全文获取类型
收费全文 | 2045篇 |
免费 | 378篇 |
国内免费 | 236篇 |
专业分类
2659篇 |
出版年
2023年 | 12篇 |
2022年 | 36篇 |
2021年 | 43篇 |
2020年 | 53篇 |
2019年 | 67篇 |
2018年 | 37篇 |
2017年 | 35篇 |
2016年 | 45篇 |
2015年 | 75篇 |
2014年 | 143篇 |
2013年 | 118篇 |
2012年 | 190篇 |
2011年 | 209篇 |
2010年 | 148篇 |
2009年 | 135篇 |
2008年 | 126篇 |
2007年 | 96篇 |
2006年 | 85篇 |
2005年 | 72篇 |
2004年 | 58篇 |
2003年 | 54篇 |
2002年 | 52篇 |
2001年 | 43篇 |
2000年 | 41篇 |
1999年 | 35篇 |
1998年 | 18篇 |
1997年 | 13篇 |
1995年 | 16篇 |
1994年 | 13篇 |
1992年 | 17篇 |
1991年 | 35篇 |
1990年 | 28篇 |
1989年 | 31篇 |
1988年 | 19篇 |
1987年 | 34篇 |
1986年 | 24篇 |
1985年 | 17篇 |
1984年 | 20篇 |
1983年 | 43篇 |
1982年 | 25篇 |
1979年 | 20篇 |
1978年 | 14篇 |
1977年 | 13篇 |
1976年 | 17篇 |
1975年 | 20篇 |
1974年 | 14篇 |
1973年 | 16篇 |
1971年 | 12篇 |
1970年 | 11篇 |
1969年 | 11篇 |
排序方式: 共有2659条查询结果,搜索用时 0 毫秒
1.
Frank H. T. Rhodes 《Journal of the history of biology》1987,20(2):139-157
Conclusion The major tenets of the recent hypothesis of punctuated equilibrium are explicit in Darwin's writing. His notes from 1837–1838 contain references to stasis and rapid change. In the first edition of the Origin (1859), Darwin described the importance of isolation of local varieties in the process of speciation. His views on the tempo of speciation were influenced by Hugh Falconer and also, perhaps, by Edward Suess (1831–1914). It is paradoxical that, although both topics were recorded in his unpublished notes of 1837–1838, the second was not explicitly and fully discussed until the fourth edition of the Origin (1866). While no wholly satisfactory explanation of this paradox suggests itself, it seems probable that Falconer's work on the persistence of fossil species of elephant helped Darwin to see the wider significance of the tempo of evolution for his general theory. 相似文献
2.
3.
4.
5.
Judith M. Furze Michael J. C. Rhodes Richard J. Robins 《Plant Cell, Tissue and Organ Culture》1987,8(1):17-25
A cell-wall digestion medium has been devised to isolate protoplasts from suspension cultures of Humulus lupulus. Conditions have been developed for colony formation from protoplasts and the plating efficiency determined in three types of agar and by two culture methods. Viable calli were produced only when protoplasts embedded in Seaplaque agarose were incubated in a defined liquid medium. HPLC analysis showed that none of the isolated colonies accumulated -acids. 相似文献
6.
R H Rhodes 《The American journal of anatomy》1987,179(4):369-384
The surfaces of the brain offer metabolic and mechanical support to the underlying parenchyma. Mouse, rat, and monkey brains were fixed by immersion in a glutaraldehyde fixative or glutaraldehyde with cetylpyridinium chloride, followed by block staining for complex carbohydrates using alcian blue with OsO4 postfixation, or OsO4 postfixative solution containing ruthenium red, or alcian blue and then ruthenium red-OsO4 treatment. The ependyma in these species had a glycocalyx extending into the ventricular fluid as a finely filamentous network when stained with alcian blue or with alcian blue followed by ruthenium red-OsO4. Mice in the middle age range had stained material in this glycocalyx resembling the hyaluronic acid reported in the ocular vitreous body. Similar material was seen in the arachnoidal space of these mice and in the inner connective tissue matrix of the dura mater. Both the mouse and monkey had a cell-free zone, termed the inner dural matrix zone, between the thick fibrous dura and its innermost cellular layer. This zone contained filamentous and globular alcian blue-stained material. The complex carbohydrates of the mouse ependymal glycocalyx and inner dural matrix zone underwent changes developmentally. Aged rats were injected intraventricularly with latex beads, which, along with extravasated erythrocytes, were seen to adhere to the ependymal glycocalyx. A similar adhesion of erythrocytes was seen in the mouse and monkey ependymal glycocalyx and in the filamentous network of the mouse and monkey inner dural matrix zone. The ependymal glycocalyx, formed in part of complex carbohydrates, is much thicker than previously demonstrated. Some activities related to the ependymal lining of the ventricles, including the movement of cells or particles, the penetration of metabolites or serum-protein fractions (e.g., immunoglobulins), and cell-surface hydration, probably depend in part on complex carbohydrates that provide a sticky, electrically negative, hydrophilic environment. The complex carbohydrates in the inner dural matrix zone might provide mechanical buffering. Complex carbohydrates in the arachnoidal space may help to maintain a loose tissue that needs not only to be hydrated, but also to be open enough to provide cerebrospinal fluid circulation. 相似文献
7.
