全文获取类型
收费全文 | 40898篇 |
免费 | 2789篇 |
国内免费 | 3611篇 |
专业分类
47298篇 |
出版年
2024年 | 129篇 |
2023年 | 519篇 |
2022年 | 863篇 |
2021年 | 1141篇 |
2020年 | 1152篇 |
2019年 | 1519篇 |
2018年 | 1336篇 |
2017年 | 1032篇 |
2016年 | 1082篇 |
2015年 | 1303篇 |
2014年 | 2095篇 |
2013年 | 2549篇 |
2012年 | 1584篇 |
2011年 | 2027篇 |
2010年 | 1560篇 |
2009年 | 1948篇 |
2008年 | 2003篇 |
2007年 | 2214篇 |
2006年 | 1962篇 |
2005年 | 1766篇 |
2004年 | 1564篇 |
2003年 | 1431篇 |
2002年 | 1340篇 |
2001年 | 1070篇 |
2000年 | 1006篇 |
1999年 | 954篇 |
1998年 | 935篇 |
1997年 | 798篇 |
1996年 | 842篇 |
1995年 | 761篇 |
1994年 | 672篇 |
1993年 | 681篇 |
1992年 | 618篇 |
1991年 | 572篇 |
1990年 | 493篇 |
1989年 | 396篇 |
1988年 | 402篇 |
1987年 | 340篇 |
1986年 | 291篇 |
1985年 | 357篇 |
1984年 | 418篇 |
1983年 | 268篇 |
1982年 | 327篇 |
1981年 | 234篇 |
1980年 | 197篇 |
1979年 | 149篇 |
1978年 | 132篇 |
1977年 | 77篇 |
1976年 | 61篇 |
1974年 | 36篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
Synthesis and in vitro bioactivity of C-terminal deleted analogs of human growth hormone-releasing factor 总被引:4,自引:0,他引:4
N Ling A Baird W B Wehrenberg N Ueno T Munegumi P Brazeau 《Biochemical and biophysical research communications》1984,123(2):854-861
A series of C-terminal deleted analogs of human growth hormone-releasing factor (hGRF) with either an amidated or a free carboxylic acid C-terminus were synthesized by solid phase methodology. Their capacity to release growth hormone was tested on rat anterior pituitary cells in monolayer culture. A gradual decrease of bioactivity down to 23% relative to hGRF was noted when the C-terminal amino acids were deleted to hGRF (1-34)OH. Further deletions, however, did not decrease the bioactivity because the potencies of the fragments, hGRF(1-31)NH2, (1-30)NH2 and (1-29)NH2 remained at about 50% of that of hGRF. Continual deletion of residues to hGRF(1-23)NH2, (1-22)NH2 and (1-21)NH2 still yielded bioactive fragments with full intrinsic activity despite very low potency. Only with the deletion down to hGRF(1-19)NH2 did the bioactivity completely disappear. Thus, together with the data published in a previous paper (1), the minimal biologically active core of hGRF with full intrinsic activity comprises the fragment (3-21). 相似文献
92.
Otolith and somatic mass of two Gadidae ( Merlangius merlangus and Trisopterus minutus ) were compared in order to analyse the sex-specific relationship between otolith and somatic growth at age. In the present study, otolith mass appeared a reliable indicator of age in both species. Otolith growth reflected somatic growth, but the relationship between these characters varied and differed between species and sexes. 相似文献
93.
Summary Density-dependent regulation of cell growth in tissue culture is a well-known phenomenon but the mechanism of regulation remains obscure. Here we explore the effects of cell density and metabolite flux on the collective dynamics of a cell population. The intracellular dynamics are modelled by positive feedback kinetic mechanisms of the kind known to apply to yeast cells. Several experimental observations related to glycolytic oscillations are predicted and it is suggested that the general conclusions may be applicable in a broader context. 相似文献
94.
95.
