全文获取类型
收费全文 | 1225569篇 |
免费 | 98914篇 |
国内免费 | 1349篇 |
专业分类
1325832篇 |
出版年
2021年 | 17615篇 |
2020年 | 12530篇 |
2019年 | 16085篇 |
2018年 | 17434篇 |
2017年 | 16243篇 |
2016年 | 27768篇 |
2015年 | 41883篇 |
2014年 | 49853篇 |
2013年 | 76065篇 |
2012年 | 31746篇 |
2011年 | 19487篇 |
2010年 | 42389篇 |
2009年 | 44140篇 |
2008年 | 19170篇 |
2007年 | 16901篇 |
2006年 | 22663篇 |
2005年 | 24080篇 |
2004年 | 23257篇 |
2003年 | 21020篇 |
2002年 | 19258篇 |
2001年 | 25901篇 |
2000年 | 23178篇 |
1999年 | 25274篇 |
1998年 | 24446篇 |
1997年 | 24129篇 |
1996年 | 23798篇 |
1995年 | 21930篇 |
1994年 | 21753篇 |
1993年 | 20810篇 |
1992年 | 22451篇 |
1991年 | 21158篇 |
1990年 | 19781篇 |
1989年 | 20974篇 |
1988年 | 19224篇 |
1987年 | 18181篇 |
1986年 | 17288篇 |
1985年 | 19457篇 |
1984年 | 20081篇 |
1983年 | 17807篇 |
1982年 | 19089篇 |
1981年 | 18494篇 |
1980年 | 17312篇 |
1979年 | 15975篇 |
1978年 | 15875篇 |
1977年 | 15303篇 |
1976年 | 14575篇 |
1975年 | 13943篇 |
1974年 | 14348篇 |
1973年 | 14746篇 |
1972年 | 12433篇 |
排序方式: 共有10000条查询结果,搜索用时 7 毫秒
881.
John G. Neuhoff Katherine L. Long Rebecca C. Worthington 《Evolution and human behavior》2012,33(4):318-322
Listeners consistently perceive approaching sounds to be closer than they actually are and perceptually underestimate the time to arrival of looming sound sources. In a natural environment, this underestimation results in more time than expected to evade or engage the source and affords a “margin of safety” that may provide a selective advantage. However, a key component in the proposed evolutionary origins of the perceptual bias is the appropriate timing of anticipatory motor behaviors. Here we show that listeners with poorer physical fitness respond sooner to looming sounds and with a larger margin of safety than listeners with better physical fitness. The anticipatory perceptual bias for looming sounds is negatively correlated with physical strength and positively correlated with recovery heart rate (a measure of aerobic fitness). The results suggest that the auditory perception of looming sounds may be modulated by the response capacity of the motor system. 相似文献
882.
C-Type Virus Released from Cultured Human Rhabdomyosarcoma Cells 总被引:44,自引:0,他引:44
R. M. McALLISTER M. NICOLSON M. B. GARDNER R. W. RONGEY S. RASHEED P. S. SARMA R. J. HUEBNER M. HATANAKA S. OROSZLAN R. V. GILDEN A. KABIGTING L. VERNON 《Nature: New biology》1972,235(53):3-6
RD-114 virus, released from human rhabdomyosarcoma cells, has all the characteristics of a mammalian C-type virus. Immunological tests indicate that it differs from all known C-type viruses and is the most likely candidate for a human C-type virus yet described. 相似文献
883.
Populations of the Australian sleepy lizard, Tiliqua rugosa, near Mt. Mary, South Australia carry natural infestations of two tick species Aponomma hydrosauri and Amblyomma limbatum. In field experiments at two sites, 18 km apart, lizards with experimentally increased tick loads had smaller home ranges,
moved shorter distances in a day, and were found basking more but moving less often than lizards from which ticks were experimentally
removed. The results were consistent for adult lizards in two years, and for sub-adults in a third year. Laboratory trials
showed that juvenile lizards that had tick infestations had lower sprint speeds than uninfested siblings, and that adults
with tick infestations had less endurance than those that were uninfested. The results contrast with those of a previous survey
that showed that lizards with high tick loads had greater body size and remained longer at a site, but indicate that there
may be a balance, for lizards, between the fitness advantages in occupying habitats with high-quality resources, and the costs
from parasites that also prefer those habitats.
