全文获取类型
收费全文 | 76248篇 |
免费 | 5684篇 |
国内免费 | 8265篇 |
出版年
2023年 | 1379篇 |
2022年 | 1777篇 |
2021年 | 2103篇 |
2020年 | 2141篇 |
2019年 | 3123篇 |
2018年 | 2642篇 |
2017年 | 2405篇 |
2016年 | 2338篇 |
2015年 | 2381篇 |
2014年 | 3800篇 |
2013年 | 4974篇 |
2012年 | 2932篇 |
2011年 | 3596篇 |
2010年 | 2811篇 |
2009年 | 3667篇 |
2008年 | 3816篇 |
2007年 | 4001篇 |
2006年 | 3642篇 |
2005年 | 3138篇 |
2004年 | 2749篇 |
2003年 | 2571篇 |
2002年 | 2297篇 |
2001年 | 1920篇 |
2000年 | 1597篇 |
1999年 | 1534篇 |
1998年 | 1349篇 |
1997年 | 1273篇 |
1996年 | 1200篇 |
1995年 | 1209篇 |
1994年 | 1179篇 |
1993年 | 991篇 |
1992年 | 966篇 |
1991年 | 947篇 |
1990年 | 727篇 |
1989年 | 734篇 |
1988年 | 650篇 |
1987年 | 635篇 |
1986年 | 579篇 |
1985年 | 848篇 |
1984年 | 1080篇 |
1983年 | 841篇 |
1982年 | 899篇 |
1981年 | 707篇 |
1980年 | 722篇 |
1979年 | 639篇 |
1978年 | 498篇 |
1977年 | 449篇 |
1976年 | 411篇 |
1974年 | 264篇 |
1973年 | 273篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Interspecific relations of African crocidurine shrews (Mammalia: Soricidae) based on multivariate analysis of mandibular data 总被引:1,自引:0,他引:1
P. M. BUTLER R. S. THORPE F.L.S. M. GREENWOOD 《Zoological Journal of the Linnean Society》1989,96(4):373-412
Eighteen graded non-metric characters and 30 measurements of the mandible and lower dentition were used to investigate the grouping of African species of the genera Crocidura, Suncus and Sylvisorex. Seventy-seven taxa (species, subspecies and synonyms), including four European species, were represented by one to nine mandibles (147 mandibles in all), and the means of four Pleistocene African species were also included. The metric data were subjected to principal component and canonical variate analyses, and the non-metric data to principal component analysis. Grouping was detected by comparing the results of the three analyses. Six groups are recognized within the genus Crocidura , typified by C. fumosa, C. turba, C. russula, C. fuscomurina, C. hirta and C. flavescens. Sylvisorex is divisible into two groups, typified by S. lunaris and S. granti. The African species of Suncus (but not the introduced species, S. murinus ) form a single group, which is linked to the Sylvisorex granti group. Suncus is closely related to, or convergent with, the Crocidura fuscomurina group. The first non-metric principal component, the second metric principal component and the second canonical variate are significantly correlated. It is considered that these together represent a general trend of evolutionary advance. Phylogenetic relations are discussed in the light of this; Crocidura may be diphylefic. 相似文献
952.
M. N. B. NAIR H. Y. MOHAN RAM 《Botanical journal of the Linnean Society. Linnean Society of London》1989,100(4):323-336
NAIR, M. N. B. AND MOHAN RAM, H. Y., 1989. Vestured pits and vestured vessel member walls in some Indian dicotyledonous woods. The woods of 144 taxa belonging to 38 families of angiosperms were examined for vestured pits and vestured vessel member walls using scanning electron microscopy. Vestured pits are present in 66 taxa (belonging to ten families) and vestured vessel member walls occur in only six taxa (belonging to three families). In Ehretiaceae and Euphorbiaceae vestures are present only in certain vessel members. In Wrighlia tinctoria , perforation plates containing vestures have been observed in addition to the presence of vestured pits. A classification of vestured pits based on their morphology and distribution is proposed by us. In all the types of vestured pits, vestures are present on the margin of the outer pit aperture or on the pit chamber wall. Occasionally, they are present in the pit canal, on the margin and in the vicinity of the inner pit aperture and rarely over the inner walls of the vessel members. The functions of vestured pits are not clear, although several suggestions are made. Whether or not these structures affect wood processing is not presently understood. It appears that vestured pits and vestured vessel member walls have diagnostic rather than phylogenetic value. 相似文献
953.
对我国52种微茎类吸虫的18项成虫形态学特征进行主成分分析,结果表明:卵巢位置、子宫延伸位置等7项性状对第一主成分贡献较大,提示描述器官位置的指标是重要的分类依据。52个虫种在前三个主成分上的排序图显示应将其划分成4个亚科。 相似文献
954.
955.
