首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   122422篇
  免费   7431篇
  国内免费   18篇
  2022年   780篇
  2021年   1708篇
  2020年   1429篇
  2019年   1598篇
  2018年   3117篇
  2017年   2759篇
  2016年   3799篇
  2015年   4980篇
  2014年   5267篇
  2013年   6925篇
  2012年   7999篇
  2011年   7206篇
  2010年   4789篇
  2009年   3902篇
  2008年   5803篇
  2007年   5581篇
  2006年   5488篇
  2005年   4599篇
  2004年   4618篇
  2003年   4120篇
  2002年   3820篇
  2001年   3138篇
  2000年   2965篇
  1999年   2383篇
  1998年   1144篇
  1997年   853篇
  1996年   854篇
  1995年   810篇
  1994年   722篇
  1992年   1481篇
  1991年   1370篇
  1990年   1287篇
  1989年   1352篇
  1988年   1158篇
  1987年   1167篇
  1986年   1070篇
  1985年   1107篇
  1984年   916篇
  1983年   813篇
  1982年   671篇
  1979年   948篇
  1978年   692篇
  1975年   784篇
  1974年   864篇
  1973年   831篇
  1972年   759篇
  1971年   677篇
  1970年   729篇
  1969年   790篇
  1968年   739篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Summary Our field surveys conducted in 10 Hungarian grape producing areas have revealed that the nutrient contents of vine leaves of different grape varieties were closely correlated with the EUF-nutrient contents of different soil types with different contents of clay. Resulting from these relationships the following EUF-nutrient values are considered as required for optimal nutrition of the vine-stock to attain grape yields of 10–12 t/ha in the areas under investigation:  相似文献   
992.
Although the seed coat, through its thickness and permeability, often regulates seed germination, very little is known about the control of its development. Using soybean (Glycine max [L.] Merrill) explants, podbearing cuttings in which defined solutions can be substituted for the roots, we have demonstrated that cytokinin and mineral nutrients moving through the xylem can control soybean seed coat development. Lack of cytokinin and minerals in the culture solution, causes a thicker, less permeable seed coat to develop. The seeds with thickened coats will imbibe water rapidly if scarified; furthermore, these scratched seeds also germinate and produce normal plants. Inasmuch as stress (e.g. drought) decreases mineral assimilation and cytokinin production by the roots, the resulting delay in germination could be an adaptive response to stress.  相似文献   
993.
The distribution of the two glutamate-synthase (GOGAT) activities known to exist in higher plants (NADH dependent, EC 2.6.1.53; and ferredoxin dependent, EC 1.4.7.1) was studied in non-chlorophyllous and chlorophyllous cultured tissue as well as in young leaves of Bouvardia ternifolia. The NADH-GOGAT was present in all three tissues. Using a sucrose gradient we found it in both the soluble and the plastid fraction of non-chlorophyllous and chlorophyllous tissue, but exclusively in the chloroplast fraction of the leaves. Ferredoxin-GOGAT was found only in green tissues and was confined to the chloroplasts. Ferredoxin-GOGAT activity increased in parallel with the chlorophyll content of the callus during the greening process in Murashige-Skoog medium (nitrate and ammonium as the nitrogen sources), while NADH-GOGAT was not affected by the greening process in this medium. Furthermore, both activities were differentially affected by either nitrate or ammonium as the sole nitrogen source in the medium during this process. It is suggested that each GOGAT activity is a different entity or is differently regulated.Abbreviations GOGAT glutamate synthase - MS Murashige-Skoog (1962) medium - PMSF phenylmethylsulfonyl fluoride  相似文献   
994.
The expression of specific and non-specific rat liver messenger RNAs has been studied during 3'-methyl-4-(dimethylamino)azobenzene (3'-MeDAB) carcinogenesis, using cDNA probes complementary to mRNAs encoding aldolase A and B, L-type pyruvate kinase, albumin, alpha-fetoprotein, transferrin and an unidentified 2.7 X 10(3)-base mRNA. mRNAs specific for undifferentiated cells, such as those encoding aldolase A and the unidentified 2.7 X 10(3)-base species were re-expressed very early, being easily detectable at the 1st week of 3'-MeDAB treatment. They reached a maximum of expression at the 4th week. Simultaneously the levels of aldolase B and L-type pyruvate kinase mRNAs dramatically decreased as compared to controls, but remained responsive to induction by a high-carbohydrate diet. Albumin and transferrin mRNA levels were only slightly modified in the course of the carcinogenic diet. At the terminal stage of hepatocarcinogenesis, i.e. in malignant hepatoma cells, expression and inducibility of aldolase B and L-type pyruvate kinase mRNAs were similar to those in normal adult rats while mRNAs specific for undifferentiated or foetal stages were also synthesized. The very early changes in gene expression for aldolases A and B, L-type pyruvate kinase and the 2.7 X 10(3)-base mRNA species could indicate that carcinogenic diet modifies gene control mechanisms long before inducing hepatoma.  相似文献   
995.