Received: 02 March 1999 / Accepted: 07 October 1999 相似文献
884.
G. F. Katekak 《Biologia Plantarum》1985,27(2-3):92-99
The chemical and stereochemical properties of phytotropins are described in terms of a receptor model, based on their ability
to bind to a receptor isolated from maize coleoptiles. Their root gravitropic properties are correlated with their ability
to bind to the receptor. It is suggested that their ability to inhibit auxin transport, without more, may not be enough to
explain their physiological effects. 相似文献
885.
886.
Infections of one and two Hymenolepis diminuta established in newly weaned rats continued to grow for the duration of the experiment (238 days), whereas infections of 5 worms per rat became asymptotic around Day 55 postinfection and remained at or below this level thereafter as shown by biomass and mean weight per worm measurements. Infections of 50 worms established in newly weaned rats became asymptotic around Day 28 postinfection and thereafter worms were lost from the rats. Initially the biomass fell with the loss of worms, but by Day 56 a new lower biomass persisted for the remainder of the infection period. This level was maintained, despite diminishing numbers of worms, due to the growth of surviving individuals to a weight exceeding the original weight at maturity by a factor of more than 2. Experiments using rats that were mature at the time of infection demonstrated that the same response occurred, but approximately 3 weeks earlier. 相似文献
887.
Ice crystal formation temperature was determined in the region of the crown in one group of 7-day-old intact unhardened high-salt plants of winter wheat (Triticum aestivum L. cv. Weibulls Starke II) with TA (Thermal Analysis) and DTA (Differential Thermal Analysis) methods. After exposure of another group of plants, grown for the first 7 days in the same way as the first group, to various sub-zero temperatures (-1 to 5°C), influx in roots of Rb+(86Rb+) and Ca2+(45Ca2+) and contents of K+ and Ca2+ were determined at intervals during 7 days of recovery. Ice crystal formation in the crown tissue was probably extracellular and took place at about -4°C. There was a large loss of K+ from the roots after treatment at sub-zero temperatures. This loss increased as the temperature of the sub-zero treatment decreased. During recovery, roots of plants exposed to -1, -2 and -3°C gradually reabsorbed K+. Reabsorption of K+ in roots of plants exposed to -4°C was greatly impaired. Rb+ influx decreased and Ca2+ influx increased after sub-zero temperature treatments of the plants. Active Rb+ influx mechanisms and active extrusion of Ca2+ were impaired or irreversibly damaged by the exposure. While Rb+ influx mechanisms were apparently repaired during recovery in plants exposed to temperatures down to -3°C, Ca2+ extrusion mechanisms were not. The temperature for ice crystal formation in the region of the crown tissue coincides with the temperature at which the plants lost the ability to reabsorb K+ and to repair Rb+ influx mechanisms during the recovery period. Plants were lethally damaged at temperatures below ?4°C. 相似文献
888.
889.
890.
By the freeze-fracture method it is shown that metabolic depletion of erythrocytes affects three levels of cell organization: the microstructural (erythrocyte form), microstructural (micro-relief of erythrocyte surface) and ultrastructural (ultrastructural state of erythrocyte plasma membranes) ones. As it is established, the size of spikes on the echinocyte surface and that of membrane vesicles budding from a cell coincide with each other. The structural modification of the membrane precedes the stage of erythrocyte crenation. The following model of vesicle budding process is suggested: reduction of ATP level and dephosphorylation of actin-spectrin network--structural modification of the protein and lipid membrane phases with the formation of regions disconnected from the spectrin framework--protrusion of these anomalous regions in the form of spikes--budding of spikes as spherical vesicles. 相似文献