全文获取类型
收费全文 | 31697篇 |
免费 | 2621篇 |
国内免费 | 3696篇 |
专业分类
38014篇 |
出版年
2024年 | 121篇 |
2023年 | 685篇 |
2022年 | 819篇 |
2021年 | 985篇 |
2020年 | 1069篇 |
2019年 | 1421篇 |
2018年 | 1111篇 |
2017年 | 1176篇 |
2016年 | 1227篇 |
2015年 | 1297篇 |
2014年 | 1678篇 |
2013年 | 2270篇 |
2012年 | 1273篇 |
2011年 | 1547篇 |
2010年 | 1250篇 |
2009年 | 1656篇 |
2008年 | 1731篇 |
2007年 | 1698篇 |
2006年 | 1581篇 |
2005年 | 1451篇 |
2004年 | 1302篇 |
2003年 | 1147篇 |
2002年 | 1050篇 |
2001年 | 825篇 |
2000年 | 700篇 |
1999年 | 757篇 |
1998年 | 593篇 |
1997年 | 560篇 |
1996年 | 508篇 |
1995年 | 507篇 |
1994年 | 487篇 |
1993年 | 379篇 |
1992年 | 350篇 |
1991年 | 339篇 |
1990年 | 273篇 |
1989年 | 248篇 |
1988年 | 229篇 |
1987年 | 210篇 |
1986年 | 170篇 |
1985年 | 198篇 |
1984年 | 208篇 |
1983年 | 140篇 |
1982年 | 170篇 |
1981年 | 122篇 |
1980年 | 122篇 |
1979年 | 104篇 |
1978年 | 83篇 |
1977年 | 45篇 |
1976年 | 40篇 |
1974年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Many biomedical experiments require the qualitative and quantitative localization of trace elements with high sensitivity
and good spatial resolution. The feasibility of measuring the chemical form of the elements, the time course of trace element
metabolism, and conducting experiments in living biological systems are also important requirements for biological trace element
research. Nuclear analytical techniques that employ ion or photon beams have grown in importance in the past decade and have
led to several new experimental approaches. Some of the important features of these methods are reviewed here along with their
role in trace element research. Examples of their use are given to illustrate potential for new research directions. It is
emphasized that the effective application of these methods necessitates a closely integrated multidisciplinary scientific
team. 相似文献
92.
Paternal inheritance of chloroplast DNA in Larix 总被引:2,自引:0,他引:2
Restriction enzyme analysis was used to determine the inheritance of chloroplast DNA in conifers. The plant material studied included five individual trees of European larch (Larix decidua Mill.) and Japanese larch (Larix leptolepis Sieb. & Zucc.) and six hybrids from controlled crosses between these species. The chloroplast DNA fragment patterns generated by Bam-HI and Bcl-I were species-specific. Paternal inheritance of chloroplast DNA patterns was found in most Larix crosses. One hybrid showed maternal chloroplast DNA patterns. In addition, two other hybrids had mixed Bam-HI patterns suggesting recombination between maternal and paternal chloroplast DNA. The mechanisms favoring paternal inheritance in conifers are not known. Paternal inheritance of chloroplast DNA is suggested it to be a general phenomenon in conifers. 相似文献
93.
94.
D. GODFREY J. N. LYTHGOE† ‡ D. A. RUMBALL § 《Biological journal of the Linnean Society. Linnean Society of London》1987,32(4):427-433
Some striped animals are camouflaged in their natural environment, whereas others are conspicuous. Mammals are known to have spatial frequency analysers in their visual mechanism, and it is suggested that the spatial characteristics of a striped pattern are different in camouflaged and conspicuous animals. Fourier analysis of the stripes of the zebra shows spatial frequencies in the pattern that are unlikely to be present so strongly in their natural background scene. A similar analysis of the camouflaging stripes of a tiger show that the distribution of spatial frequencies are similar to that in the background scene. 相似文献
95.
Differential thermal analysis indicated that the frost resistance of winter rape leaves ( Brassica napus L. var. oleifera L. cv. Gòrczanski), collected from plants grown in the cold (5/2°C), relies mainly on their ability to supercool to −9 to −11°C, i.e. consists in freezing avoidance. Initiation of ice formation in the cold-acclimated leaves resulted in the death of more than 50% of the cells as determined with a conductivity method. The development of freezing tolerance appeared to be an attribute of the second stage of plant hardening and was induced by the exposure of plants to a slightly subzero temperature (−5°C) for 18 h. Such a treatment brought about a sudden and persistent water potential decrease in the leaves, despite the fact that they had reabsorbed water from the medium prior to water potential measurements. Water potential changes were associated with a higher growth capability of the leaves as checked by determinations of disk area increments. It is suggested that the increased frost tolerance of the cold-grown winter rape leaves, subjected to subfreezing temperature, is related to the decreased water potential of the tissue caused by changes in turgor and/or in osmotic pressures of the cells. 相似文献
96.
