全文获取类型
收费全文 | 2421篇 |
免费 | 186篇 |
国内免费 | 1篇 |
专业分类
2608篇 |
出版年
2023年 | 8篇 |
2022年 | 24篇 |
2021年 | 44篇 |
2020年 | 25篇 |
2019年 | 46篇 |
2018年 | 62篇 |
2017年 | 47篇 |
2016年 | 72篇 |
2015年 | 114篇 |
2014年 | 136篇 |
2013年 | 150篇 |
2012年 | 214篇 |
2011年 | 227篇 |
2010年 | 118篇 |
2009年 | 98篇 |
2008年 | 158篇 |
2007年 | 151篇 |
2006年 | 162篇 |
2005年 | 130篇 |
2004年 | 116篇 |
2003年 | 99篇 |
2002年 | 91篇 |
2001年 | 25篇 |
2000年 | 27篇 |
1999年 | 38篇 |
1998年 | 42篇 |
1997年 | 21篇 |
1996年 | 21篇 |
1995年 | 28篇 |
1994年 | 8篇 |
1993年 | 12篇 |
1992年 | 15篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1973年 | 3篇 |
1971年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有2608条查询结果,搜索用时 15 毫秒
11.
Mari Källersjö 《Nordic Journal of Botany》1985,5(6):527-542
Within South African Asteraceae-Anthemideae there is a group of genera containing furanosesquiterpenes rather than the common polyacetylenes. Of these genera, Asaemia (Harv.) Ham. ex Benth. & Hook., Athanasia L., Eumorphia DC., Gymno-pcnfzia Benth., Phymaspermum Less. and Sfilpnophyfon Less. have been investigated morphologically especially with respect to fruit structure. As a result of the investigations Stilpnophyton has been reduced to synonomy under Athanasia L. emend. Källersjö (with 36 spp.) and five species of Athanasia , together with Phaeocephalus S. Moore., are placed in the revived genus Hymenolepis Cass. (with 7 spp.). Brachymerk DC. and four misplaced species of Aihanasia are included in Phymaspermum Less. emend. Källersjö (with 17 spp.). Nine other misplaced species of Athanasia and one Pentzia Thunb. species have been described as a new genus Inulanihera Källersjö (with 10 spp.), a group without furanosesquiterpenes. The two monotypic genera Asaemia and Gymnopentzia , and Eumorphia (with 6 spp.) remain unchanged. The interrelationships of the genera possessing furanosesquiterpenes are shown in a cladogram. There are 25 new combinations in Afhanasia, Znulanthera, Hymenolepis and Phymaspermum . 相似文献
12.
Emilio Román Galán Jaun A. Galbis Pérez Mariá A. Arévalo Arévalo 《Carbohydrate research》1983,116(2):255-262
The reaction between 2-(benzylamino)-2-deoxy-d-glycero-l-gluco-heptose and 5,5-dimethyl-1,3-cyclohexanedione yields 1-benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-2-(d-galacto-pentitol-1-yl)-indol-4-one (2). Acid-catalyzed, intramolecular dehydration of 2 under kinetically controlled conditions gives 1-benzyl-4,5,6,7-tetrahydro-2-α-d-lyxofuranosyl-6,6-dimethylindol-4-one; the anomeric configuration of this compound is only suggested. When the dehydration reaction is conducted under thermodynamically controlled conditions, it produces a 1:1 mixture of the α- and β-d-lyxopyranosyl compounds. The structures of the new compounds were elucidated by chemical and physical methods. 相似文献
13.
14.
