全文获取类型
收费全文 | 4729篇 |
免费 | 645篇 |
国内免费 | 397篇 |
出版年
2024年 | 13篇 |
2023年 | 150篇 |
2022年 | 109篇 |
2021年 | 200篇 |
2020年 | 223篇 |
2019年 | 241篇 |
2018年 | 219篇 |
2017年 | 208篇 |
2016年 | 241篇 |
2015年 | 268篇 |
2014年 | 275篇 |
2013年 | 366篇 |
2012年 | 269篇 |
2011年 | 241篇 |
2010年 | 182篇 |
2009年 | 242篇 |
2008年 | 242篇 |
2007年 | 296篇 |
2006年 | 221篇 |
2005年 | 200篇 |
2004年 | 168篇 |
2003年 | 151篇 |
2002年 | 138篇 |
2001年 | 120篇 |
2000年 | 86篇 |
1999年 | 93篇 |
1998年 | 71篇 |
1997年 | 60篇 |
1996年 | 66篇 |
1995年 | 48篇 |
1994年 | 58篇 |
1993年 | 39篇 |
1992年 | 40篇 |
1991年 | 28篇 |
1990年 | 27篇 |
1989年 | 32篇 |
1988年 | 22篇 |
1987年 | 15篇 |
1986年 | 18篇 |
1985年 | 7篇 |
1984年 | 13篇 |
1983年 | 13篇 |
1982年 | 10篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 8篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1975年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有5771条查询结果,搜索用时 15 毫秒
21.
P. Guo A. Malhotra S. Creer C. E. Pook 《Journal of Zoological Systematics and Evolutionary Research》2009,47(4):378-384
Trimeresurus (in its widest sense) is a very diverse and widespread radiation of Asian pitvipers, which has been subject to numerous taxonomic revisions, some of which have been based on characteristics of the skull. In order to evaluate the taxonomic utility of such characters, we conducted a comparison of the skulls of 57 specimens representing nine genera, and two currently unassigned species that are very closely related to each other. A canonical variate analysis reveals three distinct phenetic groups: the Protobothrops group, a group containing Ovophis monticola , ' Ovophis ' okinavensis , and ' Trimeresurus ' gracilis , and finally a group comprising the remaining species and characterized by considerable overlap between most genera with the exception of the monotypic Peltopelor and Himalayophis . Agreement between phenetic similarity based on skull characteristics and phylogenetic relationships based on molecular evidence varies between different groups: the morphological similarity of the skull of Protobothrops sieversorum to the other Protobothrops species is congruent with their recent synonymization while the phenetic similarity among the species within the second group does not reflect current molecular phylogenetic relationships and indicates that convergent or parallel evolution may be responsible for at least some of the phenetic similarity detected among skulls of the Asian pit vipers examined. A test of phylogenetic independence, however, indicates that there is still a significant phylogenetic signal that can be recovered from several skull characteristics. Thus, we conclude that skull morphology can contribute to an overall understanding of pitviper taxonomy, but that it would be unwise to rely on skull characteristics alone. 相似文献
22.
23.
S. Han M. H. U. Khan Y. Yang K. Zhu H. Li M. Zhu O. Amoo S. U. Khan C. Fan Y. Zhou 《Plant biology (Stuttgart, Germany)》2020,22(4):709-721
- The CLE (CLAVATA3/ESR) gene family, encoding a group of small secretory peptides, plays important roles in cell‐to‐cell communication, thereby controlling a broad spectrum of development processes. The CLE family has been systematically characterized in some plants, but not in Brassica napus.
- In the present study, 116 BnCLE genes were identified in the B. napus genome, including seven unannotated, six incorrectly predicted and five multi‐CLE domain‐encoding genes. These BnCLE members were separated into seven distinct groups based on phylogenetic analysis, which might facilitate the functional characterization of the peptides.
- Further characterization of CLE pre‐propeptides revealed 31 unique CLE peptides from 45 BnCLE genes, which may give rise to distinct roles of BnCLE and expansion of the gene family. The biological activity of these unique CLE dodecamer peptides was tested further through in vitro peptide assays. Variations in several important residues were identified as key contributors to the functional differentiation of BnCLE and expansion of the gene family in B. napus. Expression profile analysis helped to characterize possible functional redundancy and sub‐functionalization among the BnCLE members.
- This study presents a comprehensive overview of the CLE gene family in B. napus and provides a foundation for future evolutionary and functional studies.
24.
25.
Richard O. Prum 《Evolution; international journal of organic evolution》1994,48(5):1657-1675
Phylogenetic analyses of lekking, lek spatial organization, and cooperative and coordinated lek display in the manakins (Aves: Pipridae) demonstrate that variation in social behavior in the group has a strong, phylogenetic component. Two of the three classes of social behavior examined also show significant phylogenetic constraints. Current adaptive plasticity models are insufficient to explain the phylogenetic variation in these behaviors in the manakins. These findings support the conclusion that vertebrate reproductive social behavior has an evolutionary history, and that it is not determined solely by adaptive individual plasticity to current conditions. The evolution of social behavior, particularly through sexual selection, can have historical consequences that can limit subsequent behavioral adaptation. 相似文献
26.
The absorption of lead from loops of small intestinein situ was investigated in rats in which iron absorption was increased by stimuli varying in type, intensity, or duration. Lead
absorption was increased by a short period of severe iron restriction before any change in hematological indices became apparent.