Berthold Lausen Torsten Hothorn Frank Bretz Martin Schumacher 《Biometrical journal. Biometrische Zeitschrift》2004,46(3):364-374
The identification and assessment of prognostic factors is one of the major tasks in clinical research. The assessment of one single prognostic factor can be done by recently established methods for using optimal cutpoints. Here, we suggest a method to consider an optimal selected prognostic factor from a set of prognostic factors of interest. This can be viewed as a variable selection method and is the underlying decision problem at each node of various tree building algorithms. We propose to use maximally selected statistics where the selection is defined over the set of prognostic factors and over all cutpoints in each prognostic factor. We demonstrate that it is feasible to compute the approximate null distribution. We illustrate the new variable selection test with data of the German Breast Cancer Study Group and of a small study on patients with diffuse large B‐cell lymphoma. Using the null distribution for a p‐value adjusted regression trees algorithm, we adjust for the number of variables analysed at each node as well. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
96.
Growth rate differences between resident native brook trout and non-native brown trout 总被引:1,自引:0,他引:1
Between species and across season variation in growth was examined by tagging and recapturing individual brook trout Salvelinus fontinalis and brown trout Salmo trutta across seasons in a small stream (West Brook, Massachusetts, U.S.A.). Detailed information on body size and growth are presented to (1) test whether the two species differed in growth within seasons and (2) characterize the seasonal growth patterns for two age classes of each species. Growth differed between species in nearly half of the season- and age-specific comparisons. When growth differed, non-native brown trout grew faster than native brook trout in all but one comparison. Moreover, species differences were most pronounced when overall growth was high during the spring and early summer. These growth differences resulted in size asymmetries that were sustained over the duration of the study. A literature survey also indicated that non-native salmonids typically grow faster than native salmonids when the two occur in sympatry. Taken together, these results suggest that differences in growth are not uncommon for coexisting native and non-native salmonids. 相似文献
97.
F E CONSTABLE J JONES K S GIBB Y M CHALMERS R H SYMONS 《The Annals of applied biology》2004,144(2):205-218
Surveys were conducted in four Chardonnay vineyards for 3 to 6 years and one Shiraz vineyard for 3 years to determine the yearly percentage of grapevines affected by Australian grapevine yellows disease (AGYd), restricted growth disease (RGd) and late season leaf curl disease (LSLCd). In each of the Chardonnay vineyards in each year, all three diseases were characterised by remission of disease in some grapevines, recurrence of disease in other grapevines and new observations of disease in previously unaffected grapevines. The pattern of temporal incidence of each disease was different between vineyards for the survey period. Although Koch's postulates have not been fulfilled, phytoplasmas are considered to be the most likely cause of AGYd. While some grapevines exhibited a combination of AGYd and RGd or AGYd and LSLCd, both RGd and LSLCd can occur independently of AGYd. Statistical analyses using log-linear models also indicated that RGd and LSLCd were not always associated with AGYd. Thus, it is possible that phytoplasmas are not the cause of RGd or LSLCd and their association is coincidental. Expression of AGYd in Shiraz grapevines occurred later in the season compared to Chardonnay. Very little recurrence of AGYd was observed in the Shiraz grapevines indicating that the variety Shiraz responds differently to phytoplasma infections, assuming that AGYd in Shiraz is a phytoplasma caused disease. RGd and LSLCd were not observed on any grapevines in the Shiraz vineyard. 相似文献
98.
99.
The effects of arsenate on the growth characteristics of five isolates of the freshwater alga, Chlorella vulgaris Beij., were examined. Two field isolates originated from arsenic-contaminated sites in Yukon, Canada and Kyushi, Japan; two reference isolates were obtained from the University of Texas Culture Collection. One isolate was selected for arsenic-tolerance in the laboratory. All five strains survived in culture solutions containing high arsenate concentrations. Arsenate (1–25 mM As) reduced photosynthesis and cell growth, as reflected by induced lag periods, slower growth rates, and lower stationary cell yields. Field isolates had shorter lag periods, higher growth rates, and enhanced cell yields compared to lab isolates when exposed to the same arsenic concentrations. Growth of the phosphorus-limited field strains was stimulated by the addition of arsenic. The cell yield of phosphorus-limited C. vulgaris Yukon, when treated with arsenic, was two times that of the phosphorus-limited control. This pattern was not evident when photosynthesis was used as a measure of cell response. 相似文献
100.