Site-specific aflatoxin B1 adduction of sequence-positioned nucleosome core particles 总被引:2,自引:0,他引:2
The question of how the presence of nucleosomal packing of DNA modifies carcinogen interaction at specific sites cannot be answered by studies on whole chromatin or bulk nucleosomes because of the heterogeneity of DNA sequences in the particles. We have circumvented this problem by using nucleosomes that are homogenous in DNA sequence and hence in DNA-histone contact points. A cloned DNA fragment containing a sea urchin 5 S gene which precisely positions a histone octamer was employed. By using 32P end-labeled DNA and genotoxins that allow cleavage at sites of attack, the frequency of adduction at every susceptible nucleotide can be determined on sequencing gels. The small methylating agent dimethyl sulfate and the bulky alkylating agent aflatoxin B1-dichloride (AFB1-Cl2) were used to probe the influence of DNA-histone interactions on DNA alkylation patterns in the sequence-positioned core particle. We find dimethyl sulfate to bind with equal preference to naked or nucleosomal DNA. In contrast, AFB1-Cl2 binding is suppressed an average of 2.4-fold at guanyl sites within nucleosomes compared with AFB1-Cl2 affinity at the corresponding site in naked DNA. The DNA is more accessible in regions near the particle boundary. We observe no other histone-imposed localized changes in AFB1-Cl2 sequence specificity. Further, sites of DNase I cleavage or proposed DNA bending show neither enhanced nor reduced AFB1-Cl2 adduction to N7-guanine. Since AFB1-Cl2 binding sites lie in the major groove, nucleosomal DNA appears accessible to AFB1-Cl2 at all points of analysis but with an access which is uniformly restricted in the central 100 nucleotides of the core particle. The data available do not indicate further localized or site-specific perturbations in DNA interactions with the two carcinogens studied. 相似文献
15.
Consequences of herbivory in the mountain birch (Betula pubescens ssp tortuosa): importance of the functional organization of the tree 总被引:3,自引:0,他引:3
Summary Three types of experiments indicate that the functional organization of the mountain birch may influence the ways in which the tree responds to simulated or natural herbivory. The first experiment showed that herbivory to both short and long shoot leaves affects plant development but, because growth largely proceeds by resources of the previous year, is manifested only in the year following the damage. The second experiment showed that even partial damage to a single long shoot leaf caused the axillary bud of that leaf to produce a shorter shoot the next year. Therefore, the value of a leaf depends also on the organ which it is subtending. In the third experiment we manipulated the apical dominance of shoots in ramets and caused improvement to leaf quality in extant shoots. Ramets within a tree responded individually, probably mediated by disturbance of the hormonal control because removal of apical buds elicited the response although removal of the same number of basal buds did not. Induced amelioration is a different response to induced resistance. The two responses are triggered by different cues and may occur in the same plant. By altering hormonal balance of shoots it is potentially possible for herbivores to induce amelioration of food quality. The ways in which herbivory is simulated may explain variability of results obtained when herbivory-induced responses in plants have been studied. 相似文献
16.
Summary We have investigated the effects of high and low temperature on the synthesis and secretion of cellulases and other enzymes by two common and readily available strains ofTrichoderma
reesei. While some effects were similar in both strains QM9414 and RUT-C30 (a reduction in cellulase production but stimulation of xylanase production at high temperature, and alterations in expression of the cellulase complex at low temperature), some specific differences between the strains were determined, most significantly an enhanced specific secretion rate (secretion/growth) at low growth temperature for QM9414. 相似文献
17.
Shinji Yamasaki Zaw Lin Hiromasa Shirai Akito Terai Yuichi Oku Hideaki Ito Mari Ohmura Tadahiro Karasawa Teizo Tsukamoto Hisao Kurazono Yoshifumi Takeda 《Microbiology and immunology》1996,40(5):345-352
To identify the type of Verotoxins (VT) produced by Verocytotoxin-producing Escherichia coli (VTEC), a sensitive bead-enzyme-linked immunosorbent assay and polymerase chain reaction with common and specific primers to various VTs (VT1, VT2, VT2vha, VT2vhb, and VT2vp1) were developed. Together with colony hybridization tests with oligo- and polynucleotide probes, these methods were applied to VTEC isolates to type the VT produced. The toxin types of 26 of 37 strains were identified, but the reaction profiles in assays of the remaining 11 strains suggested the existence of new VT2 variants. The application of these identification procedures may be useful as a tool for clinical and epidemiological studies of VTEC infection. 相似文献
18.
