全文获取类型
收费全文 | 41101篇 |
免费 | 3622篇 |
国内免费 | 22篇 |
专业分类
44745篇 |
出版年
2023年 | 240篇 |
2022年 | 521篇 |
2021年 | 1098篇 |
2020年 | 635篇 |
2019年 | 814篇 |
2018年 | 966篇 |
2017年 | 802篇 |
2016年 | 1250篇 |
2015年 | 2091篇 |
2014年 | 2319篇 |
2013年 | 2535篇 |
2012年 | 3470篇 |
2011年 | 3187篇 |
2010年 | 2047篇 |
2009年 | 1773篇 |
2008年 | 2535篇 |
2007年 | 2500篇 |
2006年 | 2254篇 |
2005年 | 2107篇 |
2004年 | 1946篇 |
2003年 | 1838篇 |
2002年 | 1706篇 |
2001年 | 359篇 |
2000年 | 257篇 |
1999年 | 369篇 |
1998年 | 433篇 |
1997年 | 298篇 |
1996年 | 266篇 |
1995年 | 228篇 |
1994年 | 222篇 |
1993年 | 243篇 |
1992年 | 227篇 |
1991年 | 219篇 |
1990年 | 200篇 |
1989年 | 175篇 |
1988年 | 184篇 |
1987年 | 175篇 |
1986年 | 133篇 |
1985年 | 136篇 |
1984年 | 174篇 |
1983年 | 129篇 |
1982年 | 159篇 |
1981年 | 155篇 |
1980年 | 143篇 |
1979年 | 106篇 |
1978年 | 112篇 |
1977年 | 101篇 |
1976年 | 86篇 |
1974年 | 76篇 |
1973年 | 86篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
A hemagglutinating activity was detected in a synaptic vesicle-enriched fraction prepared from adult rat brain, using trypsinized glutaraldehyde-fixed rabbit erythrocytes. The specific activity of the fraction, in two series of experiments, was 7.5 and 16-fold higher than in the other subcellular fractions. The activity was absent from the synaptosome cytosol. In a study using twenty-five different carbohydrates and glycoproteins, best inhibitors were N-acetylneuraminic acid and N-glycolylneuraminic acid, together with bovine submaxillary mucin and a glycopeptide fraction prepared from rabbit erythrocyte membranes. The activity was thermolabile and very sensitive to proteolytic enzymes (but insensitive to neuraminidase) indicating that a protein (agglutinin) is responsible for the activity. Experiments using detergents and high ionic strength showed that the agglutinin is tightly bound to membranes, inactivated by the so-called non denaturing detergents, and stable in diluted sodium dodecyl sulphate. Hypotheses are discussed on the possible function of the agglutinin. 相似文献
72.
Paul H. Ward Andrew F. Ippoliti Daniel H. Simmons James V. Maloney Jr 《The Western journal of medicine》1988,149(1):58-65
An edited summary of an Interdepartmental Conference arranged by the Department of Medicine of the UCLA School of Medicine, Los Angeles. The Director of Conferences is William M. Pardridge, MD, Professor of Medicine.Several specialists have recently recognized that gastrointestinal reflux causes complications resulting in significant disease. It causes discomfort, indigestion, esophagitis, Barrett''s esophagus, and carcinoma of the esophagus. Pediatricians attribute many early pulmonary problems, and even some sudden deaths in infants, to the reflux of gastric contents. Otolaryngologists now recognize that many cases of nonbacterial, nonspecific pharyngitis and laryngitis are due to the reflux of gastrc acid secretions. Contact granuloma and cancer of the larynx may, in some instances, be secondary to nocturnal reflux. Thoracic surgeons and pulmonologists believe chronic tracheobronchitis and some cases of pulmonary disease are attributable to recurrent bathing of the respiratory epithelium by aspirated gastric contents. An awareness of the many complications of gastrointestinal reflux should lead to a multidisciplined attack on the factors responsible for these diseases. 相似文献
73.
74.
Summary Although spinose teeth of holly leaves have been widely cited as an example of a physical defense against herbivores, this assumption is based largely on circumstantial evidence and on general misinterpretation of a single, earlier experiment. We studied the response of third and fifth instar larvae of the fall webworm, Hyphantria cunea Drury, a generalist, edge-feeding caterpillar, to intact American holly leaves and to leaves that had been modified by blunting the spines, by removing sections of leaf margin between the spines, or by removing the entire leaf margin. The results suggest that the thick glabrous cuticle and tough leaf margin of Ilex opaca are more important than the spinose teeth in deterring edge-feeding caterpillars. Microscopic examination of mature leaves revealed that the epidermis is thickened at the leaf margin, and that the leaf is cirucumscribed by a pair of fibrous veins. In simple choice tests neither domesticated rabbits nor captive whitetailed deer discriminated between spinescent holly foliage and foliage from which spines were removed. Nevertheless, we found little evidence of herbivory by mammals in the field, either on small experimental trees or in the forest understory. While it is possible that spinose teeth contribute to defense by reducing acceptibility of holly relative to other palatable plant species, we suggest that the high concentrations of saponins and poor nutritional quality of holly foliage may be more important than spines in deterring vertebrate herbivores. The degree of leaf spinescence and herbivory was compared at different heights with the tree canopy to test the prediction that lower leaves should be more spinescent as a deterrent to browsers. Leaves on lower branches of mature forest trees were slightly more spinescent than were upper leaves, and juvenile trees were slightly more spinescent than were mature trees. However, there was no relationship between degree of spinescence and feeding damage. The greater spinescence of holly leaves low in the canopy is probably an ontogenetic phenomenon rather than a facultative defense against browsers.The investigation reported in this paper (No. 87-7-8-77) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Dirctor 相似文献
75.
