全文获取类型
收费全文 | 11819篇 |
免费 | 1025篇 |
国内免费 | 1689篇 |
出版年
2024年 | 8篇 |
2023年 | 184篇 |
2022年 | 232篇 |
2021年 | 308篇 |
2020年 | 333篇 |
2019年 | 413篇 |
2018年 | 374篇 |
2017年 | 371篇 |
2016年 | 447篇 |
2015年 | 405篇 |
2014年 | 551篇 |
2013年 | 774篇 |
2012年 | 520篇 |
2011年 | 562篇 |
2010年 | 446篇 |
2009年 | 652篇 |
2008年 | 674篇 |
2007年 | 688篇 |
2006年 | 692篇 |
2005年 | 572篇 |
2004年 | 517篇 |
2003年 | 488篇 |
2002年 | 429篇 |
2001年 | 389篇 |
2000年 | 318篇 |
1999年 | 347篇 |
1998年 | 276篇 |
1997年 | 239篇 |
1996年 | 231篇 |
1995年 | 207篇 |
1994年 | 215篇 |
1993年 | 213篇 |
1992年 | 201篇 |
1991年 | 175篇 |
1990年 | 154篇 |
1989年 | 114篇 |
1988年 | 112篇 |
1987年 | 96篇 |
1986年 | 84篇 |
1985年 | 91篇 |
1984年 | 87篇 |
1983年 | 57篇 |
1982年 | 74篇 |
1981年 | 56篇 |
1980年 | 41篇 |
1979年 | 32篇 |
1978年 | 20篇 |
1977年 | 15篇 |
1976年 | 26篇 |
1974年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Decoupling of reproductive rates and parental expenditure in a polyandrous butterfly 总被引:6,自引:2,他引:4
Current theory postulates that the operational sex ratio (OSR)determines the relative degree of mating competition in thetwo sexes and is in turn influenced by a sexual difference inthe potential reproductive rate (PRR) denned as 1/time out,where time out is the time an individual must spend recoveringfrom a bout of mating activity and/or caring for offspring.In bushcricket mating systems where males provide females witha nuptial gift, relative energy expenditure in offspring influencesthe PRR of males and females and underlies a diet-mediated shiftin the OSR. Here we investigated if there is a similar positiverelationship between relative parental nutrient expenditurein offspring and PRR in the polyandrous butterfly Pieris napi,where female fecundity is strongly dependent on male nuptialgifts at mating. By varying the amount of nutrients femalesreceive at mating and relating this to number of offspring produced,we show that male P. napi have, on average, a nutrient expenditurein offspring equaling that of females. In spite of this, themale reproductive rate is 813 times higher than thatof females. Hence the relative degree of parental expenditurein offspring is largely decoupled from the degree of matingcompetition in P. napi. Two alternative explanations are advancedto account for the difference between the butterfly and thebushcricket mating systems. 相似文献
972.
James W. Blackburn 《Bioremediation Journal》1998,1(4):265-282
Bioremediation has been applied in laboratory-scale testing to simulate field-scale bioremediation applications. The simulations have focused on the development of concentration-time profile data in the laboratory to be modeled and used for predicting field-scale performance, particularly the time of treatment and the treatment concentration endpoints. This review reports on more than a dozen examples of bioremediation where both the laboratory-scale and field-scale data are provided. These data have been analyzed to examine how well the laboratory-scale kinetics predict the kinetics at field scale. In most cases, the laboratory-scale kinetics exceed field-scale kinetics and underpredict the time of treatment to a given endpoint in excess of 100% and by as much as 11,900%. In some cases, the laboratory-scale kinetic rate constants fall between ±100% of the field-scale kinetics, leading to predictions within 50 to 200% of the time of treatment to a given concentration level. Not enough examples nor details within the references exist to permit a detailed explanation of why these differences exist. However, several hypotheses are given in this review as to why scaleup is often problematic, and comments from other investigators on this topic are presented. 相似文献
973.
Effects of climate change on nitrogen dynamics in upland soils. 2. A soil warming study 总被引:7,自引:0,他引:7
Philip Ineson David G. Benham Janet Poskitt Anthony F. Harrison Kenneth Taylor Clive Woods 《Global Change Biology》1998,4(2):153-161
A new warming technique has been developed in a field experimental study of the potential effects of climatic change on N leaching from hill land plant/soil systems. Thermocouple compensating cable has been utilized to provide a small cross-section, flexible, low voltage heating cable, mounted on a framework of stainless steel mesh, to provide uniform heating at the vegetation/soil interface of zero-tension lysimeters and surrounding turf. We describe a specially designed heat controller capable of maintaining a temperature differential of 3 °C above ambient at a soil depth of 0.8 cm. The equipment raises temperatures down the soil profile and within the grass sward, whilst tracking normal diurnal temperature variation. Results presented here illustrate the efficacy of the warming technique, together with the consequences for the release of nitrate from lysimeters. The responses of soil solution concentrations of nitrate varied markedly between soil types, but showed a significant decrease in the brown earth during the first 5 months of additional heating. This suggests that increased nutrient release is masked by plant uptake in this soil, but the responses in the other two soils were less marked. 相似文献
974.
