全文获取类型
收费全文 | 40682篇 |
免费 | 189篇 |
国内免费 | 330篇 |
专业分类
系统科学 | 492篇 |
丛书文集 | 794篇 |
教育与普及 | 105篇 |
理论与方法论 | 136篇 |
现状及发展 | 16617篇 |
研究方法 | 1585篇 |
综合类 | 20562篇 |
自然研究 | 910篇 |
出版年
2013年 | 474篇 |
2012年 | 775篇 |
2011年 | 1710篇 |
2010年 | 498篇 |
2009年 | 447篇 |
2008年 | 894篇 |
2007年 | 1081篇 |
2006年 | 1030篇 |
2005年 | 998篇 |
2004年 | 902篇 |
2003年 | 714篇 |
2002年 | 744篇 |
2001年 | 1154篇 |
2000年 | 1178篇 |
1999年 | 819篇 |
1992年 | 722篇 |
1991年 | 547篇 |
1990年 | 580篇 |
1989年 | 542篇 |
1988年 | 536篇 |
1987年 | 542篇 |
1986年 | 555篇 |
1985年 | 715篇 |
1984年 | 552篇 |
1983年 | 429篇 |
1982年 | 399篇 |
1981年 | 428篇 |
1980年 | 508篇 |
1979年 | 1170篇 |
1978年 | 899篇 |
1977年 | 920篇 |
1976年 | 699篇 |
1975年 | 745篇 |
1974年 | 1079篇 |
1973年 | 911篇 |
1972年 | 985篇 |
1971年 | 1085篇 |
1970年 | 1410篇 |
1969年 | 1149篇 |
1968年 | 1031篇 |
1967年 | 1077篇 |
1966年 | 1002篇 |
1965年 | 737篇 |
1959年 | 387篇 |
1958年 | 715篇 |
1957年 | 508篇 |
1956年 | 399篇 |
1955年 | 375篇 |
1954年 | 405篇 |
1948年 | 257篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
991.
Native mushrooms play an important, though often underestimated, role in deer, elk, and caribou diets in North America. Mushrooms are often noted as an unusual or anomalous food in the diets of cervids; yet they often dominate diets in the late summer and fall in forested areas of western North America and throughout the year in the southeastern U.S. Mushrooms are particularly high in protein (16-19%), phosphorus (average 0.75%), and potassium (average 2%). Also, mushroom production is generally greatest in fall. Therefore, they are a highly nutritious food in late season when other native forages may marginally meet basal nutrient requirements of ungulates. 相似文献
992.
Even though every cell in a multicellular organism contains the same genes, the differing spatiotemporal expression of these
genes determines the eventual phenotype of a cell. This means that each cell type contains a specific epigenetic program that
needs to be replicated through cell divisions, along with the genome, in order to maintain cell identity. The stable inheritance
of these programs throughout the cell cycle relies on several epigenetic mechanisms. In this review, DNA methylation and histone
methylation by specific histone lysine methyltransferases (KMT) and the Polycomb/Trithorax proteins are considered as the
primary mediators of epigenetic inheritance. In addition, non-coding RNAs and nuclear organization are implicated in the stable
transfer of epigenetic information. Although most epigenetic modifications are reversible in nature, they can be stably maintained
by self-recruitment of modifying protein complexes or maintenance of these complexes or structures through the cell cycle. 相似文献
993.
Richard L. Wallace J. S. Griffith Jr. D. M. Daley Patrick J. Connolly G. B. Beckham 《西北部美国博物学家》2011,44(2)
Cottus greenei, a potentially threatened species endemic to Idaho, was collected from 49 localities in 25 springs/streams in south central Idaho. Most localities were along the north bank of the Snake River in waters of the Thousand Springs formation, Gooding County. One population was found in a spring in the main Snake River. Another sculpin, Cottus bairdi, was collected with C. greenei at 23 locations in 16 springs/streams. Confusion concerning the type locality of Cottus greenei is discussed. 相似文献
994.
Data were gathered concerning the seasonal activity pattern of a population of Columbian ground squirrels ( Spermophilus columbianus ) in the Idaho Primitive Area. Adult females were significantly more active in June of all years than were adult males. A relationship between ground squirrel activity and temperature is postulated in which the squirrels alter their activity so as to avoid high temperatures and possible heat stress. 相似文献
995.
