全文获取类型
收费全文 | 1369852篇 |
免费 | 128274篇 |
国内免费 | 1476篇 |
专业分类
1499602篇 |
出版年
2021年 | 17887篇 |
2019年 | 16199篇 |
2018年 | 17831篇 |
2017年 | 16542篇 |
2016年 | 28156篇 |
2015年 | 42497篇 |
2014年 | 50539篇 |
2013年 | 76839篇 |
2012年 | 37271篇 |
2011年 | 25927篇 |
2010年 | 43701篇 |
2009年 | 45107篇 |
2008年 | 24501篇 |
2007年 | 22388篇 |
2006年 | 27575篇 |
2005年 | 28620篇 |
2004年 | 27554篇 |
2003年 | 25373篇 |
2002年 | 23397篇 |
2001年 | 36043篇 |
2000年 | 33441篇 |
1999年 | 32876篇 |
1998年 | 25776篇 |
1997年 | 25751篇 |
1996年 | 25200篇 |
1995年 | 23385篇 |
1994年 | 23146篇 |
1993年 | 22402篇 |
1992年 | 28797篇 |
1991年 | 27355篇 |
1990年 | 25646篇 |
1989年 | 26392篇 |
1988年 | 24184篇 |
1987年 | 23077篇 |
1986年 | 21926篇 |
1985年 | 23800篇 |
1984年 | 23357篇 |
1983年 | 20712篇 |
1982年 | 21103篇 |
1981年 | 20151篇 |
1980年 | 18908篇 |
1979年 | 19502篇 |
1978年 | 18346篇 |
1977年 | 17542篇 |
1976年 | 16688篇 |
1975年 | 16123篇 |
1974年 | 16533篇 |
1973年 | 16809篇 |
1972年 | 14243篇 |
1971年 | 13090篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
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
992.
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
993.
The discrimination of the angular velocity of ventrodorsal and dorsoventral movement of an acoustic image was studied in nine test subjects. The experiments were performed using an apparent movement produced by consecutive activation of loudspeakers located along an arc in the vertical plane. The differential thresholds were measured by the minimum increment method. As the velocity of an acoustic image movement in opposite directions increased, the values of its mean absolute differential thresholds increased monotonically. Regression lines plotted by linear approximation of these values did not differ significantly. 相似文献
994.
Heat stress reduces maize yield and several lines of evidence suggest that the heat lability of maize endosperm ADP-glucose pyrophosphorylase (AGPase) contributes to this yield loss. AGPase catalyzes a rate-limiting step in starch synthesis. Herein, we present a novel maize endosperm AGPase small subunit variant, termed BT2-TI that harbors a single amino acid change of residue 462 from threonine to isoleucine. The mutant was isolated by random mutagenesis and heterologous expression in a bacterial system. BT2-TI exhibits enhanced heat stability compared to wildtype maize endosperm AGPase.The TI mutation was placed into another heat-stable small subunit variant, MP. MP is composed of sequences from the maize endosperm and the potato tuber small subunit. The MP-TI small subunit variant exhibited greater heat stability than did MP. Characterization of heat stability as well as kinetic and allosteric properties suggests that MP-TI may lead to increased starch yield when expressed in monocot endosperms. 相似文献
995.
S. Rai Karamjit 《Journal of genetics》1999,78(3):163-169
Beginning in the mid-1950s, much progress has been made in studying various aspects of the genetics of mosquitoes, particularly involving several species of three principal genera,Aedes, Culex andAnopheles, that transmit important human diseases. Here I discuss selected areas of research involving formal genetics; genome structure, organization and evolution at the interspecific and intraspecific level; and evolutionary genetics of theAedes scutellaris group. Information and insights gained from in-depth analyses of these areas, particularly transmission genetics, cytogenetics and genetics of chromosomal rearrangements, and of mutagen-induced sexual sterility, have proved invaluable for the development of the theory and evaluation of feasibility of genetic control of natural populations. As a result, mosquitoes represent some of the best studied taxa at various levels of genetic organization. Recent developments in molecular genetics offer exciting possibilities for extension of these concepts. 相似文献
996.
997.
998.
Thirteen polymorphic microsatellite loci were developed for blowflies for use in studies of genetic differentiation in wild populations of Lucilia illustris, to detect the possible occurrence of bottlenecks and to study changes in genetic variation in laboratory populations of Lucilia sericata following artificial bottlenecks. In this preliminary study it was revealed that heterozygosity was lower than expected in wild populations and genetic variation had been lost in the laboratory population despite being kept at a large size. 相似文献
999.
1000.