Carbon Dioxide Variation in a Hardwood Forest Stream: An Integrative Measure of Whole Catchment Soil Respiration 总被引:8,自引:3,他引:5
The concentration of CO2 in stream water is a product of not only instream metabolism but also upland, riparian, and groundwater processes and as
such can provide an integrative measure of whole catchment soil respiration. Using a 5-year dataset of pH, alkalinity, Ca2+, and Mg2+ in surface water of the West Fork of Walker Branch in eastern Tennessee in conjunction with a hydrological flowpath chemistry
model, we investigated how CO2 concentrations and respiration rates in stream, bedrock, and soil environments vary seasonally and interannually. Dissolved
inorganic carbon concentration was highest in summer and autumn (P < 0.05) although the proportion as free CO2 (pCO2) did not vary seasonally (P > 0.05). Over the 5 years, pCO2 was always supersaturated with respect to the atmosphere ranging from 374 to 3626 ppmv (1.0- to 10.1-fold greater than atmospheric
equilibrium), and CO2 evasion from the stream to the atmosphere ranged from 146 to 353 mmol m−2 d−1. Whereas pCO2 in surface water exhibited little intra-annual or interannual variation, distinct seasonal patterns in soil and bedrock pCO2 were revealed by the catchment CO2 model. Seasonally, soil pCO2 increased from a winter low of 8167 ppmv to a summer high of 27,068 ppmv. Driven by the seasonal variation in gas levels,
evasion of CO2 from soils to the atmosphere ranged from 83 mmol m−2 d−1 in winter to 287 mmol m−2 d−1 in summer. The seasonal variation in soil CO2 tracked soil temperature (r
2= 0.46, P < 0.001) and model-derived estimates of CO2 evasion rate from soils agreed with previously reported fluxes measured using chambers (Pearson correlation coefficient =
0.62, P < 0.05) supporting the model assumptions. Although rates of CO2 evasion were similar between the stream and soils, the overall rate of evasion from the channel was only 0.4% of the 70,752
mol/d that evaded from soils due to the vastly different areas of the two subsystems. Our model provides a means to assess
whole catchment CO2 dynamics from easily collected and measured stream-water samples and an approach to study catchment scale variation in soil
ecosystem respiration.
Received 24 July 1997; accepted 14 November 1997. 相似文献
975.
Chick parasitism by blowflies affects feeding rates in a Mediterranean population of blue tits 总被引:1,自引:0,他引:1
Sylvie Hurtrez-Boussès Jacques Blondel Philippe Perret Jacques Fabreguettes & François Renaud Renaud 《Ecology letters》1998,1(1):17-20
Offspring fitness depends on interactions between parental care and environmental constraints. It has been suggested that in altricial birds parents are able to compensate for the detrimental effects of ectoparasites by improving food provisioning. We tested this prediction in a population of blue tits highly parasitized by blowfly larvae. The frequency of parental feeding visits was significantly higher in parasitized broods than in broods experimentally deparasitized. Despite a strong increase in parental care, chicks of parasitized broods were lighter, smaller, and more anaemic than chicks in deparasitized broods. Parents invest more in feeding parasitized young but cannot fully compensate for the negative effects of parasites, hence young are in poor condition at fledging. 相似文献
976.
在温室条件下研究了水分亏缺对银合欢根瘤的水分关系、固氮酶活性(乙炔还原活性)、呼吸活性以及蔗糖代谢有关酶活性的影响。随着土壤含水量的下降,根瘤水势也相继下降。土壤干旱不但显著地抑制了根瘤乙炔还原的活性,而且对根瘤的呼吸活性、ATP的产生以及催化蔗糖降解的碱性转化酶和蔗糖合成酶的活性也具强烈的抑制作用。然而,根瘤可溶性总糖含量则不受土壤干旱的影响。用呼吸抑制剂DNP处理根瘤后,其固氮酶活性、呼吸活性及ATPI含量都受到极显著的抑制。这都表明,水分胁迫对根瘤呼吸活性及ATP产生的抑制可解释干旱条件下固氮酶活性的下降。 相似文献
977.
