全文获取类型
收费全文 | 656346篇 |
免费 | 79464篇 |
国内免费 | 328篇 |
出版年
2018年 | 5480篇 |
2016年 | 7159篇 |
2015年 | 9866篇 |
2014年 | 11632篇 |
2013年 | 16565篇 |
2012年 | 18562篇 |
2011年 | 19029篇 |
2010年 | 12763篇 |
2009年 | 12074篇 |
2008年 | 16945篇 |
2007年 | 17504篇 |
2006年 | 16607篇 |
2005年 | 16013篇 |
2004年 | 15589篇 |
2003年 | 15371篇 |
2002年 | 14931篇 |
2001年 | 33215篇 |
2000年 | 33608篇 |
1999年 | 26090篇 |
1998年 | 8342篇 |
1997年 | 9068篇 |
1996年 | 8463篇 |
1995年 | 8128篇 |
1994年 | 7971篇 |
1993年 | 8142篇 |
1992年 | 21610篇 |
1991年 | 21404篇 |
1990年 | 20446篇 |
1989年 | 20165篇 |
1988年 | 18607篇 |
1987年 | 17659篇 |
1986年 | 16396篇 |
1985年 | 16295篇 |
1984年 | 13250篇 |
1983年 | 11527篇 |
1982年 | 8625篇 |
1981年 | 7658篇 |
1980年 | 7365篇 |
1979年 | 12504篇 |
1978年 | 9886篇 |
1977年 | 8957篇 |
1976年 | 8212篇 |
1975年 | 9242篇 |
1974年 | 9592篇 |
1973年 | 9484篇 |
1972年 | 8524篇 |
1971年 | 7868篇 |
1970年 | 6726篇 |
1969年 | 6549篇 |
1968年 | 5819篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Interactions between water availability and elevated atmosphericCO2 concentrations have the potential to be important factorsin determining future forage supply from temperate pastures.Using large turves from an established pasture, the responseof these communities at 350 or 700 l l1 CO2 to a soilmoisture deficit and to recovery from the deficit in comparisonto turves that were well-watered throughout was measured. Priorto this experiment the turves had been exposed to the CO2 treatmentsfor 324 d. Net CO2 exchange continued at elevated CO2 even when the volumetricsoil moisture content was less than 0.10 m3 m3 soil;at the same moisture deficit gas exchange at ambient CO2 waszero. The additional carbon fixed by the elevated CO2 turveswas primarily allocated below-ground as shown by the maintenanceof root length density at the same level as in well-wateredturves. When the dry turves were rewatered there was compensatorygrowth at ambient CO2 so that the above-ground growth rate exceededthat of turves that had not experienced a moisture deficit.At the start of this experiment, the turves that were growingat 700 l I1 CO2 had a greater proportion of legume (principallywhite clover, Trifolium repens L.) in the harvested herbage.There was a trend for the legume content at elevated CO2 tobe reduced under a soil moisture deficit. The results indicate different strategies in response to soilmoisture deficits depending on the CO2 concentration. At ambientCO2, growth stopped, but plants were able to respond stronglyon rewatering; while at elevated CO2 growth continued (particularlybelow-ground), but no additional growth was evident on rewatering.Ecosystem gas exchange measurements taken at the end of theexperiment (after 429 d of exposure to CO2) showed 33% moreCO2 was fixed at elevated CO2 with only a small (12%) and nonsignificantdownward regulation. Key words: Carbon dioxide, climate change, grassland, gas exchange, soil moisture deficit 相似文献
52.
53.
