全文获取类型
收费全文 | 22942篇 |
免费 | 2117篇 |
国内免费 | 2541篇 |
出版年
2024年 | 56篇 |
2023年 | 477篇 |
2022年 | 468篇 |
2021年 | 826篇 |
2020年 | 887篇 |
2019年 | 996篇 |
2018年 | 856篇 |
2017年 | 753篇 |
2016年 | 930篇 |
2015年 | 941篇 |
2014年 | 1100篇 |
2013年 | 1793篇 |
2012年 | 1018篇 |
2011年 | 918篇 |
2010年 | 738篇 |
2009年 | 986篇 |
2008年 | 1108篇 |
2007年 | 1070篇 |
2006年 | 1056篇 |
2005年 | 975篇 |
2004年 | 908篇 |
2003年 | 864篇 |
2002年 | 825篇 |
2001年 | 763篇 |
2000年 | 712篇 |
1999年 | 554篇 |
1998年 | 475篇 |
1997年 | 413篇 |
1996年 | 436篇 |
1995年 | 366篇 |
1994年 | 390篇 |
1993年 | 383篇 |
1992年 | 344篇 |
1991年 | 266篇 |
1990年 | 256篇 |
1989年 | 215篇 |
1988年 | 182篇 |
1987年 | 165篇 |
1986年 | 121篇 |
1985年 | 191篇 |
1984年 | 161篇 |
1983年 | 93篇 |
1982年 | 94篇 |
1981年 | 92篇 |
1980年 | 59篇 |
1979年 | 68篇 |
1978年 | 46篇 |
1977年 | 45篇 |
1976年 | 34篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
91.
For three acid soils from Santa Catarina, Brazil, lime application and time of incubation with lime had little effect on the
adsorption of added phosphorus. In two soils with high contents of exchangeable aluminium, solution P and isotopically exchangeable
P were decreased by incubating with lime for 1 month: phosphorus was probably adsorbing on freshly precipitated aluminium
hydrous oxides. In one soil with less exchangeable aluminium, P in solution was increased by liming. After 23 months lime
increased solution and exchangeable P possibly due to crystallization of aluminium hydrous oxides reducing the number of sites
for P adsorption. All these changes were however small.
In a pot experiment, lime and phosphorus markedly increased barley shoot and root dry matter and P uptake. Although liming
reduced P availability measured by solution P, isotopically exchangeable P and resin extractable P, it increased phosphorus
uptake by reducing aluminium toxicity and promoting better root growth. The soil aluminium saturation was reduced by liming,
but the concentration of aluminium in roots changed only slightly. The roots accumulated aluminium without apparently being
damaged. 相似文献
92.
Five plant species were cultivated on a soil from the Neckar alluvial fan near Heidelberg (FRG) polluted by the emissions
of a cement plant. Thallium, cadmium and lead concentrations in seedlings and mature plants were determined by atomic absorption
analysis. AdditionallyBrassica napus L.napus was grown on soils containing 5 different concentrations of heavy metals, achieved by mixing two similar soils, from the
same area but with different metal concentrations.
Thallium and cadmium were shown to be taken up by roots whilst lead which was also absorbed, was deposited mainly on the plant
surface. However during cultivation in the winter months, a remarkable deposit of lead via the roots was found. Thallium in
the soil from a anthrorogen source was more available to plants than thallium of geological origin.
During the lifetime of a plant concentrations of thallium and cadmium were always highest in the seedling. The decrease in
metal concentration with maturity depended on the plant species and the element, but was not a function of the metal concentration
in the soil. 相似文献
93.
Floral determination in the terminal bud of the short-day plant Nicotiana tabacum cv. Maryland Mammoth has been investigated. Plants grown continuously in short days flowered after producing 31.4±1.6 (SD) nodes while plants grown continuously in long days did not flower and produced 172.5±9.5 nodes after one year. At various ages, expressed as number of leaves that were at least 1.0 cm in length above the most basal 10-cm leaf, one of three treatments was performed on plants grown from seed in short days: 1) whole plants were shifted from short days to long days, 2) the terminal bud was removed and then rooted and grown in long days, and 3) the terminal bud was removed and then rooted and grown in short days. Whole plants flowered only when shifted from short days to long days at age 15 or later. Only rooted terminal buds from plants at age 15 or older produced plants that flowered when grown in long days. Only terminal buds from plants at age 15 or older that were rooted and grown in short days produced the same number of nodes as they would have produced in their original locations while buds from younger plants produced more nodes than they would have in their original locations. Thus, determination for floral development in the terminal bud, as assayed by rooting, is simultaneous with the commitment to flowering as assayed by shifting whole plants to non-inductive conditions.Abbreviations LD
long day(s)
- SD
short day(s)
- DN
dayneutral 相似文献
94.
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
95.
96.
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
97.
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
98.
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
99.
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
100.
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