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   828篇
  免费   52篇
  国内免费   115篇
  995篇
  2024年   3篇
  2023年   13篇
  2022年   18篇
  2021年   28篇
  2020年   34篇
  2019年   38篇
  2018年   24篇
  2017年   27篇
  2016年   24篇
  2015年   24篇
  2014年   43篇
  2013年   35篇
  2012年   37篇
  2011年   38篇
  2010年   35篇
  2009年   33篇
  2008年   53篇
  2007年   44篇
  2006年   40篇
  2005年   57篇
  2004年   33篇
  2003年   33篇
  2002年   30篇
  2001年   22篇
  2000年   21篇
  1999年   22篇
  1998年   17篇
  1997年   11篇
  1996年   13篇
  1995年   11篇
  1994年   12篇
  1993年   16篇
  1992年   8篇
  1991年   13篇
  1990年   5篇
  1989年   11篇
  1988年   5篇
  1987年   8篇
  1986年   5篇
  1985年   8篇
  1984年   5篇
  1983年   10篇
  1982年   12篇
  1981年   2篇
  1980年   3篇
  1979年   2篇
  1978年   3篇
  1976年   3篇
  1975年   1篇
  1974年   2篇
排序方式: 共有995条查询结果,搜索用时 9 毫秒
1.
2.
3.
The histopathology of two populations of Sarisodera hydrophila Wouts and Sher, 1971 was examined on Salix lasiolepis Benth. (willow), Populus fremontii Wats. (cottonwood), and Lyonothamnus floribundus Gray (ironwood) using light microscopy as well as scanning and transmission electron microscopy. Sarisodera hydrophila induces formation of a single uninucleate hypertrophied cell (giant cell) which varies only slightly among the three hosts. The giant cell is enclosed by the root stele and contacts phloem, vascular cambium, and xylem. The single hypertrophied nucleus of the giant cell is ameboid or lobulate in shape, generally with a single nucleolus. The cell is characterized by a wall which is separated into two distinct regions about 2 μm and 13 μm thick; the thicker region occurs adjacent to the nematode head. Cell wall ingrowths, such as those associated with host responses to certain other plant-parasitic nematodes, were not observed in giant cells induced by S. hydrophila. However, a high frequency of pit fields with plasmodesmata occurred in the thinner portion of the cell wall which is adjacent to vascular elements. Roots of the three hosts simultaneously infected with S. hydrophila and Meloidogyne sp. resulted in adjacent responses characteristic of each nematode, supporting the view that the specific type of host response is a function of the nematode rather than the host. The varying expressions of host responses among Heteroderoidea may be useful in testing congruency with existing interpretations of phylogeny.  相似文献   
4.
ABSTRACT. The germinal micronucleus divides six times during conjugation of Paramecium caudatum : this includes two meiotic divisions and one mitosis of haploid nuclei during mating, and three mitoses of a fertilization nucleus (synkaryon). Microsurgical removal of the macronucleus showed that micronuclei were able to divide repeatedly in the absence of the macronucleus, after metaphase of meiosis I of the micronucleus and also after synkaryon formation. When the macronucleus was removed after the first division of synkaryon, in an extreme case the synkaryon divided five times and produced 32 nuclei, compared to three divisions and eight nuclei produced in the presence of the macronucleus. Treatment with actinomycin D (100 μ /ml) inhibited the morphological changes of the macronucleus during conjugation and induced a multimicronucleate state in exconjugants. However, in other cells, it induced production of a few giant micronuclei. We conclude that the micronucleus is able to undergo repeated divisions at any stage of conjugation in the absence of the macronucleus once the factor(s) for induction of the micronuclear division has been produced by the macronucleus. The macronucleus may also produce a regulatory factor required to stop micronucler division.  相似文献   
5.