Evidence for a ferredoxin-dependent choline monooxygenase from spinach chloroplast stroma 总被引:18,自引:5,他引:13 下载免费PDF全文
Chenopods synthesize betaine in the chloroplast via a two-step oxidation of choline: choline → betaine aldehyde → betaine. Our previous experiments with intact chloroplasts, and in vivo18O2 labeling studies, led us to propose that the first step is mediated by a monooxygenase which uses photosynthetically generated reducing power (C Lerma, AD Hanson, D Rhodes [1988] Plant Physiol 88: 695-702). Here, we report the detection of such an activity in vitro. In the presence of O2 and reduced ferredoxin, the stromal fraction from spinach (Spinacia oleracea) chloroplasts converted choline to betaine aldehyde at rates similar to those in intact chloroplasts (20-50 nanomoles per hour per milligram protein). Incorporation of 18O from 18O2 by the in vitro reaction was demonstrated by fast atom bombardment mass spectrometry. Ferredoxin could be reduced either with thylakoids in the light, or with NADPH plus ferredoxin-NADP reductase in darkness; NADPH alone could not substitute for ferredoxin. No choline-oxidizing activity was detected in the stromal fraction of pea (Pisum sativum L.), a species that does not accumulate betaine. The spinach choline-oxidizing enzyme was stimulated by 10 millimolar Mg2+, had a pH optimum close to 8, and was insensitive to carbon monoxide. The specific activity was increased threefold in plants growing in 200 millimolar NaCl. Gel filtration experiments gave a molecular weight of 98 kilodaltons for the choline-oxidizing enzyme, and provided no evidence for other electron carriers which might mediate the reduction of the 98-kilodalton enzyme by ferredoxin. 相似文献
8.
Human T cell responses to the Epstein-Barr nuclear antigen-1 (EBNA-1) as evaluated by synthetic peptides 总被引:1,自引:0,他引:1
A panel of synthetic peptides derived from Epstein-Barr virus (EBV) nuclear antigen 1 (EB-NA-1) was used to examine human T cell responses to this antigen. In six of seven normal persons with past EBV infection, T cell precursors specific for five peptides (P27, amino acid residues 83-101;P62, 148-166;E31, 353-367;E41, 368-381; and E11, 461-474) were detectable. The precursor frequencies were in the range of 1:20,000 to less than 1:100,000 peripheral blood mononuclear cells as determined by limiting dilution analyses. Only two of these peptides were predicted as alpha-helices; all peptides were glycine-rich. Four other peptides were not reactive in the seven individuals tested. T cell responses were not detectable in donors without prior EBV infection. Infectious mononucleosis patients investigated 4-6 weeks after diagnosis had likewise no detectable peptide-specific T cell precursors. Thus, it appears that T cells recognizing peptides from EBNA-1 arise and persist in people with past EBV infection. 相似文献
9.
10.
C Foy M Gallagher T Rhodes J Setters P Philips C Donaldson J Bond M Moore S Naji 《BMJ (Clinical research ed.)》1990,300(6731):1048-1049
OBJECTIVE--To assess the impact of HIV on procedures to control infection in general practices. DESIGN--A postal questionnaire survey. SETTING--General practices throughout Britain. SUBJECTS--5359 General practitioners, 3429 (63.9%) of whom returned the questionnaire. MAIN OUTCOME MEASURE--Response to questionnaire on knowledge about HIV and policies for controlling infection. RESULTS--Most doctors (2018) had started to wear gloves when taking blood. Almost half (1510) had not resheathed needles previously but a further 776 had adopted this policy because of HIV. Over half of the doctors did not know or were unsure about the risk of infection from needlestick injuries, and 1759 had no practice policy for controlling infection. CONCLUSIONS--Many doctors are uncertain about measures to control infection in general practice. More information and advice are needed to help doctors develop policies to protect patients and staff. 相似文献