A period of hypoxia, which markedly increased iron absorption, did not influence absorption of lead. An extended period of
moderate iron restriction resulted in a marked reduction in liver iron stores and increased iron absorption throughout the
17-wk experiment. Under these conditions lead absorption was initially also increased, but after 12 wk, when iron intake had
become adequate to meet essential requirements, lead absorption was similar to that in iron-supplemented rats. These results
are discussed in the light of evidence for a receptor-mediated absorption process for iron. 相似文献
27.
Daniel Pérusse 《Human nature (Hawthorne, N.Y.)》1994,5(3):255-278
Most research on mate choice in modern societies is based on data that may or may not reflect actual mating behavior (e.g.,
stated preferences, personal advertisements). In the present study, real-life matings were reported by a large representative
sample of men and women (N = 1,133). These data were used to test an evolutionary model in which mate choice is hypothesized to depend on resources
potentially contributed to reproduction by each sex. Consistent with the model, it was found that (a) men (but not women) of higher social status acquire more mating partners, suggesting that male status is an important criterion
in female choice; (b) women’s (but not men’s) number of partners decreases linearly with age, suggesting that female reproductive potential is
an important criterion in male choice; and (c) women (but not men) display a significant relationship between marital dissolution and promiscuity, suggesting that female
sexual exclusivity is an important criterion in male choice. These results are discussed in relation to understanding mate
choice mechanisms from behavioral data.
Daniel Pérusse is an assistant professor in the Department of Anthropology at the Université de Montréal. His research interests
include the evolutionary biology of human social and reproductive behavior, sexual selection theory, and biocultural evolution.
His current research bears on human socialization processes and psychosocial development from an evolutionary and behavior-genetic
perspective. Recent publications include “Cultural and Reproductive Success in Industrial Societies: Testing the Relationship
at the Proximate and Ultimate Levels” (Behavioral and Brain Sciences 16(2):267–322, 1993) and “Human Parental Behavior: Evidence for Genetic Influence and Potential Implications for Gene-Culture
Theory” with M. C. Neale, A. Heath, and L. J. Eaves (Behavior Genetics, in press). 相似文献
28.
Hisataka Shikama Motoyuki Yajima Michio Ui 《Biochimica et Biophysica Acta (BBA)/General Subjects》1980,631(2):278-288
Hepatic glycogen metabolism was studied in rats during the period of transition from the fed to fasted states. Glycogenic activity was measured in vivo based on the incorporation of [14C]glucose into liver glycogen. Its changes were almost parallel to the changes in glucogen synthase activity. Progressive accumulation of liver glycogen that occurred in the fed state was associated with a proportional increase in glycogenic activity. Within 4 h after the cessation of food intake, glycogenic activity showd a precipitous fall from the peak to its nadir without significant changes in glycogen content. Meanwhile, the glucose concentration in the portal vein decreased. Upon further development of fasting, glycogenic activity displayed a progressive regain, reciprocally as glycogen contents gradually decreased. The precipitous fall of glycogenic activity during the transition from the fed to fasted states was associated with a transient increase in plasma glucagon, and was partly overcome by the injection of anti-glucagon serum. It is concluded that the fall of portal venous concentration of glucose and secretion of glucagon act as a signal to initiate liver glycogen metabolism characteristics of the fasted or postabsorptive state. 相似文献
29.
《Archives of animal nutrition》2013,67(4):339-345
Three experiments were carried out with male broiler chickens reared from day‐ old to 6 weeks of age on semi‐purified diets containing 10% fresh (Expt. 1 and 3) or oxidized (Expt. 2) re‐esterified triglycerides with a fatty acid composition similar to that of soya bean oil containing increasing concentrations of either a mixture of d‐α‐, γ‐, δ‐tocopherylacetate (d‐tocopherols) of natural source or dl‐α‐ tocopheryl acetate (dl‐tocopherol). In Expt. 1 and 2 the mixture of d‐tocopherols consisted of 35.7% d‐α‐, 45.3% d‐γ‐ and 19.0% d‐δ‐, while in Expt. 3 the distribution was 25.3% d‐α‐, 28.1% d‐γ‐and 10.8% d‐γ‐ in 35.8% re‐ esterified triglycerides. The relative biopotency of d‐α‐: γ‐: δ‐tocopherol was anticipated to be 100: 25: 1, whereas that of dl‐a‐tocopherol was 74% relative to d‐ a‐tocopherol. The experiments demonstrate that the results obtained for the biological activity depend on the response parameters chosen. With respect to gain in weight, feed conversion, relative organ weight, packed cell volume (PCV), ELP (erythrocytelipidperoxidation), plasma activities of glutamate‐oxaloacetate‐transaminase (GOT), creatine kinase (CK) and glutathione peroxidase (GSH‐Px) and plasma Na+ concentration, the mixture of natural source tocopherols was identical to that of dl‐α‐tocopheryl acetate, although the concentration of a‐ tocopherol was only about one third of that of dl‐α‐tocopherol. Differences between natural source and synthetic tocopherols were expectedly observed with respect to plasma concentrations of α‐, γ‐, δ‐tocopherol. Differences between the two forms as to muscular dystrophy, in vitro haemolysis and potassium concentration in plasma were ambigous. It is suggested that the function of d‐α‐, γ‐, δ‐tocopherol in erythrocyte fragility and skeletal muscle structure should be compared to that of dl‐α‐tocopherol in future investigations. 相似文献
30.