全文获取类型
收费全文 | 11651篇 |
免费 | 1181篇 |
国内免费 | 1545篇 |
专业分类
14377篇 |
出版年
2024年 | 39篇 |
2023年 | 170篇 |
2022年 | 429篇 |
2021年 | 637篇 |
2020年 | 516篇 |
2019年 | 597篇 |
2018年 | 562篇 |
2017年 | 467篇 |
2016年 | 522篇 |
2015年 | 721篇 |
2014年 | 901篇 |
2013年 | 941篇 |
2012年 | 1090篇 |
2011年 | 922篇 |
2010年 | 607篇 |
2009年 | 549篇 |
2008年 | 621篇 |
2007年 | 566篇 |
2006年 | 465篇 |
2005年 | 412篇 |
2004年 | 445篇 |
2003年 | 434篇 |
2002年 | 430篇 |
2001年 | 344篇 |
2000年 | 207篇 |
1999年 | 171篇 |
1998年 | 123篇 |
1997年 | 73篇 |
1996年 | 80篇 |
1995年 | 59篇 |
1994年 | 62篇 |
1993年 | 43篇 |
1992年 | 29篇 |
1991年 | 35篇 |
1990年 | 24篇 |
1989年 | 24篇 |
1988年 | 9篇 |
1987年 | 14篇 |
1986年 | 4篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 1篇 |
1950年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Plant somatic cells have the capability to switch their cell fates from differentiated to undifferentiated status under proper
culture conditions, which is designated as totipotency. As a result, plant cells can easily regenerate new tissues or organs
from a wide variety of explants. However, the mechanism by which plant cells have such remarkable regeneration ability is
still largely unknown. In this study, we used a set of meristem-specific marker genes to analyze the patterns of stem cell
differentiation in the processes of somatic embryogenesis as well as shoot or root organogenesis in vitro. Our studies furnish preliminary and important information on the patterns of the de novo stem cell differentiation during various types of in vitro organogenesis. 相似文献
2.
Xiao-Guo Xiang Wei-Tao Jin De-Zhu Li André Schuiteman Wei-Chang Huang Jian-Wu Li Xiao-Hua Jin Zhen-Yu Li 《PloS one》2014,9(1)
Collabieae (Orchidaceae) is a long neglected tribe with confusing tribal and generic delimitation and little-understood phylogenetic relationships. Using plastid matK, psaB, rbcL, and trnH-psbA DNA sequences and morphological evidence, the phylogenetic relationships within the tribe Collabieae were assessed as a basis for revising their tribal and generic delimitation. Collabieae (including the previously misplaced mycoheterotrophic Risleya) is supported as monophyletic and nested within a superclade that also includes Epidendreae, Podochileae, Cymbidieae and Vandeae. Risleya is nested in Collabiinae and sister to Chrysoglossum, a relationship which, despite their great vegetative differences, is supported by floral characters. Ania is a distinct genus supported by both morphological and molecular evidence, while redefined Tainia includes Nephelaphyllum and Mischobulbum. Calanthe is paraphyletic and consists four clades; the genera Gastrorchis, Phaius and Cephalantheropsis should be subsumed within Calanthe. Calanthe sect. Ghiesbreghtia is nested within sect. Calanthe, to which the disputed Calanthe delavayi belongs as well. Our results indicate that, in Collabieae, habit evolved from being epiphytic to terrestrial. 相似文献
3.
4.
Tandem duplication and divergence of a sea urchin protein belonging to the troponin C superfamily 总被引:1,自引:0,他引:1
M Q Xiang P A Bédard G Wessel M Filion B P Brandhorst W H Klein 《The Journal of biological chemistry》1988,263(32):17173-17180
The Spec1 and Spec2 proteins of the sea urchin Strongylocentrotus purpuratus are related to calmodulin, troponin C, and myosin light chains by sequence similarity in their four calcium binding domains. These domains, the EF-hands, are distinct helix-loop-helix structures of about 40 amino acids. The Spec1 and Spec2 genes are expressed specifically in aboral ectoderm cells of the developing embryo; however, the function of the Spec proteins in these cells is unknown. To find conserved regions of the proteins that might be important for structure and function, Spec homologues from Lytechinus pictus, a distantly related sea urchin, were sought. L. pictus embryos do not synthesize detectable amounts of the 14,000-17,000-Da Spec proteins as determined by two-dimensional gel electro-phoresis, but do synthesize three 34,000-Da proteins that cross-react with Spec1 antibodies and display a similar ontogenetic pattern of expression. cDNA clones were isolated by hybridization to a synthetic oligonucleotide corresponding to the EF-hand. One clone, LpS1, encodes an mRNA with developmental properties like those of the S. purpuratus Spec mRNAs. However, LpS1 contains an open reading frame for a protein of 34,000 Da rather than 17,000 Da, and antibodies raised against part of the LpS1 reading frame demonstrate that LpS1 encodes a 34,000-Da protein in L. pictus embryos. The sequence of LpS1 reveals the presence of eight EF-hand domains, which share structural homology with the Spec1 or Spec2 EF-hands; however, little else in the protein sequence is conserved. The results support the hypothesis that the LpS1 gene arose from a duplication of an ancestral Spec gene and that the overall structural features of the Spec family of proteins are more conserved than the amino acid sequences. 相似文献
5.