Circumnutation in Phaseolus vulgaris. I. Growth, osmotic potential and cell ultrastructure in the free-moving part of the shoot 总被引:1,自引:0,他引:1
Circumnutation of Phaseolus vulgaris L. cv. Blanc de Juillet twining shoots was examined at the cellular level. Cell growth was monitored inside the free-moving part of the shoot as a function of time and position with respect to the terminal bud. Complete elongation of epidermal cells required >7 days. A 60% increase in length of the cells in the bending zone was observed. Scanning and transmission electron microscopy studies showed the elasticity of cell walls, the extent of plasmodesmata and endoplasmic reticulum of epidermal cells. The osmotic potential in epidermal cells of the bending zone displayed significant differences only between the concave and convex sides. The pattern of cell growth in twining shoots is discussed. Circumnutation could be seen as a consequence of periodic turgor changes inducing periodic growth. 相似文献
76.
Growth Inhibition, Turgor Maintenance, and Changes in Yield Threshold after Cessation of Solute Import in Pea Epicotyls 总被引:1,自引:0,他引:1 下载免费PDF全文
The dependence of stem elongation on solute import was investigated in etiolated pea seedlings (Pisum sativum L. var Alaska) by excising the cotyledons. Stem elongation was inhibited by 60% within 5 hours of excision. Dry weight accumulation into the growing region stopped and osmotic pressure of the cell sap declined by 0.14 megapascal over 5 hours. Attempts to assay phloem transport via ethylenediaminetetraacetate-enhanced exudation from cut stems revealed no effect of cotyledon excision, indicating that the technique measured artifactual leakage from cells. Despite the drop in cell osmotic pressure, turgor pressure (measured directly via a pressure probe) did not decline. Turgor maintenance is postulated to occur via uptake of solutes from the free space, thereby maintaining the osmotic pressure difference across the cell membrane. Cell wall properties were measured by the pressure-block stress relaxation technique. Results indicate that growth inhibition after cotyledon excision was mediated primarily via an increase in the wall yield threshold. 相似文献
77.
Evidence Against the Involvement of Ionically Bound Cell Wall Proteins in Pea Epicotyl Growth 总被引:1,自引:1,他引:0
Ionically bound cell wall proteins were extracted from 7 day old etiolated pea (Pisum sativum L. cv Alaska) epicotyls with 3 molar LiCl. Polyclonal antiserum was raised in rabbits against the cell wall proteins. Growth assays showed that treatment of growing region segments (5-7 millimeters) of peas with either dialyzed serum, serum globulin fraction, affinity purified immunoglobulin, or papain-cleaved antibody fragments had no effect on growth. Immunofluorescence microscopy confirmed antibody binding to cell walls and penetration of the antibodies into the tissues. Western blot analysis, immunoassay results, and affinity chromatography utilizing Sepharose-bound antibodies confirmed recognition of the protein preparation by the antibodies. Experiments employing in vitro extension as a screening measure indicated no effect upon extension by antibodies, by 50 millimolar LiCl perfusion of the apoplast or by 3 molar LiCl extraction. Addition of cell wall protein to protease pretreated segments did not restore extension nor did addition of cell wall protein to untreated segments increase extension. It is concluded that, although evidence suggests that protein is responsible for the process of extension, the class(es) of proteins which are extracted from pea cell walls with 3 molar LiCl are probably not involved in this process. 相似文献
78.
