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   7444篇
  免费   889篇
  国内免费   3篇
  8336篇
  2023年   20篇
  2022年   55篇
  2021年   133篇
  2020年   57篇
  2019年   94篇
  2018年   75篇
  2017年   89篇
  2016年   165篇
  2015年   277篇
  2014年   309篇
  2013年   352篇
  2012年   516篇
  2011年   518篇
  2010年   283篇
  2009年   279篇
  2008年   428篇
  2007年   465篇
  2006年   427篇
  2005年   444篇
  2004年   423篇
  2003年   444篇
  2002年   403篇
  2001年   93篇
  2000年   77篇
  1999年   118篇
  1998年   123篇
  1997年   91篇
  1996年   90篇
  1995年   79篇
  1994年   86篇
  1993年   74篇
  1992年   65篇
  1991年   67篇
  1990年   67篇
  1989年   62篇
  1988年   51篇
  1987年   72篇
  1986年   45篇
  1985年   62篇
  1984年   67篇
  1983年   59篇
  1982年   92篇
  1981年   61篇
  1980年   71篇
  1979年   51篇
  1978年   33篇
  1977年   43篇
  1976年   28篇
  1975年   31篇
  1974年   24篇
排序方式: 共有8336条查询结果,搜索用时 0 毫秒
131.
Multiple stress factors in honey bees are causing loss of bee colonies worldwide. Several infectious agents of bees are believed to contribute to this problem. The mechanisms of honey bee immunity are not completely understood, in part due to limited information about the types and abundances of hemocytes that help bees resist disease. Our study utilized flow cytometry and microscopy to examine populations of hemolymph particulates in honey bees. We found bee hemolymph includes permeabilized cells, plasmatocytes, and acellular objects that resemble microparticles, listed in order of increasing abundance. The permeabilized cells and plasmatocytes showed unexpected differences with respect to properties of the plasma membrane and labeling with annexin V. Both permeabilized cells and plasmatocytes failed to show measurable mitochondrial membrane potential by flow cytometry using the JC-1 probe. Our results suggest hemolymph particulate populations are dynamic, revealing significant differences when comparing individual hive members, and when comparing colonies exposed to diverse conditions. Shifts in hemocyte populations in bees likely represent changing conditions or metabolic differences of colony members. A better understanding of hemocyte profiles may provide insight into physiological responses of honey bees to stress factors, some of which may be related to colony failure.  相似文献   
132.
Pallister Killian syndrome (OMIM: # 601803) is a rare multisystem disorder typically caused by tissue limited mosaic tetrasomy of chromosome 12p (isochromosome 12p). The clinical manifestations of Pallister Killian syndrome are variable with the most common findings including craniofacial dysmorphia, hypotonia, cognitive impairment, hearing loss, skin pigmentary differences and epilepsy. Isochromosome 12p is identified primarily in skin fibroblast cultures and in chorionic villus and amniotic fluid cell samples and may be identified in blood lymphocytes during the neonatal and early childhood period. We performed genomic expression profiling correlated with interphase fluorescent in situ hybridization and single nucleotide polymorphism array quantification of degree of mosaicism in fibroblasts from 17 Caucasian probands with Pallister Killian syndrome and 9 healthy age, gender and ethnicity matched controls. We identified a characteristic profile of 354 (180 up- and 174 down-regulated) differentially expressed genes in Pallister Killian syndrome probands and supportive evidence for a Pallister Killian syndrome critical region on 12p13.31. The differentially expressed genes were enriched for developmentally important genes such as homeobox genes. Among the differentially expressed genes, we identified several genes whose misexpression may be associated with the clinical phenotype of Pallister Killian syndrome such as downregulation of ZFPM2, GATA6 and SOX9, and overexpression of IGFBP2.  相似文献   
133.
Summary The relative hydraulic conductivities of major and minor longitudinal veins, and the apoplastic permeability of the bundle sheaths surrounding all longitudinal and transverse veins were investigated in representatives of the C3, C4/NAD-ME, C4/NAD-ME/PCK intermediate, C4/PCK and C4/NADP-ME photosynthetic types. Using the Hagen-Poiseuille equation and measurements of tracheary element diameters, the number of elements in each vein type and the numbers of each vein type, we calculated that 87–99% of the water flow in a longitudinal direction would be expected to occur in the major veins. The permeability of the mestome sheaths and parenchymatous bundle sheaths surrounding the veins was tested using the negatively-charged, fluorescent dye, trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate (PTS). This dye proved nontoxic to plant tissue at a concentration of 0.5%, according to a deplasmolysis test with onion epidermal strips. The PTS concentration achieved in the tested grass leaves was about 0.035%, well below the toxic limit. When a solution of PTS was fed to the leaves by means of a basal cut, the dye moved into the veins of all orders. From there, it moved outward into the surrounding tissues, indicating that the sheaths surrounding the veins of all orders in all species tested were permeable. Therefore, contrary to previous predictions based on structural observations and some tracer studies, bundle sheaths with suberized cell walls do not function as endodermal layers.  相似文献   
134.
