全文获取类型
收费全文 | 1555篇 |
免费 | 101篇 |
国内免费 | 194篇 |
专业分类
1850篇 |
出版年
2023年 | 27篇 |
2022年 | 24篇 |
2021年 | 22篇 |
2020年 | 49篇 |
2019年 | 48篇 |
2018年 | 57篇 |
2017年 | 51篇 |
2016年 | 79篇 |
2015年 | 59篇 |
2014年 | 62篇 |
2013年 | 108篇 |
2012年 | 57篇 |
2011年 | 61篇 |
2010年 | 61篇 |
2009年 | 67篇 |
2008年 | 67篇 |
2007年 | 89篇 |
2006年 | 61篇 |
2005年 | 69篇 |
2004年 | 56篇 |
2003年 | 50篇 |
2002年 | 50篇 |
2001年 | 42篇 |
2000年 | 34篇 |
1999年 | 26篇 |
1998年 | 45篇 |
1997年 | 23篇 |
1996年 | 37篇 |
1995年 | 43篇 |
1994年 | 32篇 |
1993年 | 29篇 |
1992年 | 40篇 |
1991年 | 21篇 |
1990年 | 31篇 |
1989年 | 26篇 |
1988年 | 20篇 |
1987年 | 14篇 |
1986年 | 13篇 |
1985年 | 13篇 |
1984年 | 21篇 |
1983年 | 9篇 |
1982年 | 11篇 |
1981年 | 12篇 |
1980年 | 8篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 1篇 |
1976年 | 7篇 |
1975年 | 1篇 |
1973年 | 2篇 |
排序方式: 共有1850条查询结果,搜索用时 15 毫秒
51.
The effects of the consumption of flea faeces and non-viable eggs on larval development in the cat flea Ctenocephalides felis (Bouché) (Siphonaptera: Pulicidae) were investigated. Only 13.3% of larvae developed into adults when fed a diet of male or female flea faeces alone; however, 90% of larvae developed into adults when fed on flea faeces supplemented with non-viable flea eggs. When fed with non-viable eggs alone, larvae did not develop into adults. Nevertheless, non-viable eggs may provide critical supplemental nutrients, lacking in flea faeces and required for larval development. None of the larvae fed on flea faeces or non-viable eggs alone formed a cocoon. A diet of flea faeces alone significantly extended the second as well as third larval stadia compared to larvae fed on diets containing non-viable eggs. It is suggested that the cannibalism of fertile eggs may limit population growth in the cat flea. 相似文献
52.
The relevance of porphyrins as therapeutic drugs targeted to mitochondria has been widely recognized. In this work, we studied the action of meso-tetrakis porphyrins (TMPyP) on respiring rat liver mitochondria. Mn(III)TMPyP exerted a protective effect against lipid peroxidation induced by Fe(II) or the azo initiator 4,4-azobis(4-cyanopentanoic acid) (ABCPA), which partition in the hydrophobic phospholipid moiety, and 2,2-azobis(2-amidinepropane)dihydrochloride (ABAP), which partitions in the aqueous phase. In contrast, Fe(III)TMPyP itself induced an intense lipid peroxidation, accompanied by mitochondrial permeability transition. Both mesoporphyrins studied promoted a release of mitochondrial state-4 respiration, in the concentration range of 1.0–20 M. Based on the relative effects of Mn(III)TMPyP against ABAP and ABCPA-induced lipid peroxidation, we believe that meso-tetrakis porphyrins must concentrate preferably at membrane–water interfaces. 相似文献
53.
Controls on Soil Carbon Dioxide and Methane Fluxes in a Variety of Taiga Forest Stands in Interior Alaska 总被引:21,自引:0,他引:21
CO2 and CH4 fluxes were monitored over 4 years in a range of taiga forests along the Tanana River in interior Alaska. Floodplain alder
and white spruce sites and upland birch/aspen and white spruce sites were examined. Each site had control, fertilized, and
sawdust amended plots; flux measurements began during the second treatment year. CO2 emissions decreased with successional age across the sites (alder, birch/aspen, and white spruce, in order of succession)
regardless of landscape position. Although CO2 fluxes showed an exponential relationship with soil temperature, the response of CO2 production to moisture fit an asymptotic model. Of the manipulations, only N fertilization had an effect on CO2 flux, decreasing flux in the floodplain sites but increasing it in the birch/aspen site. Landscape position was the best
predictor of CH4 flux. The two upland sites consumed CH4 at similar rates (approximately 0.5 mg C m−2 d−1), whereas the floodplain sites had lower consumption rates (0–0.3 mg C m−2 d−1). N fertilization and sawdust both inhibited CH4 consumption in the upland birch/aspen and floodplain spruce sites but not in the upland spruce site. The biological processes
driving CO2 fluxes were sensitive to temperature, moisture, and vegetation, whereas CH4 fluxes were sensitive primarily to landscape position and biogeochemical disturbances. Hence, climate change effects on C-gas
flux in taiga forest soils will depend on the relationship between soil temperature and moisture and the concomitant changes
in soil nutrient pools and cycles.
