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   15809篇
  免费   131篇
  15940篇
  2023年   130篇
  2013年   632篇
  2012年   131篇
  2011年   142篇
  2010年   156篇
  2009年   317篇
  2008年   172篇
  2007年   169篇
  2006年   125篇
  2005年   133篇
  2002年   116篇
  1999年   101篇
  1996年   147篇
  1995年   146篇
  1994年   114篇
  1993年   233篇
  1992年   201篇
  1991年   203篇
  1990年   225篇
  1989年   394篇
  1988年   346篇
  1987年   454篇
  1986年   392篇
  1985年   390篇
  1984年   323篇
  1983年   259篇
  1982年   307篇
  1981年   326篇
  1980年   316篇
  1979年   311篇
  1978年   365篇
  1977年   408篇
  1976年   426篇
  1975年   454篇
  1974年   306篇
  1973年   171篇
  1972年   177篇
  1971年   168篇
  1970年   169篇
  1969年   138篇
  1968年   138篇
  1967年   132篇
  1964年   165篇
  1963年   110篇
  1962年   107篇
  1961年   111篇
  1960年   116篇
  1959年   112篇
  1958年   131篇
  1957年   163篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Populations of pathogenic Pseudomonas syringae pv. syringae were monitored on apparently healthy leaves, blossoms, and fruit from two apple orchards with known histories of blister bark and a pear orchard with a known history of blossom blast. Populations on blossoms and fruits were higher on pears than on apples. Yellow-pigmented, non-pathogenic bacteria might have suppressed or masked the presence of P. syringae pv. syringae on apple trees. Populations of P. syringae pv. syringae on apple and pear leaves fluctuated sharply but higher levels generally occurred during the 1984/85 growing season than during the drier 1983/84 season. This investigation indicates that the resident phase of P. syringae pv. syringae is probably a major source of inoculum for apple blister bark and pear blossom blast in South Africa.  相似文献   
52.
Surface electromyography provides a very valuable set of information when used diagnostically with pain-related disorders. Unfortunately, most researchers and clinicians limit their investigation to only one level of diagnostic information available concerning the neuromuscular system. This article develops and encourages the clinician/researcher to consider three levels of diagnostic information: emotional, organ-related, and postural aspects. The theoretical background and diagnostic procedures used to investigate each is presented. Differential treatment considerations, given differential diagnostic findings, are discussed.  相似文献   
53.
In cyanobacteria, the glutamine synthetase-L-glutamine-2-oxoglutarate aminotransferase (GS-GOGAT) pathway is the major ammonia-assimilating route. The GS ofAnabaena doliolum was synthesized more under N2-fixing conditions, followed by ammonium, nitrate, and nitrite as nitrogen sources. The activities of both the glutamine synthetase, Mg2+-dependent biosynthetic and Mn2+-dependent -glutamyl transferase were optimum at pH 7. The active site of the enzyme bears sulfhydryl (-SH) groups; this was confirmed with the-SH group inhibitors, para-chloromercuribenzoate (pCMB) and N-ethylmaleimide (NEM). The biosynthetic and -glutamyl transferase activities showed specificity for the divalent cations, Mg2+ and Mn2+, respectively. The other divalent cations Co2+, Cu2+, and Ni2+ were poor substitutes. This enzyme also required these divalent cations to stabilize its structure and function under extreme conditions such as high and low temperatures and urea denaturation. The glutamate analogl-methionine-d,l-sulfoximine, inactivated the enzyme, whereas the GOGAT inhibitor, azaserine, had no effect on the enzyme activity. The GS enzyme required de novo protein synthesis.  相似文献   
54.
55.
The Tamang women tea-labouren of Jalpaiguri district show no significant difference in age at menopause among their three «migration status» groups, viz., MO, M1 and M2+. The mean age at menopause is 47.3±4.26 and the median age is 47.68 years. A significant difference occurs between the observed mean age at menopause and 45 years, which is generally taken to be the menopausal age by convention. These Tamang women seem to be similar to other hill women in respect of age at menopause.  相似文献   
56.
