全文获取类型
收费全文 | 12126篇 |
免费 | 809篇 |
国内免费 | 1533篇 |
出版年
2024年 | 36篇 |
2023年 | 164篇 |
2022年 | 185篇 |
2021年 | 317篇 |
2020年 | 417篇 |
2019年 | 412篇 |
2018年 | 412篇 |
2017年 | 374篇 |
2016年 | 493篇 |
2015年 | 499篇 |
2014年 | 706篇 |
2013年 | 1224篇 |
2012年 | 561篇 |
2011年 | 561篇 |
2010年 | 424篇 |
2009年 | 545篇 |
2008年 | 624篇 |
2007年 | 552篇 |
2006年 | 513篇 |
2005年 | 445篇 |
2004年 | 403篇 |
2003年 | 414篇 |
2002年 | 346篇 |
2001年 | 343篇 |
2000年 | 297篇 |
1999年 | 236篇 |
1998年 | 222篇 |
1997年 | 209篇 |
1996年 | 212篇 |
1995年 | 185篇 |
1994年 | 183篇 |
1993年 | 207篇 |
1992年 | 165篇 |
1991年 | 184篇 |
1990年 | 142篇 |
1989年 | 128篇 |
1988年 | 115篇 |
1987年 | 109篇 |
1986年 | 88篇 |
1985年 | 100篇 |
1984年 | 125篇 |
1983年 | 78篇 |
1982年 | 77篇 |
1981年 | 77篇 |
1980年 | 53篇 |
1979年 | 68篇 |
1978年 | 50篇 |
1977年 | 41篇 |
1976年 | 34篇 |
1973年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
51.
I. Papastylianou 《Plant and Soil》1987,104(1):23-29
The increasing need for protein at low cost has created a need to evaluate the biological nitrogen fixing potential of legumes
in Cyprus. In field studies which were conducted over the growing years of 1982–3 and 1983–4, legumes which are traditionally
grown in the country were evaluated for dry matter and nitrogen yield and biological nitrogen fixation (BNF). The legumes
studied were medic (Medicago truncatula Gearth), ochrus vetch (Lathyrus ochrus L.), bitter vetch (Vicia ervilia L.) and faba bean (Vicia faba L. var major) in the first year and in addition chickpea (Cicer arietinum L.), woollypod vetch (Vicia dasycarpa Ten.) and tickbean (Vicia faba L. var minor) in the second year.
Using the A-value method with barley and oats as reference crops, nitrogen (N) fixed by the various legumes in the first year
was 30–50% and from 55–67% of total N yield for the two reference crops, respectively. In the second year the estimates of
N fixed ranged from 70 to 80% with similar results obtained for the two reference crops barley and ryegrass. However, in the
second year chickpea, which had limited nodulation, fixed only 40% of its N yield.
Estimates of nitrogen from the atmosphere (Ndfa) obtained by the difference method (DM) were 10 to 14% lower than those from
the A-value method. These results were obtained after correcting for the amount of N derived from the applied fertilizer.
The two methods were highly correlated (r=0.98) for estimates of amount of BNF. The rates of N2 fixation of uninoculated legumes which are nodulated by the indigenous populations of Rhizobium in Cyprus are comparable
to those of legumes inoculated with selected strains of Rhizobium in other countries. An exception was the amount of N fixed
by chickpea. The appearance of the first nodules at late stages of growth may be the reason for the low BNF of this crop. 相似文献
52.
The fossil evidence of the “robust” australopithecines is reviewed with an emphasis on the taxonomic divisions and evolutionary relationships among this group of hominids. The hypodigms of A. robustus, A. crassidens and A. boisei are described, and the significance of morphological variation within and between these species is assessed. Phylogenetic relationships among the “robust” australopithecines are examined using maximum parsimony analysis, and evolutionary scenarios are evaluated in the light of recent discoveries in East Africa. 相似文献
53.
54.
Cladocera in space and time: Analysis of lake sediments 总被引:6,自引:6,他引:0
Wolfgang Hofmann 《Hydrobiologia》1987,145(1):315-321
Shells of Bosminidae and Chydoridae are quantitatively preserved in lake sediments. The chronological deposition of these
remains provides the means for longterm observation of these Cladocera, both in terms of species and communities.