The effect of varied supply of P (2.5× 10−5 to 6× 10−4 M) and Zn (0 to 10−6 M) on uptake and concentrations of P and Zn was studied in cotton ( Gossypium hirsutum L. cv. Deltapine 15/21) grown in nutrient solution under controlled environmental conditions. At a given Zn supply, increasing levels of P had no significant effect on the concentrations of total Zn in plants. However, increasing levels of P induced or enhanced visual Zn deficiency symptoms when the Zn concentration in the nutrient solution was low. The concentrations of water-soluble Zn in roots and shoots constituted 60% of the total Zn concentrations for plants grown with low P and 30% for plants grown with high P. The concentration of water-soluble Zn in leaves, but not total Zn, was closely correlated with visual Zn deficiency symptoms, levels of chlorophyll, super oxide dismutase and membrane permeability. The critical deficiency concentration of water-soluble Zn in cotton leaves was in the range of 6 to 7 μg (g dry weight)−1 or about 1.0 μg (g fresh weight)−1 . The results show that high P concentrations in plant tissue decrease the physiological availability of Zn. Water-soluble Zn in the tissue appears to be a suitable indicator for Zn nutritional status in general and phosphorus-induced Zn deficiency in particular. Also in field-grown orange trees (Citrus sinensis) visual Zn deficiency symptoms in leaves were closely related to the concentration of water-soluble Zn. 相似文献
97.
Genetic diversity within and among 11 juvenile populations of green alder (Alnus crispa) in Canada 总被引:1,自引:0,他引:1
Germinated seeds from 11 populations of green alder [ Alnus crispa (Ait.) Pursh] sampled in four Canadian provinces were analysed for electrophoretically demonstrable diversity of 10 enzymes encoded by 15 structural loci. Of these, nine were polymorphic, and on average, 52% of the loci per population were polymorphic. Assuming a diploid model of expression, average level of expected heterozygosity was 0.11 with nearly all populations in Hardy-Weinberg equilibrium for the set of polymorphic loci analysed. No significant inbreeding and associated subpopulation structuring were noted. Rates of gene flow appeared high within and among populations. Although little divergence was observed among populations, genetic and geographical distances between populations were related. Discriminant and cluster analyses revealed a pattern of genetic variation associated with geography. Populations from northern Quebec were poorly differentiated, whereas western populations from Alberta exhibited a larger degree of genetic differentiation. Introgresive hybridization with the sympatric species Alnus sinuata (Regel) Rydberg and partial isolation in the West are suggested as an explanation for this larger differentiation. The occurrence and significance of rare alleles is discussed in relation to the importance of geographical distance in the process of population differentiation in this species. 相似文献
98.
Summary Investigations into iron deficiency have been hindered by the lack of a satisfactory diagnostic tissue test, which in turn
results from the total iron content of plant tissue commonly being an unreliable index of the iron status. Our measurements
of chlorotic and normal leaves of field grown groundnut (Arachis hypogaea L.) showed that total iron was unsatisfactory as the measure of iron status of plant tissue. It was found that iron status
was better assessed from an estimate of the ferrous iron content of fresh leaf materials obtained by extraction with o-phenanthroline.
Extractable iron content increased with leaf age. Chlorotic buds or the first fully opened leaf always contained less than
6μg extractable-Fe/g fresh tissue.
Approved for publication as ICRISAT Journal Article No. 307. 相似文献
99.
H.-R. Roth G. Dolf G. Stranzinger 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,74(1):42-48
Summary A statistical approach to the interpretation of data from gene assignment with somatic cell hybrids is presented. The observed data are analysed under a variety of hypotheses. The fit to the hypotheses is compared by means of the likelihood obtained under a given hypothesis. Two of these hypotheses are related to fundamental questions: is a gene responsible for the enzyme observation and if so, is that gene located on a specific chromosome or could it change its position and be sometimes on chromosome j and, in another hybrid line, on chromosome k? The other hypotheses concern the assignment of the gene to just one of the chromosomes.To improve the traditional data analysis approach we considered additional information: the uncertainties and possible errors of laboratory methods in all our calculations and the length of the donor chromosomes in connection with one specific hypothesis.This method allows us to account for the reliability of the investigation methods and the nature of the hybrid lines involved. Data can be evaluated at different error probabilities within a realistic range in order to compare and discuss results. 相似文献
100.
A. Lebeda T. Jendrulek 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,75(1):194-199
Summary In gene-for-gene systems, specificity of hostparasite interactions is most often estimated qualitatively using the symbols +, –, (i.e. susceptibility and/or resistance). In large sets (interaction patterns) it becomes impossible to analyze numerous data by mere comparison. This is overcome by application of cluster analysis. In our experiments the methods in question were used to estimate the data obtained in a study on interactions between more than 220 Lactuca sativa cultivars and 12 Bremia lactucae physiological races (isolates) of Czechoslovak origin. The matrix of similarity coefficients was analyzed by hierarchical clustering. Similarity and/or dissimilarity of host R-genotypes was graphically expressed using the method of two principal components. The results obtained are related to genetic constitution of race specific resistance of the host and the possibility of predicting effective resistance sources. 相似文献