S M Kappes M D Bishop J W Keele M C T Penedo H C Hines M D Grosz G A Hawkins R T Stone S L F Sunden C W Beattie 《Animal genetics》1994,25(3):133-140
Seven bovine erythrocyte antigen loci and three serum protein loci were tentatively assigned to chromosomes or synteny groups by linkage analysis to previously assigned microsatellite DNA markers. The erythrocyte antigen locus EAB was mapped to synteny group U27; EAC to chromosome 18, synteny group U9; EAL to chromosome 3, synteny group U6; EAS to chromosome 21, synteny group U4; EAZ to chromosome 10, synteny group U5; EAR' to chromosome 16, synteny group U1; and EAT' to chromosome 19, synteny group U21. The vitamin D binding protein (GC) and albumin (ALB) loci were assigned to chromosome 6, synteny group U15 and post-transferrin 2 (PTF 2) to chromosome 19, synteny group U21. 相似文献
54.
Terence T. Yen 《Obesity (Silver Spring, Md.)》1994,2(5):472-480
In several species of obese animals, a group of phenethanolamine β-agonists stimulates lipolysis and thermogenesis, resulting in the loss of body fat and weight. Brown adipose tissue is considered to be the major target tissue for the antiobesity activity of these compounds. Independent of this antiobesity activity, some of these compounds are also antidiabetic and increase muscle mass. Based on the pharmacological profile of these com-pounds, a npeceptor was proposed and character-ized in mouse, rat, and humans. The 133-receptor in brown adipose tissue has been suggested to mediate the antiobesity activity of these 13-agonists. Whether this receptor is responsible for the antidiabetic activ-ity and whether there is a linkage between the antiobesity/antidiabetic activity and the nutrient par-titioning activity is not clear. Clinical trials with these mixed 13-agonists showed marginal antiobesity effects when caloric intake of subjects was restricted. Insulin sensitivity was also improved in some of the trials designed to test the antidiabetic activity of these compounds. Side effects included tachycardia and tremor. To eliminate these side effects, a second generation of compounds was selected for its agonist activity on rat D3-receptors. Clinical trials with these compounds have shown lit-tle increase of energy expenditure even at high doses. Successful development of an antiohesity and antidi-abetic drug from this class of compounds will require the elucidation of the physiological role of the human 133-receptor and the regulatory mecha-nism between fuel efficiency and feeding behavior. 相似文献
55.
56.
A recent meta-analysis from the Antiplatelet Trialists'' Collaboration recommended that antilatelet treatment either alone or, for greater effect, in addition to other proved forms of thromboprophylaxis should be considered for patients at high risk of thromboembolism. This paper argues that the current evidence does not justify the adoption of aspirin or other antiplatelet treatment for venous thromboprophylaxis, especially when more effective alternatives exist. Furthermore, several issues relating to this latest meta-analysis need to be debated. 相似文献
57.
58.
59.
Serum samples of Meishan (13 animals) and Meishan x Wild Boar crosses (361 animals) were analysed by means of two-dimensional electrophoresis. Some new variants in protease inhibitor systems PO1A, PO1B and PI2 are reported. 相似文献
60.
T. Friedel F. G. Barth 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1997,180(3):223-233
Spiders can use air particle movements to localize moving prey. We studied the responses of 32 wind-sensitive interneurones
in the hunting spider Cupiennius salei to prey stimuli.
Stimulation with a tethered flying fly or with artificial air pulses activated plurisegmental interneurones that responded
to changes in air movement velocity and were thus well suited to represent the highly fluctuating air stream typical of prey
stimuli. In most interneurones (n = 18) the responses to the stimulation of different legs were not significantly different from each other.
Different interneurones had different response characteristics and their latencies largely overlapped suggesting that there
is parallel processing of the signals by populations of interneurones with different response characteristics.
In two interneurones the number of spikes and the spiking pattern elicited by stimulation of each of the eight legs markedly
differed depending on the leg stimulated. These neurones may play an important role in directional information processing.
Stimulation of the adjacent legs from front to back or from back to front revealed two interneurones sensitive to the direction
of successive stimulation of the legs. These neurones may be able to detect the motion of an air movement source in a preferred
direction and thus act as nearfield motion detectors to localize a moving prey item.
Accepted: 28 September 1996 相似文献