C D Warren P F Daniel B Bugge J E Evans L F James R W Jeanloz 《The Journal of biological chemistry》1988,263(29):15041-15049
Eleven oligosaccharides were purified form the urine of sheep with swainsonine toxicosis induced by the feeding of Astragalus lentiginosus. Oligosaccharides were extracted by charcoal adsorption, chromatographed on Bio-Gel P-2, and partially fractionated by preparative-layer chromatography. Separation into individual compounds was completed by semi-preparative high pressure liquid chromatography. Structures were determined by a combination of high pressure liquid chromatography and exo- and endo- glycosidase action, methanolysis followed by gas-liquid chromatography, methylation analysis, and high resolution nuclear magnetic resonance spectroscopy. Two homologous series of oligosaccharides were identified: (a) alpha-D-Manp-(1----6)-beta-D-Manp-(1----4)-D-GlcpNAc, alpha-D-Manp(1----3)-[alpha-D-Manp-(1----6)]-beta-D-Manp+ ++-(1----4)-D-GlcpNAc, alpha-D-Manp-(1----2)-alpha-D-Manp(1----3)-[alpha-D-Manp+ ++-(1----6)]-beta-D-Manp-(1----4)-D-GlcpNAc, and alpha-D-Manp-(1----2)-alpha-D-Manp-(1----2)-alpha-D-Manp+ ++-(1----3)-[alpha- D-Manp-(1----6)]-beta-D-Manp-(1----4)-D-GlcpNAc (minor series); (b) alpha-D-Manp-(1----6)-beta-D-Manp-(1----4)-beta-D-GlcpNAc- (1----4)-D-GlcpNAc, alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-beta-D-Manp -(1----4)-beta-D-GlcpNAc-(1----4)-D-GlcpNAc, alpha-D-Manp(1----3)-alpha-D-Manp-(1----6)-beta-D-Manp -(1----4)-beta-D-GlcpNAc- (1----4)-D-GlcpNAc, alpha-D-Manp-(1----6)-alpha-D-Manp-(1----6)-beta-D-Manp++ +-(1----4)-beta-D-GlcpNAc - (1----4)-D-GlcpNAc, alpha-D-Manp-(1----3)-alpha-D-Manp-(1----6)-[alpha-D-Manp -(1----3)]-beta-D- Manp-(1----4)-beta-D-GlcpNAc-(1----4)-D-GlcpNAc, alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-alpha-D-Man p-(1----6)-beta-D- Manp-(1----4)-beta-D-GlcpNAc-(1----4)-D-GlcpNAc, and alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-alpha-D-Man p-(1----6)- [alpha-D-Manp-(1----3)]-beta-D-Manp-(1----4)-beta-D-GlcpNAc- (1----4)-D- GlcpNAc (major series). 相似文献
79.
A small group of chimpanzees was allowed to manipulate different quantities of novel objects in order to enhance their captive conditions. This procedure led to a general increase in the manipulation frequency by the apes. It decreased their inactivity and their self-grooming, and almost eliminated the abnormal behaviors displayed by one individual. The distribution of novel objects did not affect the total frequency of social interactions and did not increase the aggressivity of the animals, even though dominant individuals secured most of the objects when only a small number of them were available. Following their familiarization with the novel objects, the chimpanzees' manipulation frequency decreased whereas self-grooming and abnormal behaviors were increased. Suggestions are made on how to maximize the use of a limited bank of toys to occupy captive chimpanzees. 相似文献
80.
Terry L. Riss Kenneth P. Karey B. Daniel Burleigh David Farker David A. Sirbasku 《In vitro cellular & developmental biology. Plant》1988,24(11):1099-1106
Summary A serum-free clonal density growth assay was developed for the quantification of the biological activity of human recombinant
insulin-like growth factor I (IGF-I). The assay measures IGF-I stimulated growth of Balb/c 3T3 cells cultured over 4 d on
poly-d-lysine-coated plastic surfaces in a serum-free medium formulation composed of a 1∶1 (vol/vol) mixture of Ham's F12 and Dulbecco's
modified Eagle's media, supplemented with 3.0 ng/ml bovine basic fibroblast growth factor (bFGF), 10 μg/ml human transferrin,
100 μg/ml ovalbumin, and 1.0 μM dexamethanose. Low-temperature trypsinization of serum-supplemented stock cultures combined with the use of poly-d-lysine-coated plates made it unnecessary to use serum or fibronectin to promote cell attachment and survival. Serum-free
growth conditions were optimized with respect to the concentrations of the supplements. Addition of IGF-I resulted in 3.5-fold
more cells than control cultures without IGF-I after 4 d. Deletion of bFGF resulted in no IGF-I stimulation of growth. The
concentrations of various preparations of IGF-I required to achieve one-half maximal stimulation of cell number (ED50), ranged between 1.25 and 4.7 ng/ml. In parallel assays, IGF-I was 6.6 times more potent than human recombinant insulin-like
growth factor II and 32 times more potent than insulin. When cells were seeded into medium containing IGF-I, transferrin,
ovalbumin, and dexamethasone but no bFGF, growth was minimal. Dose-response addition of bFGF showed an ED50, of 0.9 ng/ml. The methods reported are useful to monitor the biological potency of recombinant and natural-source growth
factors as well as providing a new means of studying the multiple growth factor requirements of Balb/c 3T3 cells in cultures.
This work was supported by a contract from IMCERA Bioproducts, Inc. 相似文献