Abstract: The effects of α-sialosylcholesterol (α-SC) on formation of either microfilament or glia filament of rat astrocytes were investigated using a reconstitution system. Polymerization of the depolymerized microfilament preparation that had been extracted from a crude cytoskeletal fraction of rat astrocytes, in the presence of 100 m M KCI and 10 m M MgCI2 , was suppressed in a dose-dependent manner by α-SC. α-SC inhibited polymerization of G-actin in a similar manner. The intensity of a-SC inhibition of G- actin polymerization was as great as that of microfilament polymerization, suggesting that the inhibition of microfilament polymerization by α-SC was due to the direct action of α-SC on actin, the main component of microfilament. α-SC depolymerized partly the polymerized microfilament preparation, which resembled F-actin (microfilament-like filaments). α-SC suppressed, in a dose-dependent manner, polymerization of a glia filament preparation that had been extracted from astrocyte cytoskeletons in the presence of phalloidin. An increase in the amount of added α-SC (up to 15 n M ) decreased the amount of the larger glia filament-like filaments, which were 10 nm thick and centrifuged down at 16,000 g for 30 min, and increased that of smaller ones precipitated only after centrifugation at 100,000 g for 1 h. The lower the concentration of the depolymerized glia filament extract, the greater was the inhibition by α-SC of the polymerization. α-SC repressed polymerization of vimentin, the dominant component of glia filament. Vimentin polymerization was more strongly inhibited by α-SC than polymerization of glia filament was. The findings suggested that α-SC suppressed polymerization of glia filament through a direct action on vimentin and that the glia filament-associated proteins increased its structural stability in the presence of α-SC. 相似文献
19.
Gibberellin A3 and Abscisic Acid Cause the Reorientation of Cortical Microtubules in Epicotyl Cells of the Decapitated Dwarf Pea 总被引:1,自引:0,他引:1
To determine whether or not the changes in the orientation ofmicrotubules (MTs) that are induced by GA3 and ABA result fromchanges in the rate of epicotyl elongation caused by these hormones,we examined the effects of GA3 and ABA on the orientation ofMTs in epidermal cells of decapitated epicotyls of the dwarfpea (Pisum sativum cv. Little Marvel), in which neither GA3nor ABA causes changes in the rate of epicotyl elongation. Cuttings taken from GA3-pretreated seedlings were decapitatedand treated with ABA. ABA eliminated the GA3-induced predominanceof transverse MTs and treatment with ABA resulted in a predominanceof longitudinal MTs in the decapitated cuttings. However, ABAdid not reduce the rate of epicotyl elongation in these samples.Cuttings taken from ABA-pretreated seedlings were decapitatedand treated with GA3. GA3 caused the orientation of MTs to changefrom longitudinal to transverse in the decapitated cuttings.However, GA3 had no promotive effect on elongation of theseepicotyls. The results indicate that both ABA and GA3 have the abilityto change the orientation of MTs by mechanisms that do not involvechanges in the rate of cell elongation. (Received August 18, 1992; Accepted January 18, 1993) 相似文献
20.
Noriko Arase-Fukushi Hisashi Arase Bingyan Wang Mari Hirano Kazumasa Ogasawara Robert A. Good Kazunori Ono 《Microbiology and immunology》1993,37(11):883-894
Allo-chimerism and clonal elimination of self antigen (Ag) (Ia + Mls-1a) reactive Vβ6+ T cells were analyzed and compared between allogeneic bone marrow (BM) chimeras reconstituted with BM cells which had been treated with anti-Thy-1 monoclonal antibody (mAb) plus complement (C) (T– chimeras) and BM chimeras which had been reconstituted with BM cells pretreated with anti-Thy-1 mAb alone (T+ chimeras). When lethally irradiated AKR (Mls-1a) mice were reconstituted with BM cells from B10 or B10 H-2 congenic mice, both T+ and T– chimeras were entirely free of signs of graft-versus-host reaction (GVHR). However, complete replacement of the AKR lymphoid tissues by donor BM cells was accomplished at an early stage in T+ chimeras but not in T– chimeras. On the other hand, clonal elimination of Vβ6+ T cells reactive to the recipient Ag (Mls-1a) was abolished in T+ chimeras but successfully induced in T– chimeras. The Vβ6+ T cells not eliminated in T+ chimeras showed depressed responses against Mls-1a antigens. The findings herein demonstrate that T cells which contaminate a BM inoculum survive in recipient mice after treatment with anti-Thy-1 mAb without C in vitro followed by BMT. The surviving T cells have been estimated to represent fewer than 0.5% of the BM cells inoculated. These cells appear to accelerate the full replacement of recipient lymphoid tissues by donor cells. Furthermore, the T cells which survive in the marrow inoculum influence eventually the development of a tolerant state in the T cell repertoire of the donor. 相似文献