全文获取类型
收费全文 | 1374篇 |
免费 | 83篇 |
国内免费 | 138篇 |
出版年
2023年 | 13篇 |
2022年 | 14篇 |
2021年 | 27篇 |
2020年 | 25篇 |
2019年 | 28篇 |
2018年 | 29篇 |
2017年 | 28篇 |
2016年 | 40篇 |
2015年 | 25篇 |
2014年 | 42篇 |
2013年 | 47篇 |
2012年 | 40篇 |
2011年 | 45篇 |
2010年 | 32篇 |
2009年 | 73篇 |
2008年 | 53篇 |
2007年 | 65篇 |
2006年 | 70篇 |
2005年 | 57篇 |
2004年 | 65篇 |
2003年 | 48篇 |
2002年 | 52篇 |
2001年 | 50篇 |
2000年 | 36篇 |
1999年 | 36篇 |
1998年 | 34篇 |
1997年 | 45篇 |
1996年 | 54篇 |
1995年 | 58篇 |
1994年 | 35篇 |
1993年 | 42篇 |
1992年 | 37篇 |
1991年 | 36篇 |
1990年 | 32篇 |
1989年 | 32篇 |
1988年 | 24篇 |
1987年 | 16篇 |
1986年 | 7篇 |
1985年 | 22篇 |
1984年 | 10篇 |
1983年 | 14篇 |
1982年 | 9篇 |
1981年 | 14篇 |
1980年 | 12篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 3篇 |
排序方式: 共有1595条查询结果,搜索用时 31 毫秒
991.
Ernst van der Maaten Andreas Hamann Marieke van der Maaten‐Theunissen Aldo Bergsma Geerten Hengeveld Ron van Lammeren Frits Mohren Gert‐Jan Nabuurs Renske Terhürne Frank Sterck 《Ecology and evolution》2017,7(8):2585-2594
Bioclimate envelope models have been widely used to illustrate the discrepancy between current species distributions and their potential habitat under climate change. However, the realism and correct interpretation of such projections has been the subject of considerable discussion. Here, we investigate whether climate suitability predictions correlate to tree growth, measured in permanent inventory plots and inferred from tree‐ring records. We use the ensemble classifier RandomForest and species occurrence data from ~200,000 inventory plots to build species distribution models for four important European forestry species: Norway spruce, Scots pine, European beech, and pedunculate oak. We then correlate climate‐based habitat suitability with volume measurements from ~50‐year‐old stands, available from ~11,000 inventory plots. Secondly, habitat projections based on annual historical climate are compared with ring width from ~300 tree‐ring chronologies. Our working hypothesis is that habitat suitability projections from species distribution models should to some degree be associated with temporal or spatial variation in these growth records. We find that the habitat projections are uncorrelated with spatial growth records (inventory plot data), but they do predict interannual variation in tree‐ring width, with an average correlation of .22. Correlation coefficients for individual chronologies range from values as high as .82 or as low as ?.31. We conclude that tree responses to projected climate change are highly site‐specific and that local suitability of a species for reforestation is difficult to predict. That said, projected increase or decrease in climatic suitability may be interpreted as an average expectation of increased or reduced growth over larger geographic scales. 相似文献
992.
Zn、Cd及其复合对小麦幼苗吸收Ca,Fe,Mn的影响 总被引:14,自引:2,他引:12
研究了溶液培养条件下,Cd,Zn及其复合对小麦幼苗吸收Ca,Fe,Mn的影响,结果表明,小麦幼苗对Zn,Cd的吸收随溶液中Cd^2 ,Zn^2 浓度的升高而增加,Cd,Zn同时存在时与其单独作用时幼苗对它们的吸收不同,Zn影响幼苗对Cd的吸收,Cd对Zn的吸收起抑制作用,Ca,Mn的吸收随溶液中Cd^2 ,Zn2 浓度升高而呈下降趋势,在Cd单独处理组和Zn单独处理组中Fe的吸收随Cd2 ,Zn2 浓度升高而增加,但在Zn Cd处理组中,Fe的吸收则呈下降趋势,其效应试学与作物具体部位有关。 相似文献
993.
994.
S.T. Forczek M. Matucha H. Uhlířová J. Albrechtová K. Fuksová H.P. Schröder 《Biologia Plantarum》2001,44(2):317-320
Trichloroacetic acid (TCA) belongs to secondary atmospheric pollutants affecting the forest health. Distribution of [1,2-14C]TCA-residues and TCA biodegradation were investigated in 4-year-old nursery-grown trees of Norway spruce [Picea abies (L.) Karst.] in the whole plant/soil system. Radioactivity was monitored in needles, wood, roots and soil as well as in the air. During two weeks of exposure TCA was continuously degraded, especially in the soil. Estimates of radioactivity balance showed loss of radioactivity into the atmosphere in the form of 14CO2; unincorporated [1,2-14C]TCA, chloroform, carbon monoxide and methane were not detected at all. TCA degradation to CO2 was indicated also in the spruce needles. Moreover, it was found that soil litter contained [1,2-14C]TCA unavailable to microorganisms. 相似文献
995.
