全文获取类型
收费全文 | 18380篇 |
免费 | 1564篇 |
国内免费 | 4篇 |
专业分类
19948篇 |
出版年
2023年 | 65篇 |
2022年 | 151篇 |
2021年 | 317篇 |
2020年 | 174篇 |
2019年 | 231篇 |
2018年 | 320篇 |
2017年 | 282篇 |
2016年 | 403篇 |
2015年 | 793篇 |
2014年 | 844篇 |
2013年 | 1065篇 |
2012年 | 1482篇 |
2011年 | 1452篇 |
2010年 | 881篇 |
2009年 | 865篇 |
2008年 | 1200篇 |
2007年 | 1215篇 |
2006年 | 1107篇 |
2005年 | 1121篇 |
2004年 | 989篇 |
2003年 | 926篇 |
2002年 | 900篇 |
2001年 | 189篇 |
2000年 | 138篇 |
1999年 | 190篇 |
1998年 | 275篇 |
1997年 | 159篇 |
1996年 | 144篇 |
1995年 | 144篇 |
1994年 | 124篇 |
1993年 | 115篇 |
1992年 | 99篇 |
1991年 | 82篇 |
1990年 | 91篇 |
1989年 | 87篇 |
1988年 | 77篇 |
1987年 | 55篇 |
1986年 | 74篇 |
1985年 | 76篇 |
1984年 | 97篇 |
1983年 | 75篇 |
1982年 | 87篇 |
1981年 | 104篇 |
1980年 | 88篇 |
1979年 | 56篇 |
1978年 | 66篇 |
1977年 | 53篇 |
1976年 | 38篇 |
1974年 | 56篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
32.
Brian M. Boom 《Brittonia》1989,41(2):129-130
Pagamea aracaënsis Boom, endemic to Serra Aracá, Brazil is described and illustrated, and its relationship toP. anisophylla Standley & Steyerm, is discussed. 相似文献
33.
34.
35.
Cyprian Weaver Robert L. Sorenson Brian Kobienia 《In vitro cellular & developmental biology. Plant》1988,24(2):108-116
Summary The purpose of this study was to develop a nonenzymatic method of isolating adult islets using atrophied pancreata from copper-deficient
rats and to analyze their morphologic characteristics and behavior in culture. This unusual model of isolation was studied
because islets remain intact in the course of dietary copper deficiency while the acinar glandular component of the pancreas
undergoes selective atrophy and lipomatosis. Small fragments containing islets were readily microdissected from atrophied
glands and placed in culture. Within 24 h the fragments congealed into small irregular- to spherical-shaped masses within
which the darker profile of islets could be distinguished. Within a period of 3 to 5 d, islet tissue began to bud from the
lipocytic mass until by Day 7 spherical aggregates of intact islet tissue separated from the residual fragments. Subsequent
to further in vitro treatment, these islets could be maintained as free viable spherical masses if periodically agitated,
as attached stationary islets which developed monolayer growth if left undisturbed and as aggregated masses of islet tissue
forming megaislets if combined in small groups. Grouped islets treated with actinomycin D and cycloheximide did not exhibit
aggregation when incubated with these inhibitors. This suggests that megaislet formation was an active process requiring protein-RNA
synthesis rather than passive clumping or aggregation that can accompany metabolically altered or dying islets undergoing
cellular shedding and adhesion. Immunohistochemical localization demonstrated that insulin, glucagon, somatostatin, and pancreatic
polypeptide-immunoreactive cell types were present within the islets derived from this technique. The cellular topography
of these islets was not unlike that described by others for islets cultured from enzymatic isolation. This culture model may
serve as a resource for mature, viable islets isolated without mechanical or enzymatic disaggregation which can have attenuating
effects on islet function.
This work was supported by a research grant from the Diabetes Research and Education Foundation. 相似文献
36.
37.