Using data for 27 Texas counties from 1978-1987, it is shown that the incidence rates of suicide, homicide, and rape are significantly higher in counties whose drinking water supplies contain little or no lithium than in counties with water lithium levels ranging from 70-170 micrograms/L; the differences remain statistically significant (p less than 0.01) after corrections for population density. The corresponding associations with the incidence rates of robbery, burglary, and theft were statistically significant with p less than 0.05. These results suggest that lithium has moderating effects on suicidal and violent criminal behavior at levels that may be encountered in municipal water supplies. Comparisons of drinking water lithium levels, in the respective Texas counties, with the incidences of arrests for possession of opium, cocaine, and their derivatives (morphine, heroin, and codeine) from 1981-1986 also produced statistically significant inverse associations, whereas no significant or consistent associations were observed with the reported arrest rates for possession of marijuana, driving under the influence of alcohol, and drunkenness. These results suggest that lithium at low dosage levels has a generally beneficial effect on human behavior, which may be associated with the functions of lithium as a nutritionally-essential trace element. Subject to confirmation by controlled experiments with high-risk populations, increasing the human lithium intakes by supplementation, or the lithiation of drinking water is suggested as a possible means of crime, suicide, and drug-dependency reduction at the individual and community level. 相似文献
956.
Versieck Jacques Vanballenberghe Lidia Wittoek Ann Vermeir Gerda Vandecasteele Carlo 《Biological trace element research》1990,(1):683-689
A method is described for the determination of mercury in human blood serum and packed blood cells employing neutron activation
analysis. Great attention was devoted to the collection and manipulation of the samples. The accuracy and precision of the
method were tested by analyzing biological reference materials and by comparing the concentrations measured in a number of
serum samples to those obtained by another, independent technique (cold vapor atomic absorption spectrometry) in the same
samples. The article reports the levels measured in blood serum and packed blood cells samples from 15 adult volunteers, as
well as the figures determined in a “second-generation” biological reference material (freeze-dried human serum), prepared
and conditioned at the University of Ghent. 相似文献
957.
Multivariate statistical analysis has been applied to time series measurements of aerosol elemental composition from PIXE
analysis of filter samples, and principal components have been resolved that represent distinct particle types in an external
mixture in the atmosphere. In this study, it is argued that a combination of chemical and statistical analyses of the data
may be more powerful in determining chemical species in atmospheric aerosols than studies that employ mainly direct chemical
analysis of chemical species in unresolved mixtures of aerosol particle samples. Sulfur is generally associated with mineral
dust elements. It is reasoned that the association may represent sulfuric acid coatings on particles that can lead to mineral
dissolution and solubilization of significant amounts of aluminum, iron, and other metals.
Upon wet or dry deposition to the surface, the fluxes of these metals in biologically-available form may be sufficient to
affect primary productivity in the world ocean and cause ecological damage in lakes. As a consequence, the fluxes of biogenic
trace gases to the atmosphere may be changed, possibly leading to changes in the tropospheric concentration of ozone. The
inputs to lakes of soluble aluminum, which is toxic to fish, may be partly by deposition directly from the atmosphere, thus
not limited to leaching of soils by acid deposition. Human inhalation of soluble aluminum and other solulilized mineral metals
may account, in part, for the observed geographic pattern of deaths attributed to chronic obstructive pulmonary disease (COPD)
that show high rates in cities of the Western US and the southeast region, but low in most of the midwest and northeast. 相似文献
958.
Krishnan S. S. McNeill K. G. Mernagh J. R. Harrison J. E. 《Biological trace element research》1990,(1):415-421
Two new facilities for in vivo activation analysis of patients have been designed, developed, and constructed at Toronto General
Hospital. One of these is for the determination of body calcium for the diagnosis of osteoporosis and other diseases associated
with bone loss. The other is for the measurement of total body nitrogen for the determination of protein status.
These facilities replace old university facilities and take into account the comfort and management of patients. In addition,
in the case of the calcium facility, the precision of the measurements has been improved because of larger detector volume
and increased neutron source strength. Both the facilities are now in routine hospital clinical use. 相似文献
959.
Edward J. B. Beeley P. A. Bennett L. G. I. Poland J. S. 《Biological trace element research》1990,(1):53-61
A microcomputer-controlled irradiation and measurement system and a microprocessor-controlled sample changer have been installed
at the SLOWPOKE-2 Facility at the Royal Military College of Canada (RMC). These systems can provide the gamut of instrumental
neutron activation analysis (INAA) techniques for the analyst. Custom software has been created for system control, data acquisition,
and off-line spectral analysis using programs that incorporate Gaussian peak-fitting methods of analysis. The design and use
of the equipment is discussed, and the performance is illustrated with results obtained from the analysis of marine sediment
and biological reference materials. 相似文献
960.
Gerhard Sandmann M. Luisa Peleato Maria F. Fillat M. Carmen Lázaro Carlos Gómez-Moreno 《Photosynthesis research》1990,26(2):119-125
Iron-dependent formation of ferredoxin and flavodoxin was determined in Anabaena ATCC 29413 and ATCC 29211 by a FPLC procedure. In the first species ferredoxin is replaced by flavodoxin at low iron levels in the vegetative cells only. In the heterocysts from Anabaena ATCC 29151, however, flavodoxin is constitutively formed regardless of the iron supply.Replacement of ferredoxin by flavodoxin had no effect on photosynthetic electron transport, whereas nitrogen fixation was decreased under low iron conditions. As ferredoxin and flavodoxin exhibited the same Km values as electron donors to nitrogenase, an iron-limited synthesis of active nitrogenase was assumed as the reason for inhibited nitrogen fixation. Anabaena ATCC 29211 generally lacks the potential to synthesize flavodoxin. Under iron-starvation conditions, ferredoxin synthesis is limited, with a negative effect on photosynthetic oxygen evolution. 相似文献