Summary The anatomy of normal and hyperhydric in vitro shoots and leaves from micropropagated simmondsia chinensis (Link.) Schn. (jojoba) was compared with that of seedlings (control plants). In vitro normal plantlets displayed good development and survived during the acclimatization stage. In vitro hyperhydric plantlets presented numerous anatomical defects, such as hypertrophy of the mesophyll and of the stem cortex, malformed non-functional stomata, epidermal discontinuity, and xylem hypolignification; they did not survice acclimatization. The study of the anatomical features of in vitro jojoba shoots and leaves allowed determination of the structural condition of the plantlets and prediction of which plantlet would survive the critical acclimatization stage.  相似文献   
135.
136.
137.
Deinococcus proteolyticus (ex Kobatake et al. 1973) Brook and Murray 1981 is one of currently 47 species in the genus Deinococcus within the family Deinococcaceae. Strain MRP(T) was isolated from feces of Lama glama and possesses extreme radiation resistance, a trait is shares with various other species of the genus Deinococcus, with D. proteolyticus being resistant up to 1.5 Mrad of gamma radiation. Strain MRP(T) is of further interest for its carotenoid pigment. The genome presented here is only the fifth completed genome sequence of a member of the genus Deinococcus (and the forth type strain) to be published, and will hopefully contribute to a better understanding of how members of this genus adapted to high gamma- or UV ionizing-radiation. Here we describe the features of this organism, together with the complete genome sequence and annotation. The 2,886,836 bp long genome with its four large plasmids of lengths 97 kbp, 132 kbp, 196 kbp and 315 kbp harbors 2,741 protein-coding and 58 RNA genes and is a part of the Genomic Encyclopedia of Bacteria and Archaea project.  相似文献   
138.
α‐Syntrophin is a component of the dystrophin scaffold‐protein complex that serves as an adaptor for recruitment of key proteins to the cytoplasmic side of plasma membranes. α‐Syntrophin knockout (KO) causes loss of the polarized localization of aquaporin4 (AQP4) at astrocytic endfeet and interferes with water and K+ homeostasis. During brain activation, release of ions and metabolites from endfeet is anticipated to increase perivascular fluid osmolarity, AQP4‐mediated osmotic water flow from endfeet, and metabolite washout from brain. This study tests the hypothesis that reduced levels of endfoot AQP4 increase retention of [14C]metabolites during sensory stimulation. Conscious KO and wild‐type mice were pulse‐labeled with [6‐14C] glucose during unilateral acoustic stimulation or bilateral acoustic plus whisker stimulation, and label retention was assayed by computer‐assisted brain imaging or analysis of [14C]metabolites in extracts, respectively. High‐resolution autoradiographic assays detected a 17% side‐to‐side difference (p < 0.05) in inferior colliculus of KO mice, not wild‐type mice. However, there were no labeling differences between KO and wild‐type mice for five major HPLC fractions from four dissected regions, presumably because of insufficient anatomical resolution. The results suggest a role for AQP4‐mediated water flow in support of washout of metabolites, and underscore the need for greater understanding of astrocytic water and metabolite fluxes.  相似文献   
139.
140.
The National Institute on Aging Interventions Testing Program (ITP) evaluates agents hypothesized to increase healthy lifespan in genetically heterogeneous mice. Each compound is tested in parallel at three sites, and all results are published. We report the effects of lifelong treatment of mice with four agents not previously tested: Protandim, fish oil, ursodeoxycholic acid (UDCA) and metformin – the latter with and without rapamycin, and two drugs previously examined: 17‐α‐estradiol and nordihydroguaiaretic acid (NDGA), at doses greater and less than used previously. 17‐α‐estradiol at a threefold higher dose robustly extended both median and maximal lifespan, but still only in males. The male‐specific extension of median lifespan by NDGA was replicated at the original dose, and using doses threefold lower and higher. The effects of NDGA were dose dependent and male specific but without an effect on maximal lifespan. Protandim, a mixture of botanical extracts that activate Nrf2, extended median lifespan in males only. Metformin alone, at a dose of 0.1% in the diet, did not significantly extend lifespan. Metformin (0.1%) combined with rapamycin (14 ppm) robustly extended lifespan, suggestive of an added benefit, based on historical comparison with earlier studies of rapamycin given alone. The α‐glucosidase inhibitor, acarbose, at a concentration previously tested (1000 ppm), significantly increased median longevity in males and 90th percentile lifespan in both sexes, even when treatment was started at 16 months. Neither fish oil nor UDCA extended lifespan. These results underscore the reproducibility of ITP longevity studies and illustrate the importance of identifying optimal doses in lifespan studies.  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