14-d-old conventional piglets were picked from normal piggery, washed with disinfectants, placed into isolators suitable for germfree work, fed a sterile diet and treated with peroral antibiotics (nalidixic acid, kanamycin, and nystatin). Beginning with day 5 or 7, Enterobacteriaceae were not found in feces. The absence of these bacteria was proved by inoculation of germfree newborn piglets with caecal content. In selectively decontaminated piglets, the white blood cell count in blood had fallen to 6 X 10(9)/L; this decrease was due to an extremely low number of granulocytes (to 0.8 X 10(9)/L). On day 35, IgG-positive cells almost disappeared from the spleen, whereas IgA cells were found in an unusually great amount. Corresponding changes in serum levels were established. The colonization resistance effect in Enterobacteriaceae-deprived piglets was confirmed; settling of introduced various E. coli strains did not occur or was delayed.  相似文献   
996.
Mathematical modelling of the course of the immune response is undoubtedly one of the most progressive and most promising areas of modern immunology. Mathematical models (along with computer programs) can be taken as "the only means of thoroughly testing and examining a large and intricate theory" (Partridge et al. 1984). The first phase of construction of mathematical models is the formulation of assumptions based on the knowledge of the facts to be modelled (manifested usually in a scheme of the presumed course of the modelled process). The first mathematical models of immune response were based on the hypothesis of a two-stage differentiation of cells participating in the humoral response, published in Prague 23 years ago (Sercarz and Coons 1962; Sterzl 1962) and illustrated by the X----Y----Z scheme. Many contemporary mathematical models still stem from this scheme which undoubtedly fits the fundamental data concerning the immune system.  相似文献   
997.
The activities of lysozyme, myeloperoxidase, and elastase were lower in PMNs and AMs from GF and AF Minnesota miniature piglets than in the leukocytes from their CONV counterparts. In the spleen and serum of gnotobiotic piglets only the levels of lysozyme were slightly reduced. Substantially depressed activities of these LEs were found also in PMNs from precolostral piglets in comparison with PMNs from their CONV mother. The bisassociation of GF piglets with Enterococcus liquefaciens and Escherichia coli caused an increase of LE activities in their AMs, spleens, and sera. Fewer LEs were released after phygocytic stimulation with zymosan from PMNs of GF, AF, and precolostral piglets than from PMNs of CONV animals of the same age. These data suggest that the antigenic-microbial stimulation is important for the development of normal lysosomal enzyme activities in PMNs and AMs from gnotobiotic animals.  相似文献   
998.
Changes in the number of bacteria and relative distribution of strains producing poly-3-hydroxybutyrate (PHB) in soil were investigated. Samples of chernozem soil were cul tivated with glucose in the presence of a mineral nitrogen source (diammonium hydrogen phos phate) or in its absence, either in a batch or a heterocontinuous cultivation system. In both cultivation systems the addition of glucose resulted in a roughly ten-fold increase of bacteria concentration and an increase in the proportion of strains able to produce PHB granules. When the nitrogen source was added simultaneously with glucose, the concentration of bacteria increased by two orders of magnitude in both cultivation systems. In the batch system changes in the concentration of strains capable of PHB production were very small under these con ditions whereas in the heterocontinuous system their number decreased by almost 50 %. The survival of bacteria in the soil suspension after 57-d starvation was associated with PHB production which differed, depending on the previous treatment of the soil samples. The concentration of bacteria decreased least pronouncedly in the control with water and most significantly during cultivation with glucose and a nitrogen source, where the initial PHB content was very low in spite of high numbers of bacteria.  相似文献   
999.
The presence ofa allotypic determinants was tested in fractions obtained by gel filtration of antigen-specific receptors isolated by immunoadsorption from lymphoid cells of antigen-stimulateda3-3 rabbits. This technique, as well as the inhibition of the reaction of isolated receptors with anti-T cell receptor antisera (anti R) by anti-a3 antibodies failed to demonstrate the presence of a allotypie determinants. The inhibitory effect of antigen-specific receptors isolated from the lymphoid cells of stimulated A/J mice on the cytotoxic effect of anti-Ia antibodies on mouse spleen cells in the presence of rabbit complement was tested. All preparations inhibited the cytotoxic reaction with the average effectivity of 60%. In order to confirm the presence of Ia determinants on the rabbit and mouse T cell receptor molecules it was shown that the reactions of three anti-R antisera with 12 different receptor preparations were inhibited by anti-la antibodies. SDS-PAGE analyses of125I-labelled mouse specific receptors and the precipitate obtained by anti-R antisera showed that T cell receptors were present in fractions with molar mass 100 and 85 kg/mol. The molar mass of the former fraction after reduction and alkylation was 45 kg/mol.  相似文献   
1000.
Agrobacterium tumefaciens C58 forms “plaques” during layer cultivation. The “plaques” were shown not to be caused by the presence of a temperate bacteriophage or by random contamination. The “plaques” and their central microcolonies were used to repeatedly isolate cultures producing an antibiotic substance against the original strainA. tumefaciens C58, other nopaline strains, some octopine strains ofA. tumefaciens and some strains of the related genusRhizobium. The substance is thus a bacteriocin; in analogy to agrocins 84 and D286 it was named agrocin C58. The agrocin is not inactivated by trypsin. Its production by strain C58 was found only on cultivation on solid but not liquid media. The producing isolate ofA. tumefaciens C58 (strain C58i2) contains neither plasmid pTiC58 nor the plasmid analogous to pAgK84 which controls the production of agrocin 84 inA. radiobacter K84.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号