Allelopathic effect of Nymphaea lotus L. on growth and yield of cultivated rice around Lake Manzala (Nile Delta) 总被引:1,自引:0,他引:1
Lotus infestation of ricefields is a major cause of crop failure and decrease of grain yield in the newly reclaimed cut-off areas around lake Manzala, Egypt. This study provides insights in the allelopathic effect of Nymphaea lotus L. on rice (Oryza sativa cavr. Giza-177). Lotus rhizome extracts were inhibitory to seed germination and seedling growth of rice. The degree of inhibition was dependent on extract type and concentration. Ethanol and water extracts were more inhibitory than chloroform extracts. The phenolic fraction of ethanol extracts showed the highest inhibitory effects. In a target (rice)-neighbour (lotus) pot experiment, rice dry mass and relative growth rate were dependent on its age and on lotus rhizome density, with decreased growth at increased lotus density. Field data on infested and non-infested ricefields demonstrated a decreased leaf area index and yield in infested fields. Identification of the potential allelochemical compounds by gas chromatography/mass spectrometry revealed the presence of allelopathic phenolics in lotus rhizomes. 相似文献
996.
997.
A method for determination of oxidation of phenolic compounds by intact wheat seedlings using o-phenylenediamine (OPD) was developed. The reaction is initiated by the addition of oxalic acid to the incubation medium. It is suggested that an endogenous peroxidase and hydrogen peroxide formed during oxidation of oxalic acid by endogenous oxalate oxidase are involved in OPD oxidation. Treatment of plants with chitooligosaccharides (1-10 mg/liter) with acetylation degree of 65% and molecular masses of 5-10 kD significantly activated OPD oxidation by wheat seedlings. 相似文献
998.
An unknown signal at 2.93 ppm in 1H-NMR spectra of rice, Oryza sativa, was assigned to the methyl groups of sulphur-methylmethionine (SMM), thereby devising a new method for the determination of this compound. Rice seedlings growing aerobically in the dark and in the light engaged for the synthesis of SMM an amount of Met corresponding to 23 and 8%, respectively, of the total seed reserves of this amino acid. In etiolated shoots, SMM reached 1.2 micromol g(-1) fresh weight, an unusually high level in vegetative tissues of wild-type plants. This is compared to a value of 0.4 micromol g(-1) fresh weight in green tissues. A decreased demand for Met during growth caused the higher accumulation of SMM in etiolated, rather than green, tissues. At the same time, dark seedlings were endowed with a readily utilizable and translocable alternative form of Met, as shown by retrieval of SMM from the coleoptile. The importance of methyl group storage in SMM is shown by comparison with choline and choline phosphate pools. 相似文献
999.
Moisture conditions and the presence of bryophytes determine fescue species abundance in a dry calcareous grassland 总被引:2,自引:0,他引:2
Festuca ovina is the abundant matrix-forming species and F. rubra a subordinate species in shallow-soil calcareous grasslands. F. pratensis is a transient species, occurring sparsely in this community. We hypothesised that the different abundances of these three species are primarily due to the differential effect of moisture conditions on their germination and early establishment, and that the effect of the pattern of rainfall intensity depends on the presence or absence of a bryophyte layer. We studied the dependence of the germination and establishment of the three fescue species on the moisture conditions both in the laboratory and in the patches of intact grassland community (microcosms). In a laboratory germination experiment, F. pratensis showed the highest, F. rubra , the intermediate and F. ovina, the lowest drought tolerance. In microcosms, the establishment of F. ovina was the highest. At the same time, the annual mortality of seedlings of F. ovina was the lowest. All three species responded positively to an increasing irrigation level. Differently from F. ovina, F. rubra showed a positive response only in plots from which the bryophyte layer had been removed, while F. pratensis responded positively to both irrigation and bryophyte removal. We conclude that moisture conditions have a differential effect on the three fescue species mainly in the seedling establishment, not in the germination phase. For the successful establishment of F. rubra and F. pratensis, the coincidence of high rainfall and local disturbance, removing bryophytes, is required. The presence or absence of bryophytes had no effect on establishment in dry years, while in rainy years the removal of bryophytes has a clear positive effect. 相似文献
1000.
Three types of microprojectile particles, 1.0-μm gold, 1.3-μm tungsten, and 1.6-μm gold, were studied for their effectiveness
on genetic transformation of black spruce via bombardment with somatic embryos as the target tissue. Different particles resulted
in different levels of transient expression of theGUS reporter gene; 1.0-μm gold particles produced the highest level of expression, and 1.6-μm gold particles produced the lowest
level. Particle type also affected stable transformation; 1.0-μm gold particles had a 10-fold higher stable transformation
efficiency than did 1.6-μm gold particles and a 2-fold higher efficiency than did 1.3-μm tungsten particles. This study indicates
that microprojectile particle type and size are important in bombardment-mediated plant transformation. 相似文献