篇 |
1977年 | 15497篇 |
1976年 | 14671篇 |
1975年 | 16218篇 |
1974年 | 17622篇 |
1973年 | 17411篇 |
1972年 | 15888篇 |
1971年 | 14395篇 |
1970年 | 12725篇 |
1969年 | 12443篇 |
1968年 | 11251篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
993.
994.
995.
The aim of this study was to investigate aging-related changes in the contribution of genetic and environmental influences to hand-grip strength in late adulthood. Subjects in this study are 152 intact twin pairs (77 monozygotic and 75 dizygotic pairs) from the National Heart, Lung, and Blood Institute Twin Study assessed repeatedly for hand-grip strength at mean ages of 63 and 73 yr. Structural equation genetic modeling was used to investigate stability and change in the genetic and environmental components of variance of hand-grip strength in late adulthood. Average decline in strength over the 10 yr of follow-up was -1.05+/-6.8 (SD) kg and was highly significant (P = 0.003). The test-retest correlation between baseline and follow-up grip strength was 0.62 (P<0.001). Bivariate genetic modeling found significant genetic and shared environmental stability in hand-grip strength over the 10 yr of follow-up, with genetic and shared environmental influences accounting for 35 and 48%, respectively, of the test-retest phenotypic correlation. We conclude from these results that stability in hand-grip strength in late adulthood is due primarily to continuity of genetic and familial influences. 相似文献
996.
997.
Scots pine (Pinus sylvestris L.) seedlings were grown for 3years in the ground in open top chambers and exposed to twoconcentrations of atmospheric CO2(ambient or ambient + 400 µmol mol-1) without addition of nutrients and water. Biomassproduction (above-ground and below-ground) and allocation, aswell as canopy structure and tissue nitrogen concentrationsand contents, were examined by destructive harvest after 3 years.Elevated CO2increased total biomass production by 55%, reducedneedle area and needle mass as indicated, respectively, by lowerleaf area ratio and leaf mass ratio. A relatively smaller totalneedle area was produced in relation to fine roots under elevatedCO2. The proportion of dry matter in roots was increased byelevated CO2, as indicated by increased root-to-shoot ratioand root mass ratio. Within the root system, there was a significantshift in the allocation towards fine roots. Root litter constituteda much higher fraction of fine roots in trees grown in the elevatedCO2than in those grown in ambient CO2. Growth at elevated CO2causeda significant decline in nitrogen concentration only in theneedles, while nitrogen content significantly increased in branchesand fine roots (with diameter less than 1 mm). There were nochanges in crown structure (branch number and needle area distribution).Based upon measurements of growth made throughout the 3 years,the greatest increase in biomass under elevated CO2took placemainly at the beginning of the experiment, when trees grownin elevated CO2had higher relative growth rates than those grownunder ambient CO2; these differences disappeared with time.Symptoms of acclimation of trees to growth in the elevated CO2treatmentwere observed and are discussed. Copyright 2000 Annals of BotanyCompany Elevated CO2, Pinus sylvestris, biomass production, allocation, fine roots, root litter, crown structure, nitrogen, C/N ratio 相似文献
998.
The occurrence of shallow-water (0.9 to 1.3 m) rhodoliths in back reef environments in southwest Puerto Rico is reported.
The rhodoliths were generally cylindrical, discoidal or irregular in shape with an average longest dimension of 7.2 cm. They
occurred at a maximum density of 524 m−2. The rhodoliths were composed of mostly coral nuclei with concentric laminations of aragonite-producing Cruoriella armorica (Peyssonneliaceae, Rhodophyta). Maximum Cruoriella accretion around coral nuclei was 30 mm although accretions of 1 to 20 mm were more common. Based on measurements of Cruoriella accretion, these shallow water rhodoliths are estimated to have minimum ages of 12 to 24 years. It is further estimated that
approximately 2% of the rhodoliths are turned over daily.
Accepted: 1 October 1999 相似文献
999.
1000.