Received 10 March 1998; accepted 29 December 1999. 相似文献
54.
The springtail Deuterosminthurus bicinctus, similarly to other members of Bourletiellidae, use their antennae, legs or heads to monopolize, stimulate, and direct female partners to spermatophores. The mating behavior of this species was examined by analysis of video recordings made on the leaves of its host plant. The characteristic stages of the behavioral sequence leading to sperm transfer were (1) preliminary courtship, highly variable in time and intensity—a male (rarely a female) endeavors to recruit a partner for further courtship, (2) push-and-retreat ritual—a rigid and rhythmical head-to-head dance of both partners, composed of some 180 phases and interspersed by several (3) free turns of a male and ended by (4) spermatophore deposition in front of a female, followed by stimulation of a female by a male to walk over the spermatophore, (5) spermatophore pick-up by a female with her gonopore, or alternately, (6) total spermatophore consumption by a female, as happened in 29% of the observed pairs. The mating ended always with (7) competition by both partners for the consumption of spermatophore residuals, usually won by the female. Comparison of mating elements and morphological features of partners engaged in mating sessions that ended either with spermatophore pick-up or consumption did not reveal any substantial differences. This suggests that changes in female motivation (hunger, state of impregnation) may be crucial for the male success in sperm transfer to a female. A digital video image is available at An erratum to this article can be found at 相似文献
55.
Saila Sillanpää 《Entomologia Experimentalis et Applicata》2008,129(3):286-294
Outbreak densities of autumnal moth, Epirrita autumnata (Borkhausen) (Lepidoptera: Geometridae), lead to high larval crowding. Phenotypic responses of E. autumnata to larval crowding and to food quality were studied by measuring growth and consumption as well as pupal weight and fecundity. Crowding may trigger increased consumption and faster development to avoid impending food shortage on good quality food. This is suggested by the result that on a good‐quality diet, the growth of crowded larvae was better than that of solitary larvae, though they did not consume more food than solitary larvae. Crowded larvae also completed the last instar earlier than solitary larvae. The fecundity of crowded autumnal moths was not lower than the fecundity of solitarily grown autumnal moths. This may provide conditions for extra rapid population build‐up of E. autumnata. During the population increase phase the crowding effect may facilitate larval performance; however, at peak density the crowding starts to have negative effects on the performance of larvae. On a poor‐quality diet, the performance of crowded and solitary larvae did not differ. The growth of larvae was better on a good‐quality diet than on a poor‐quality diet, due to higher efficiency in food utilization. Larvae feeding on low‐quality diet did not prolong their development time, but pupated at smaller size; this resulted in lower fecundity. A decrease in food quality can be seen as a cue of oncoming food shortage and resource depletion; it may be advantageous to pupate at a smaller size and ensure survival till reproduction, rather than risk prolonging development to achieve larger size and higher fecundity. 相似文献
56.
J. Orysiak P. Zmijewski A. Klusiewicz P. Kaliszewski J. Malczewska-Lenczowska J. Gajewski A. Pokrywka 《Biology of sport / Institute of Sport》2013,30(4):249-253
The aim of the study was to examine the possible relationship between I/D polymorphism of ACE gene and selected indices of aerobic capacity among male and female athletes practising winter endurance sports. Sixty-six well-trained athletes (female n = 26, male n = 40), aged 18.4 ± 2.8 years, representing winter endurance sports (cross-country skiing, n = 48; biathlon, n = 8; Nordic combined, n = 10) participated in the study. Genotyping for ACE I/D polymorphism was performed using polymerase chain reaction. Maximal oxygen consumption (VO2max), maximal running velocity (Vmax) and running velocity at anaerobic threshold (VAT4) were determined in an incremental test to volitional exhaustion on a motorized treadmill. The ACE genotype had no significant effect on absolute VO2max, relative VO2max (divided by body mass or fat free body mass), VAT4 or Vmax. No interaction effect of gender x ACE genotype was found for each of the examined aerobic capacity indices. ACE gene variation was not found to be a determinant of aerobic capacity in either female or male Polish, well-trained endurance athletes participating in winter sports. 相似文献
57.