Chydorid analysis, as based on subfossil assemblages, is an analysis of community and provides direct observation of community
dynamics over extended periods of time. It has proved to be a valuable method to obtain information on the influence of environmental
factors and time on community characteristics.
Morphological variation inBosmina (Eubosmina) has been followed for some thousand years. This is of special interest for the evaluation of taxonomic rank (species, forms)
if closely related taxa have co-existed.
Bosmina successions, as well as shifts in the chydorid fauna, are related to environmental change. Thus, cladoceran analysis of lake
sediments provides information on the developmental history of lakes and allows observation of the effects of longterm environmental
changes, such as climatic changes and eutrophication. 相似文献
55.
Carlos Eduardo Falavigna da Rocha Maria Helena Gonzaga de Carvalho Bjornber 《Hydrobiologia》1987,153(2):97-107
Cyclopid copepods collected mainly in aquatic microcosms and semiterrestrial habitats in the Juréia Ecological Reserve are studied. Hesperocyclops herbsti and Bryocyclops campaneri are described as new species and their taxonomical relationships discussed. Females of Muscocyclops operculatus (Chappuis) are redescribed and the males described for the first time. An emended diagnosis for Muscocyclops is proposed. 相似文献
56.
Intraspecific variability of Brachionus plicatilis 总被引:2,自引:1,他引:1
Minoru Sudzuki 《Hydrobiologia》1987,147(1):45-47
An extensive study of frontal margins of the lorica of Brachionus plicatilis was undertaken in an attempt to define its variability within this species. Specimens from mass cultures and from the natural environment were examined. 相似文献
57.
M. De Ridder 《Hydrobiologia》1987,150(2):123-131
The number of Rotifer taxa known from Mauritania, is raised from 8 to 90. 61% of these are cosmopolitan, 1 is temperate — boreal, 18% are thermophilic and 20% are tropicopolitan. Most of them are widely distributed in Africa, but some species have limited distributions. The presence of Keratella testudo and Wolga spinifera is remarkable and is probably related to the unusual climatic conditions in the SW Sahara. One phenotype or forma is new to science. 相似文献
58.
Ulrik Røen 《Hydrobiologia》1987,145(1):125-130
Chydorus arcticus n.sp. (Cladocera: Chydoridae: Chydorinae) is described, figured, and differentiated from the closely relatedC. sphaericus (O.F. Müller, 1785). The known distribution of the species is given, and some aspects of speciation of arctic crustaceans
are pointed out. 相似文献
59.
Lead mining in Wales originated before the Roman Occupation. The main active period was from 1750–1900 when zinc and copper were also mined and during this period only simple and inefficient ore processing methods were available. Consequently large amounts of copper, lead and zinc compounds were lost to the environment and have since become incorporated in sediments and soils. Locally, pollution may still occur from drainage from abandoned mines and by mobilization of mine tailings. This paper describes the present state of Welsh rivers and reviews the distribution of contaminants in sediments and soils. The uptake of heavy metals by plants and the consequences for human health are alto discussed. 相似文献
60.
Alvaro García-Tejedor A. Raul Castaño Federico Morán Francisco Montero 《Journal of molecular evolution》1987,26(4):294-300
Summary The most relevant properties of hypercycles were previously studied mainly from a theoretical point of view. We have developed a Monte Carlo method simulating hypercyclic organization to obtain information about the dynamics of this prebiotic organization. Nucleation, growth, and selective properties have been tested and the results obtained are in good agreement with those of the theoretical predictions. The influence of hypercyclic organization of the error threshold has also been studied. As a consequence of the emergence of a hypercycle, the value of this threshold decreases. The amount of this decrease depends on the population size. Moreover, for some interval of quality factor values, either the hypercycle organization or an error catastrophe can be produced, depending on the initial conditions. The influence of these phenomena on both the dynamic behavior and evolutionary advantages of the hypercycle, as well as their decisive roles on genome size, are discussed.Presented at the FEBS Symposium on Genome Organization and Evolution, held in Crete, Greece, September 1–5, 1986 相似文献