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   820766篇
  免费   85117篇
  国内免费   260篇
  906143篇
  2018年   8254篇
  2017年   7843篇
  2016年   11133篇
  2015年   14241篇
  2014年   16891篇
  2013年   24225篇
  2012年   27304篇
  2011年   27925篇
  2010年   19023篇
  2009年   17629篇
  2008年   24974篇
  2007年   25874篇
  2006年   24177篇
  2005年   23263篇
  2004年   22963篇
  2003年   22159篇
  2002年   21628篇
  2001年   35239篇
  2000年   34727篇
  1999年   28048篇
  1998年   10480篇
  1997年   10537篇
  1996年   10085篇
  1995年   9286篇
  1994年   8957篇
  1993年   8995篇
  1992年   22752篇
  1991年   22311篇
  1990年   21735篇
  1989年   21202篇
  1988年   19469篇
  1987年   18697篇
  1986年   17394篇
  1985年   17303篇
  1984年   14261篇
  1983年   12445篇
  1982年   9516篇
  1981年   8610篇
  1980年   7992篇
  1979年   13233篇
  1978年   10471篇
  1977年   9435篇
  1976年   9022篇
  1975年   10038篇
  1974年   10766篇
  1973年   10628篇
  1972年   9693篇
  1971年   8660篇
  1970年   7573篇
  1969年   7454篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
A 30-33 kDa electroeluted fraction of T. gondii tachyzoites improved discrimination between acute and chronic phase sera when used instead of the whole tachyzoite extract in an avidity-ELISA. In order to identify the components of these fractions, crude tachyzoite antigen was fractionated by anionic exchange chromatography. The 30-33 kDa antigen cluster eluted in the not-bound fraction could account for a large proportion of the antibody response against the 30-33 kDa electroeluted fraction. According to the N-terminal sequence data, this antigen fraction is composed mainly of SAG1 and another protein with high homology to chitin binding proteins from plants.  相似文献   
52.
The fraction of free fatty acids (FFA) is present in skeletal muscles. However, there is almost no data regarding regulation in the content of this intramuscular lipid pool. We took advantage of the isolated muscle preparation to examine whether: a) increasing exogenous concentration of FFA (500microM or 700microM, 30min) b) insulin (10.00 I.U./L, 30min), c) adrenalin (4.4 nM, 30 min), or d) contractions (200ms, tetani, 1Hz, 30min), affect the FFA content inside myocytes. Incubation of soleus (S) and extensor digitorum longus (EDL) with increasing concentrations of exogenous FFA (from 500microM to 700microM) resulted in an increase in the total FFA fraction in both muscles studied (by 280.2% and 259.1%, respectively). In contracting muscles FFA pool was significantly reduced both in S (by 73.1%) and in EDL (by 31.1%). Neither stimulation by adrenalin nor insulin affected the total content of FFA fraction in the muscles examined. We conclude that a) increased availability of exogenous FFA at the sarcolemma level results in an increase in the size of intramuscular FFA fraction b) the intracellular FFA fraction is utilized by contracting muscles with regard to the fiber composition and to a greater extent in more oxidative muscles, c) FFA fraction remains stable upon stimulation by insulin or adrenalin.  相似文献   
53.
54.
Confining an adult hen with two newly-hatched chicks induced a gradual emergence of maternal behaviour in the hen. The aim of the experiments presented here was to analyse the effects of testosterone or oestradiol treatments on the induction and maintenance of maternal behaviour in hens with no previous maternal experience. The ability of a hen to positively respond towards chicks was not altered by either testosterone or oestradiol injections. However, testosterone therapy prevented both the appearance and the maintenance of the most typical call, clucking. Testosterone injected hens responded to chicks with a contact-type call, but oestradiol treated ones did not. These results indicate that some aspects of maternal behaviour can be modulated by peripheral levels of hormones.  相似文献   
55.
56.
Abstract.  1. Pollen is considered to be an important dietary component for many species of flower-feeding herbivores. Its influence on oviposition site selection by the pollen beetle Meligethes aeneus , and on the development of its larvae was investigated.
2. The effects of pollen presence and absence on adult, egg, and larval incidence in the field, and on larval development in the laboratory were compared through the use of Synergy, a composite hybrid oilseed rape Brassica napus variety comprising male-fertile (with pollen) and male-sterile (without pollen) plants.
3. In the field, adult females were more abundant on male-fertile plants during flowering, and a greater proportion of male-fertile than male-sterile buds were accepted for oviposition. These data indicate a possible role of pollen in oviposition site selection by female pollen beetles.