糙叶败酱挥发油镇静作用的研究 总被引:4,自引:1,他引:3
本文观察了败酱科植物糙叶败酱(Patrinia scabra Bunge)根和根茎中制得的挥发油的镇静作用,井与黄花败酱挥发油做了比较。结果表明,此油灌胃给予数组小鼠,剂量0.45ml/kg,显示如下的作用:[1]能显著延长由于腹腔注射戊巴比妥钠(50mg/kg)引起的小鼠睡眠时间,但其作用强度弱于黄花败酱挥发油。(2)一次灌胃给予小鼠大剂量10.46g/kg的糙叶败酱挥发油,连续观察10天,动物外观正常,无一死亡,体重增加与对照组相似。 相似文献
6.
超结瘤大豆(Glycine m ax (L.) Merr.) nts 382 和不结瘤大豆Nod 49 的叶和根组织水提取物经Sephadex G25 过滤、洗脱,再根据洗脱物对硝酸还原酶(NR)活性的影响可划分为4 个组分(fraction)样品,即nts 382(Nod 49) F1、nts 382(Nod 49) F2、nts 382(Nod 49) F3 和nts 382(Nod 49) F4。其中, nts382 F2 和F4 抑制NR 活性作用在接种USDA110 后明显下降, 但接种的nts 382 F2 却能提高大豆Bragg 的结瘤数达一倍, 而接种的nts 382 F3 和F4 的作用不明显。NR 活性抑制因子不是刺激结瘤的因子, 刺激结瘤的因子主要分布在接种的nts382 F2 部分中。与这一现象相反, Nod 49 F2 和F4 抑制NR活性的作用在接种后更强, 且也抑制大豆nts 382 的结瘤, 其中Nod 49 F4 抑制结瘤的作用基本不能逆转。抑制结瘤因子主要分布在接过种的Nod 49 F4 部分中 相似文献
7.
Six cases of mother-cub relationships in wild giant pandas (Ailuropoda melanoleuca) were observed in the Qinling Mountains, China. It was found that panda cubs are normally left alone in the den for 4–8 h while mothers forage. The mother's absence during bouts of foraging should be considered when rescuing abandoned cubs in order to avoid adding to the decline of the wild population. © 1994 Wiley-Liss, Inc. 相似文献
8.
Zhuang XC Sun YZ Cui J Zhu JM Jiang C Xiang QL Li CS 《Journal of gravitational physiology : a journal of the International Society for Gravitational Physiology》1994,1(1):P61-P63
In the environment of microgravity, the disused atrophy of skeletal muscle, especially leg's muscle, would occur. The three purposes of this study were: 1. To observe the dynamic changes of disused atrophy of skeletal muscle under simulated weightlessness; 2. To approach the mechanism of disused atrophy of muscle; 3. To approach the countermeasures for reducing the degree of atrophy of muscle. 相似文献
9.
Zhi Jian Yu Changbae Jin Robin William Rockhold Beth Hoskins Ing Kang Ho 《Neurochemical research》1993,18(11):1203-1209
Wistar-Kyoto and spontaneously hypertensive rats received i.v. infusions of cocaine hydrochloride (60 mg/kg per day) for 3, 7, and 14 days, or saline for 7 days. Acute cocaine challenge (40 mg/kg, s.c.) was given to treated and control rats 24 hr after the termination of each infusion period. There were no strain differences in brain levels of cocaine during cocaine infusion, nor after cocaine challenges. There were no strain differences in resting levels of [3H]dopamine release. Release of [3H]dopamine decreased in nuclei accumbens of 7- and 14-day cocaine-infused animals. Release of [3H]dopamine was maximal in both brain regions 2 hr after acute cocaine challenge. After 14 days of cocaine infusion, cocaine challenge in both strains reduced [3H]dopamine release in the nucleus accumbens, but not in the striatum; the reduction being greater in Wistar-Kyoto rats. The behavioral tolerance which accompanies similar cocaine infusion regimens may be related to striatal tolerance to cocaine-induced dopamine release. 相似文献
10.
Mengyi Ba Ruonan Chen Qiuchen Huang Yanli Song Wen Li Yuanyuan Zhang Haixin Liu Xiang Xu Weidong Zhang Zhiqiang Cai Tao Sun 《化学与生物多样性》2023,20(8):e202300350
This work firstly reported a new polycaprolactone based material functionalized with guanidinium ionic liquid (PCL-GIL) as the stationary phase with high resolution performance for capillary gas chromatography (GC). It is composed of polycaprolactone (PCL) and guanidinium ionic liquid (GIL) with amphiphilic conformation. The PCL-GIL capillary column coated by static method exhibited high column efficiency of 3942 plates/m and moderate polarity. As a result, the PCL-GIL column exhibited high-resolution capability. For a mixture of 27 analytes with a wide ranging polarity and outperformed the PCL-2OH and HP-35 columns, showing its advantageous separation capability for analytes of diverse types. Moreover, the PCL-GIL column showed high resolving capability for various positional isomers and cis-/trans-isomers, including alkylbenzenes, chlorobenzenes, naphthalenes, bromonitrobenzenes, chloronitrobenzenes, benzaldehydes, phenols, alcohols, respectively. In a word, PCL derivatized by GIL units as a new type of stationary phase has a promising future in GC separations. 相似文献