Two sessions of relaxation instructions were administered under high and low expectancy conditions. Fifty-four college students scoring high on a self-report measure of anxiety served as subjects. Live and taped abbreviated progressive muscle relaxation instructions and a self-relaxation condition were equally effective in reducing within-session self-report and physiological indices of anxiety. High expectancy instructions led to greater reductions in heart rate than did low expectancy instructions. Factors controlling anxiety reduction during relaxation therapies are discussed.This experiment was submitted by the first author in partial fulfillment for a master of science degree at VPI and SU. The authors would like to thank Jean Sales and Dan Fones for their help in this investigation. The First author is now at Johns Hopkins Hospital, Baltimore. The second author is currently at the Department of Neurology, University of Wisconsin-Madison.  相似文献   
57.
Summary The ultrastructure of the somatotrophs of the caudal pituitary of the domestic fowl was studied quantitatively. Two age groups of male chickens were compared: 4–6 weeks and 24–30 weeks post-hatching. With age, somatotrophs decreased from about 40% to about 30% of the pituitary cell population. Their volume density decreased similarly. Mean volume of a somatotroph was the same in young and adult animals. Because the granule volume density of the somatotrophs was unchanged, but the somatotroph volume density of the gland declined, the granule volume density of the caudal pituitary gland dropped in parallel with that of the somatotrophs. Thus the volume of the gland comprised of somatotroph granules fell about 32%: from 6.57% to 4.45%. This lowered pool of stored hormone may be linked to the lowered circulating levels of growth hormone found in older animals by other investigators.The granule volume density of the somatotrophs was unchanged but the numerical density approximately doubled; thus the mean granule size decreased by 47% with age. The relationship of the size reduction of the granules to the lowered plasma growth hormone levels is not understood at present.Supported in part by Hatch and State funds from the New Jersey State Agricultural Experimental Station and NSF grants PCM 8,0227,27 and PCM 8,302197.  相似文献   
58.
Summary In captive African catfish, Clarias gariepinus, ovulation was induced with human chorionic gonadotropin (HCG) 4 I.U./g body weight to study the function of postovulatory follicles (POFs). Ultrastructural and enzyme-histochemical data indicate that, apart from special theca cells, the granulosa of relative young POFs (i.e., from 16 h and 28 h after HCG-injection) is capable of producing steroids. Possible functions of the synthesized steroids are discussed. Histological comparison of POFs from stripped and from unstripped fish, as well as histochemical investigation of the contents of ovulated ova and granulosa of POFs at 48 h after HCG-injection, showed that the latter structure is involved in phagocytosis of the disintegrating ovulated eggs. The polysaccharide-lipid-protein material, initially taken up by heterophagolysosomes of the granulosa cells, subsequently undergoes fatty degeneration. The granulosa cells of the POFs showed strong acid phosphatase activity and abundant granular endoplasmic reticulum from 16 h after HCG-injection onward; heterophagolysosomes appeared at 32 h. These results indicate that after ovulation the phagocytotic function of the granulosa develops progressively. Autophagolysosomes, responsible for the final disintegration of POFs, become increasingly evident in the granulosa cells with increasing time after ovulation.  相似文献   
59.
Summary Distribution of glycogen particles in semithin and ultrathin sections of biopsy samples from human muscles subjected to either short- or long-term running were investigated using PAS and Periodic Acid-ThioSemiCarbazide-Silver Proteinate (PA-TSC-SP) staining methods. Glycogen particles were predominantly found immediately under the sarcolemma or aligned along the myofibrillar Iband. After long-term exhaustive exercise type-1 fibers with a few or no glycogen particles in the core of the fibers were frequently observed. The subsarcolemmal glycogen stores of these depleted type-1 fibers were about three times as large as after exhaustive short-time exercise. Another indication of utilization of subsarcolemmal glycogen stores during anaerobic exercise was that many particles displayed a pale, rudimentary shape. This observation suggests fragmental metabolization of glycogen. Thus, depending on type of exercise and type of fiber differential and sequential glycogen utilization patterns can be observed.  相似文献   
60.
Comparisons are made of self-reported medical costs from a sample of headache patients who underwent various combinations of relaxation training and biofeedback training. The average costs for the 2 years prior to self-regulatory treatment were $955±480 (3 SEM) for 45 patients; for the 2 years after completing treatment the average costs were $52±28 (3 SEM) for patients. Within the limitations of the study, medical costs do seem to have been markedly reduced.This research was supported by a grant from NINCDS, NS-15235.  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