58.
Using Input-Output Analysis to Measure the Environmental Pressure of Consumption at Different Spatial Levels 总被引:5,自引:0,他引:5
《Journal of Industrial Ecology》2005,9(1-2):169-185
Input-output modeling is a useful tool for tracing environmental impacts of consumption. Because it includes impacts originating from production layers of infinite order (capturing the entire economy), input-output modeling is highly relevant for studies operating in a life-cycle context. In this article we show how the input-output approach can be used to enumerate the problem of sustainable consumption. Based on a literature survey including research done by the authors we present measures of the emissions of carbon dioxide at different spatial levels: nation, city, and household. Further, we take more environmental effects into account and introduce the concept of environmental efficiency by combining input-output modeling and data envelopment analysis. Finally, we discuss the policy relevance of the different measures. The article demonstrates that input-output modeling has a wide range of life-cycle oriented applications when combined with other data sources such as detailed trade statistics, foreign input-output and environmental statistics, and household expenditure data. 相似文献
59.
Yew-Hu Chien Wen-Chung Shiau 《Journal of experimental marine biology and ecology》2005,318(2):201-211
Natural carotenoids from astaxanthin containing alga Haematococcus pluvialis (H) and a non-astaxanthin carotenoid-containing alga Spirulina pacifica (S), and a synthetic astaxanthin Carophyll Pink (A) were supplemented in formulated diets at two concentrations, 50 (I) and 100 (II) mg kg−1, resulting in seven pigmented diets HI, SI, AI, HII, SII, AII, and HS (H-50 mg kg−1+S-50 mg kg−1). Formulated diet without carotenoid supplementation served as a control (C). The different diets were fed to juvenile kuruma prawn Marsupenaeus japonicus for 9 weeks. Dietary carotenoid effects on survival, growth, and pigmentation were compared by the treatment individually or collectively. A low dissolved oxygen stress test was conducted 2 weeks later and prawns' survival time and oxygen consumption rate were also compared among treatments. After 9 weeks' rearing, C-fed prawn had significantly lower survival rate than the pigmented diets-fed prawns. No difference in weight gain was found among all prawns. C-fed prawn had 66.4% less flesh astaxanthin (FA) and 75.5% less shell astaxanthin (SA) than the pigmented diets-fed prawns. I-fed (AI, HI, and SI) prawns had 31.1% less FA and 29.6% less SA than II-fed (AII, HII, SII, and HS) prawns. No significant differences were found in the comparisons by other categories. The use of these three sources of carotenoids for pigmentation in crustacean was discussed along carotenoid conversion, deposition, digestibility, and absorption. When subjected to low dissolved oxygen stress, C-fed prawn had higher oxygen consumption rate (OCR) and shorter survival time (ST) than the prawns fed the pigmented diets. No differences in OCR or ST were found in the comparisons by other categories. 相似文献
60.
W. Gibson Wood† Christine Gorka Friedhelm Schroeder‡ 《Journal of neurochemistry》1989,52(6):1925-1930
Alcohols, including ethanol, have a specific effect on transbilayer and lateral membrane domains. Recent evidence has shown that alcohols in vitro have a greater effect on fluidity of one leaflet as compared to the other. The present study examined effects of chronic ethanol consumption on fluidity of synaptic plasma membrane (SPM) exofacial and cytofacial leaflets using trinitrobenzenesulfonic acid (TNBS) labeling and differential polarized fluorometry of 1,6-diphenyl-1,3,5-hexatriene (DPH). Mice were administered ethanol or a control liquid diet for 3 weeks. Animals were killed and SPM prepared. The exofacial leaflet of SPM was significantly more fluid than the cytofacial leaflet in both groups, as indicated by limiting anisotropy of DPH. However, differences between the two leaflets were much smaller in the ethanol-treated group. Ethanol at concentrations seen clinically had a greater effect in vitro on the more fluid exofacial leaflet. This asymmetric effect of ethanol was significantly diminished in the exofacial leaflet of the ethanol-treated mice. Chronic ethanol consumption has a specific effect on membranes. Membrane functions that may be regulated by asymmetry of fluidity and lipid distribution may be altered by chronic ethanol consumption. 相似文献