4. The numbers of first instar larvae on the two plant lines did not differ; however, more second instars were found on male-fertile than on male-sterile flowers. This suggests a greater larval survival on male-fertile plants, possibly due to the more readily available food resources and better nutrition afforded by the presence of pollen.
5. Laboratory experiments confirmed that a diet which included pollen improved survival to adulthood and resulted in heavier pupae and adults; however, pollen was not obligatory for larval survival and development.
6. The pollen beetle, previously thought to be an obligate pollen feeder, is therefore more generalist in its requirements for development. These findings may relate to the nutritional and behavioural ecology of other flower-feeding herbivores.  相似文献   
57.
58.
Abstract.  1.  Myzus persicae sensu lato demonstrates considerable genetic variation in respect to adaptation to host plants. The subspecies M. persicae nicotianae shows a preference for tobacco, while M. persicae sensu stricto ( s. str. ) for other herbaceous plants. Given that winged colonisers of several aphid species play an important role in selecting host plants, here their role in the host specialisation observed in M. persicae was examined in choice and no-choice tests conducted outdoors, in performance studies, and in DC Electrical Penetration Graph (DC-EPG) studies.
2. In outdoor choice tests, 77% of spring migrants of M. persicae nicotianae chose tobacco, whereas equal proportions of M. persicae s. str. selected tobacco and pepper. In no-choice tests, spring migrants settled more quickly after alighting on host rather than on non host plants, and significantly more alate M. persicae s. str. (27%) than M. persicae nicotianae (2%) left tobacco after walking briefly on the leaf surface, whilst no significant differences were found on pepper. Cross-host transfers significantly reduced the fecundity of both summer and spring migrants of the two subspecies. Finally, the results of no-choice tests and DC-EPG studies showed that winged aphids distinguished their host through cues located on the plant surface or in subcutaneous tissues perceived prior to the initiation of feeding.
3. This study demonstrates the important role of winged colonisers in the evolution of host specialisation in M. persicae . The multifarious divergent selection that the two host forms experience, i.e. the selection against cross-migrants and their subsequent generations, is a crucial factor involved in the development and maintenance of host specialisation and promotes the parallel evolution of improved host-recognition ability.  相似文献   
59.
The daily locomotor activity pattern of Ruin lizards in the field is mainly unimodal, except for summer months when soil temperatures exceed 40 degrees C to 42 degrees C around midday. In such a situation, lizards reduce their locomotor activity around midday to avoid overheating, and thus their daily activity pattern becomes bimodal. The bimodal pattern expressed in the field is usually retained in the free-running rhythm under constant temperature and DD for a couple of weeks, after which the bimodal pattern changes into a unimodal pattern. In the present study, the authors examined whether 24-h temperature cycles (TCs) would change lizard activity from a unimodal to a bimodal pattern. Administration of TCs to unimodal lizards free-running in DD is able to entrain locomotor rhythms and to induce a bimodal pattern both in summer and autumn-winter. There are, however, striking seasonal differences in the effectiveness with which TCs achieve bimodality: (a) Numbers of lizards rendered bimodal are significantly higher in summer than in autumn-winter; (b) TCs require less time to achieve bimodality in summer than autumn-winter; (c) bimodality is retained as an aftereffect in the postentrainment free-run in summer, but not in autumn-winter; (d) TCs change activity duration in summer, but not in autumn-winter. All this demonstrates the existence of seasonal changes in responsiveness of the circadian oscillators controlling activity to the external factors inducing bimodality. Oscillators' responsiveness is high in summer, when bimodality is the survival strategy of Ruin lizards to avoid overheating around midday in open fields, and low in autumn-winter, when bimodality has no recognizable adaptive significance.  相似文献   
60.
The serum amyloid A (SAA) superfamily comprises a number of differentially expressed genes with a high degree of homology in mammalian species. SAA4, an apolipoprotein constitutively expressed only in humans and mice, is associated almost entirely with lipoproteins of the high-density range. The presence of SAA4 mRNA and protein in macrophage-derived foam cells of coronary and carotid arteries suggested a specific role of human SAA4 during inflammation including atherosclerosis. Here we underline the importance of ribosome binding site (rbs)-like sequences (also known as Shine-Dalgarno sequences) in the SAA4 cDNA for expression of recombinant SAA4 protein in Escherichia coli. In contrast to rbs sequences coded by the expression vectors, rbs-like sequences in the cDNA of target protein(s) are known to interfere with protein translation via binding to the small 16S ribosome subunit, yielding low or even no expression. Here we show that PCR mutations of two rbs-like sequences in the human SAA4 cDNA promote expression of considerable amounts of recombinant SAA4 in E.coli.  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