Robert G. Rutledge Thérèse Quellet Jiro Hattori Brian L. Miki 《Molecular & general genetics : MGG》1991,229(1):31-40
Summary The Brassica napus rapeseed cultivar Topas contains an acetohydroxyacid synthase (AHAS) multigene family consisting of five members (AHAS 1–5). DNA sequence analysis indicate that AHAS1 and AHAS3 share extensive homology. They probably encode the AHAS enzymes essential for plant growth and development. AHAS2 has diverged significantly from AHAS1 and AHAS3 and has unique features in the coding region of the mature polypeptide, transit peptide and upstream non-coding DNA, which raises the possibility that it has a distinct function. AHAS4 and AHAS5 have interrupted coding regions and may be defective. The complexity of the AHAS multigene family in the allotetraploid species B. napus is much greater than reported for Arabidopsis thaliana and Nicotiana tabacum. Analysis of the presumptive progenitor diploid species B. campestris and B. oleracea indicated that AHAS2, AHAS3 and AHAS4 originate from the A genome, whereas AHAS1 and AHAS5 originate from the C genome. Further variation within each of the AHAS genes in these species was found. 相似文献
38.
We have found and characterized an antigen associated with crystal-containing cells in the stomium and connective tissue of the anthers of Nicotiana tabacum L. (tobacco). The antigen, defined by the monoclonal antibody NtF-8B1, localizes to subcellular regions surrounding the crystals. At the light-microscope level, the antigen is detectable just after the first appearance of crystals in the connective tissue of the anther, and at approximately the same time as the appearance of crystals in the stomium. The antigen is not detectable on a Western blot, and gave inconclusive results on a test of periodate sensitivity. It is not the crystals themselves, nor is the presence of the crystals required for antibody recognition. The antigen is sensitive to heat and protease treatment, indicating that it is a protein. The antigen is not tightly membrane-bound, in spite of its localization closely surrounding the crystals. Chemical tests indicate that the druse crystals in the stomium are calcium oxalate.Abbreviations ELISA
enzyme-linked immunosorbent assay
- FITC
fluorescein isothiocyanate
This research was supported by a National Science Foundation postdoctoral fellowship to B.L.H., by National Science Foundation grants DMB-87-15799 and to W.E.F. BSR-88-18035, and by U.S. Department of Agriculture grant GAM-89-01056. The authors thank Phillip T. Evans (Louisiana State University, Baton Rouge, USA), Wilma L. Lingle, Harry T. Horner, Jr. (Iowa State University), and A. Jack Fowler, Jr., for advice and helpful discussions. 相似文献
39.
Fragments of ascidian eggs, but at random in any plane and ranging in size from 10 to 90% of the total egg volume, displayed the electrical characteristics of the intact egg, having a resting potential of -86 mV and giving rise to an action potential upon stimulation by electrical current injection. Following insemination, the fragments generated fertilization potentials, comparable to those of intact eggs, although the repolarization phase was shorter. Our data show that there are sufficient ion channels throughout the egg surface to generate action potentials and fertilization potentials in excised egg fragments, irrespective of their global origin. Furthermore, the fertilizing spermatozoon is capable of activating fertilization channels in areas of the egg plasma membrane not destined for sperm entry. 相似文献
40.
Chemical limnology of two artificial lakes used for both stormwater management and recreation 总被引:1,自引:1,他引:0
The aims of this study were to document the mainly chemical behaviour of two linked artificial lakes used for both stormwater management and recreation in the new town of Craigavon. Further, the understanding of their behaviour should help in their management and the design of other similar lakes.The lake mean total phosphorus (73 µg P l–1), nitrate (0.50 mg N l–1) and chlorophyll a (25 µg l–1) concentrations, Secchi depth (1.2 m) and the estimated total phosphorus loading (1.98 g m–2 a–1) all classify the main lake as eutrophic. An important source of the phosphorus load on the lakes is the urban area of Craigavon (52% of the total load). The interrelationships between total phosphorus, chlorophyll a and Secchi depth in the main lake are similar to those in natural ones. In addition, the lake follows the total phosphorus load — trophic state relationships (lake total phosphorus and chlorophyll a concentrations and Secchi depth) found to apply elsewhere. These two points indicate that the artificial lakes in Craigavon behave similarly to natural ones. 相似文献