L. Vanderkelen J. M. Van Herreweghe K. G. A. Vanoirbeek G. Baggerman B. Myrnes P. J. Declerck I. W. Nilsen C. W. Michiels L. Callewaert 《Cellular and molecular life sciences : CMLS》2011,68(6):1053-1064
Lysozymes are antibacterial effectors of the innate immune system in animals that hydrolyze peptidoglycan. Bacteria have evolved
protective mechanisms that contribute to lysozyme tolerance such as the production of lysozyme inhibitors, but only inhibitors
of chicken (c-) and invertebrate (i-) type lysozyme have been identified. We here report the discovery of a novel Escherichia coli inhibitor specific for goose (g-) type lysozymes, which we designate PliG (periplasmic lysozyme inhibitor of g-type lysozyme).
Although it does not inhibit c- or i-type lysozymes, PliG shares a structural sequence motif with the previously described
PliI and MliC/PliC lysozyme inhibitor families, suggesting a common ancestry and mode of action. Deletion of pliG increased the sensitivity of E. coli to g-type lysozyme. The existence of inhibitors against all major types of animal lysozyme and their contribution to lysozyme
tolerance suggest that lysozyme inhibitors may play a role in bacterial interactions with animal hosts. 相似文献
996.
Jean-Pierre Vilardaga Guillermo Romero Peter A. Friedman Thomas J. Gardella 《Cellular and molecular life sciences : CMLS》2011,68(1):1-13
The parathyroid hormone (PTH) receptor type 1 (PTHR), a G protein-coupled receptor (GPCR), transmits signals to two hormone
systems—PTH, endocrine and homeostatic, and PTH-related peptide (PTHrP), paracrine—to regulate different biological processes.
PTHR responds to these hormonal stimuli by activating heterotrimeric G proteins, such as GS that stimulates cAMP production. It was thought that the PTHR, as for all other GPCRs, is only active and signals through
G proteins on the cell membrane, and internalizes into a cell to be desensitized and eventually degraded or recycled. Recent
studies with cultured cell and animal models reveal a new pathway that involves sustained cAMP signaling from intracellular
domains. Not only do these studies challenge the paradigm that cAMP production triggered by activated GPCRs originates exclusively
at the cell membrane but they also advance a comprehensive model to account for the functional differences between PTH and
PTHrP acting through the same receptor. 相似文献
997.
提出了一种基于契约的计划生育策略. 依据该策略, 公民可在结婚时通过契约形式自由选择成功生育一个男孩, 一个女孩, 或者两个孩子作为终止生育的条件. 用概率分析和随机过程分析的手段从数学上论证了这项策略的可行性. 结果表明, 该策略在满足公民对生育子女性别或数量的基本选择权基础上, 使得一对夫妇的平均生育数量接近2, 可以总体上保证人口数量的平稳, 也不会造成男女性别比例的失衡. 相似文献
998.
G. E. Callander R. A. D. Bathgate 《Cellular and molecular life sciences : CMLS》2010,67(14):2327-2341
Since its discovery in the 1920s, relaxin has enjoyed a reputation as a peptide hormone of pregnancy. However, relaxin and
other relaxin family peptides are now associated with numerous non-reproductive physiologies and disease states. The new millennium
bought with it the sequence of the human genome and subsequently new directions for relaxin research. In 2002, the ancestral
relaxin gene RLN3 was identified from genome databases. The relaxin-3 peptide is highly expressed in a small region of the brain and in species
from teleost to primates and has both conserved sequence and sites of expression. Combined with the discovery of the relaxin
family peptide receptors, interest in the role of the relaxin family peptides in the central nervous system has been reignited.
This review explores the relaxin family peptides that are expressed in or act upon the brain, the receptors that mediate their
actions, and what is currently known of their functions. 相似文献
999.
吕韶栋 《科技情报开发与经济》2011,21(20):184-187
对北京丰台区公园式温泉度假中心的设计方案进行了简要分析。该方案遵循了"北国南园"的设计理念,保留并改造了现状绿化、现有建筑和水域,使之能融入新的规划中。 相似文献
1000.
Described is Sclerocactus blainei Welsh & Thorne, sp. nov. from Nye County, Nevada. 相似文献