离子注入技术是将某种元素的原子进行电离,并使其在电场中加速,在获得较高的速度后射入固体材料表面。在离子注入过程中,被电离的离子在电场作用下加速运动,离子靠着本身获得的动能进入基体表面,在表层中运动的离子与基体原子作用损失能量后在一定的位置停留下来。该技术自60年代问世以来,主要用于材料改性等方面。80年代中期,我国学者开始将其用于农作物育种方面的研究,大大拓宽了离子注入技术的应用领域。所用实验材料的基因及表现型见Tab3,我们将氢离子(E=35MeV)注入处于胚胎发育后期的家蚕卵内(Tab1),观察其对家蚕形态及遗传方面的影响,结果表明:(1)在家蚕胚胎发育的已4期注入氢离子,其半致死剂量LD50为1x1010~1x1011cm2这一区间之内;当剂量达到1x1012cm2时,已全部致死(Fig.1&Tab.2);(2)注入氢离子能够使家蚕在第1腹节上产生褐斑(Fig.2)的频率增高。并首次观察到因注入氢离子而导致家蚕出现非成对的褐斑(Fig.3&Tab.4)。(3)在氢离子注入剂量为1x1010cm2时,能够诱变产生大量的嵌合体家蚕,并且诱变频率高达38.5%(Fig.4&Tab.5),这样高的 相似文献
978.
Nguyen Quynh Thi Kozai Toyoki Niu Genhua Nguyen Uyen Van 《Plant Cell, Tissue and Organ Culture》1998,55(2):133-139
The photosynthetic characteristics of coffee ( Coffea arabusta) plantlets cultured in vitro in response to different CO2 concentrations inside the culture vessel and photosynthetic photon flux (PPF) were investigated preliminarily. The estimation
of net photosynthetic rate (Pn) of coffee plantlets involved three methods: (1) estimating time courses of actual Pn in situ based on measuring CO2 concentrations inside and outside the vessel during a 45-day period, (2) estimating Pn in situ at different CO2 concentrations and PPFs using the above measuring approach for 10-day and 30-day old in vitro plantlets, and (3) estimating
Pn of a single leaf at different CO2 concentrations and PPFs by using a portable photosynthesis measurement system for 45-day old in vitro coffee plantlets. The
results showed that coffee plantlets in vitro had relatively high photosynthetic ability and that the Pn increased with the increase in CO2 concentration inside the vessel. The CO2 saturation point of in vitro coffee plantlets was high (4500–5000 μmol mol-1); on the other hand, the PPF saturation point was not so high as compared to some other species, though it increased with
increasing CO2 concentration inside the vessel.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
979.
B. M. McAllan J. R. Roberts T. O'Shea 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1998,168(1):41-49
The small marsupial Antechinus stuartii experiences a synchronised life cycle that culminates in complete male mortality (within 3 weeks) following the 1 week mating
period in mid-August (late winter). There are pronounced physiological changes in male A. stuartii over the life cycle and renal function was assessed for correlation with these changes. Glomerular filtration rate and urine
and plasma electrolytes were determined in male and female A. stuartii in February, May, July and August. Females showed little change in glomerular filtration rate, except for pre-mating values
in August which decreased. In contrast, glomerular filtration rate of males decreased significantly in July and August. Plasma
sodium and chloride levels were higher in males than females and were higher in animals in July and August than in February
and May. Plasma potassium levels dropped in both males and females in July and August. Plasma osmolality was higher in animals
in February compared to animals from May and August. However, there were no significant sex or seasonal differences in urine
electrolytes, although urea concentration was higher in females than males. Urine osmolality was higher in both sexes in July
and August. There were no significant differences in total excretory rates of sodium, potassium or chloride between sexes
or between seasons. Many of the alterations in renal function are correlated with known physiological and hormonal profiles
in A. stuartii. This is the first observation of seasonal changes in glomerular filtration rate that are unrelated to dietary and water
stresses.
Accepted: 8 September 1997 相似文献
980.
温度对红点唇瓢虫实验种群的影响 总被引:4,自引:0,他引:4
在7种温度下测定了红点唇瓢虫(Chilocorus kuwanae Silvestri)发育速率,并求得各虫态的发育起点温度和有效积温。其发育速率与温度的关系能很好地用王如松等(1982)提出的模型进行拟合。由此模型估计出最低、最高临界温度和最适发育温度,分别为10.42-13.01-℃、℃33.53-37.03℃和24.99-30.12℃。卵期忍耐温度变化的能力最强。4龄幼虫最弱。温度明显地影响 相似文献