We constructed a computer model of 128 interneurons, each with multiple dendritic branches and an axonal segment. The model neurons were interconnected by gap junctions between dendritic compartments, as are known to occur in rat and guinea-pig hilar interneurons. The model contained no excitatory synapses. In the presence of low-frequency spontaneous action potentials, the model generated synchronized population bursts, when gap junction resistance was 50 M and there were at least two gap junctions per neuron on average. Population bursts occurred only when the dendrites of model neurons were electrically excitable. Consistent with experiment, somatic hyperpolarization during the population burst uncovered partial spikes. In the model, partial spikes originated in electrically active dendrites driven by coupled dendrites. This model may account for population bursts in hilar interneurons that occur in 4-aminopyridine (4AP) together with blockers of GABAA and excitatory amino acid (EAA) receptors.  相似文献   
6.
Breast Carcinoma With Osteoclast-Like Giant Cells   总被引:1,自引:0,他引:1  
The cytological and histological findings of a case of breast carcinoma with osteoclast-like giant cells are presented. A fine needle aspiration specimen demonstrated the characteristic combination of malignant epithelium and reactive multinucleated cells and enabled identification of this rare variant of breast cancer. Immunocytochemical studies using the monoclonal antibody KP1[CD68] support a histiocytic origin for the osteoclast-like cells.  相似文献   
7.
The effect of feeding by the armored scale, Rhizaspidiotus donacis (Leonardi, 1920) (Hemiptera: Diaspididae) on the growth of the plant Arundo donax L. (Poaceae) was evaluated under field conditions in its native range. The study was designed to evaluate the impact of R. donacis, a candidate agent for biological control of A. donax which is invasive in arid riparian ecosystems of the Southwestern USA and Mexico. The study was carried out at five A. donax sites in the Province of Alicante, Spain, differing in altitude and climate. At each site, 30 infested lateral shoots were selected and 15 were randomly treated monthly with imidacloprid insecticide. Shoot lengths were measured monthly over a 1-year period in a comparative growth analysis. Shoots infested with R. donacis had an over 2-fold reduced growth rate as compared to treated shoots. Growth of shoots varied by site, and the effect of R. donacis on growth was most pronounced in the late spring, when mature females produced first instar scale crawlers. The impact of R. donacis on A. donax growth under field conditions in the native range, combined with its narrow host specificity, indicate that R. donacis is a promising candidate for biological control of A. donax in North America and other areas invaded by this weed.  相似文献   
8.
The giant clam subfamily Tridacninae (family Cardiidae) is an important group of bivalve molluscs found throughout the Red Sea and Indo-Pacific, from East Africa to the Eastern Pacific biogeographic region. The Tridacna genus is currently revised with numerous cryptic species identified with molecular markers. New Tridacna records from the fringe of the known distribution areas are extremely useful to identify genetically unique species, geographic ranges, and to examine processes associated with species differentiation. While Tridacna maxima is abundant in French Polynesia (Central South Pacific Ocean) the larger fluted giant clam Tridacna squamosa was formerly reported only in the Austral Islands in the south. Following a recent survey that spanned 23 islands and atolls of the Society, Tuamotu and Gambier Archipelagos, the presence of T. squamosa between the Cook Islands and Pitcairn Islands is confirmed using both morphological and molecular information, suggesting a relic distribution across the Central Pacific Ocean. Tridacna squamosa is rare, but present throughout Tuamotu and Gambier. However, it remained undetected from the Society Islands, probably due to historical over-fishing. This species is valued by local inhabitants, and is sought after mainly as gifts and also for a limited local shell trade. The rarity of T. squamosa may call for conservation measures in the near future.  相似文献   
9.
Microfluidic technology – the manipulation of fluids at micrometer scales – has revolutionized many areas of synthetic biology. The bottom‐up synthesis of “minimal” cell models has traditionally suffered from poor control of assembly conditions. Giant unilamellar vesicles (GUVs) are good models of living cells on account of their size and unilamellar membrane structure. In recent years, a number of microfluidic approaches for constructing GUVs has emerged. These provide control over traditionally elusive parameters of vesicular structure, such as size, lamellarity, membrane composition, and internal contents. They also address sophisticated cellular functions such as division and protein synthesis. Microfluidic techniques for GUV synthesis can broadly be categorized as continuous‐flow based approaches and droplet‐based approaches. This review presents the state‐of‐the‐art of microfluidic technology, a robust platform for recapitulating complex cellular structure and function in synthetic models of biological cells.  相似文献   
10.
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