全文获取类型
收费全文 | 6926篇 |
免费 | 295篇 |
国内免费 | 502篇 |
出版年
2023年 | 59篇 |
2022年 | 105篇 |
2021年 | 136篇 |
2020年 | 131篇 |
2019年 | 131篇 |
2018年 | 136篇 |
2017年 | 126篇 |
2016年 | 148篇 |
2015年 | 170篇 |
2014年 | 331篇 |
2013年 | 392篇 |
2012年 | 368篇 |
2011年 | 393篇 |
2010年 | 315篇 |
2009年 | 335篇 |
2008年 | 363篇 |
2007年 | 356篇 |
2006年 | 324篇 |
2005年 | 315篇 |
2004年 | 280篇 |
2003年 | 246篇 |
2002年 | 154篇 |
2001年 | 127篇 |
2000年 | 148篇 |
1999年 | 150篇 |
1998年 | 116篇 |
1997年 | 97篇 |
1996年 | 129篇 |
1995年 | 117篇 |
1994年 | 100篇 |
1993年 | 141篇 |
1992年 | 125篇 |
1991年 | 102篇 |
1990年 | 89篇 |
1989年 | 84篇 |
1988年 | 81篇 |
1987年 | 77篇 |
1986年 | 83篇 |
1985年 | 96篇 |
1984年 | 64篇 |
1983年 | 47篇 |
1982年 | 79篇 |
1981年 | 57篇 |
1980年 | 68篇 |
1979年 | 57篇 |
1978年 | 35篇 |
1977年 | 32篇 |
1976年 | 35篇 |
1975年 | 16篇 |
1972年 | 14篇 |
排序方式: 共有7723条查询结果,搜索用时 15 毫秒
861.
The present study reports on effects of different light:dark periods, light intensities, N:P ratios and temperature on the specific growth rate of flagellated cells of Phaeocystis pouchetii in culture. The specific growth rate was estimated by diel changes in cellular DNA content. The cellular DNA content and cell cycle of flagellated cells of P. pouchetii are shown, and the importance of light:dark period in cell division is demonstrated. Diel patterns of the cellular DNA content showed that cell division was confined to the dark period. The cells dealt with more than one division per day by rapid divisions shortly after each other.The specific growth rates (μDNA) based on the DNA cell cycle model were in close agreement with specific growth rates (μCell) determined from cell counts. The temperature affected the specific growth rates (multiple regression, p < 0.01) and were higher at 5 °C (μ ≤ 2.2 d−1) than at 10 °C (μ ≤1.6 d−1). Increasing the light:dark period from 12:12 h to 20:4 h affected the specific growth rate of P. pouchetii at the lower temperature tested (5 °C) (multiple regression, p < 0.01), resulting in higher specific growth rates than at 10 °C. At 10 °C, the effect of light:dark period was severely reduced. Neither light nor nutrients could compensate the reduction in specific growth rates caused by elevated temperature. The specific growth rates was not affected by the N:P ratios tested (multiple regression, p = 0.21). The experiments strongly suggest that the flagellated cells have a great growth potential and could play a dominating role in northern areas at increased day length. 相似文献
862.
海拔对甘肃河西走廊玉米生育期的影响 总被引:8,自引:0,他引:8
为了研究海拔高度对玉米生育期的影响,1999年和2004年在甘肃省河西走廊的张掖市安排了4个试点的田间试验,海拔高度分别是1506.5、1706.5、2000.0和2231.5m.结果表明,608、酒单3号和中单2号3个不同熟期类型的玉米品种在同日播种条件下,随着海拔的升高成熟期延迟,海拔与品种的生育期之间呈极显著正相关,海拔每升高100m,生育期延长5~6d.海拔与播种至拔节天数之间也表现正相关,海拔与株高之间为负相关. 相似文献
863.
864.
865.
Since the response to differences in resource availability is most pronounced in smaller individuals of vascular epiphytes such as Werauhia sanguinolenta Cogn. et Marchal (Bromeliaceae), I expected variation in growth and survival of small individuals to play an important role in the dynamics of entire populations. Four annual censuses (2002–2005) of three study populations, which were located across the isthmus of Panama, allowed me to construct stage transition matrices, and to conduct growth analysis and elasticity analysis. Differences between populations were highly consistent through time, but, contrary to expectations, hardly related to the comportment of smaller plants. For example, although average mortality rates were highest at the driest site, close to the Pacific, small plants were not predominantly affected. Similarly, although the highest relative growth rates (RGR) of individuals and the highest population growth rates (λ) were found in the population with the highest moisture input, which was located close to the Atlantic coast, this was not due to a particularly strong stimulation of RGR in small plants. Elasticity analysis indicated rather small differences in the importance of the three demographic processes growth, survival, and reproduction for population growth in the three populations, but invariably identified the survival of large tanks as the single most important process determining λ. 相似文献
866.
867.
Question: What are the changes associated with the recent invasion by the non‐native legume, Cytisus scoparius? Location: Subalpine vegetation (1500 m a.s.l.) in Australia. Methods: We used multivariate techniques and regression analyses to assess vegetation and environmental changes across six study sites. Vegetation and environmental variables were investigated at three different stages of invasion: (1) recent invasion (8–10 yr), (2) mature invasion (15–16 yr) and (3) long‐term invasion (25 yr). Results: Substantial changes in floristic composition and species richness were evident after 15 yr and these changes became more pronounced after 25 yr. Changes due to invasion were associated with a dramatic loss of native species or a reduction in their abundance. No ‘new species’ were evident under invaded stands. Forbs were most affected by the establishment of C. scoparius, although all growth forms responded negatively. Dense canopy shading and an increasingly dense, homogeneous litter layer in the understorey as a result of C. scoparius were strong environmental drivers of vegetation change. Greenhouse studies confirmed the importance of these processes on the germination and growth of two native species. Conclusions: This study highlights the potential for C. scoparius to alter both vegetation and environmental processes in the subalpine region. 相似文献
868.
Estimating plant responses to climate by direct gradient analysis and geographic distribution analysis 总被引:1,自引:0,他引:1
We characterised the climatic behaviour of 53 woody species in terms of the climatic factors that play the main role in controlling
species distribution in the study area. Floristic and climatic data were obtained from 150 stands in sites under climatic
control (i.e. eu-climatopes). The sampling strategy used allowed a reliable match between floristic and climatic observations.
Different methods of frequency analysis and goodness-of-fit tests were used to identify associations between species occurrence
and climatic characteristics. The species' responses were summarised by statistics describing ecological preferences and amplitudes,
and species were grouped accordingly. A Gaussian response model was fitted to the abundance data along the main climatic gradients
for selected species and response surfaces were derived by spatial analysis for a set of indicator species. Frequency analysis
methods detected 42 indicator taxa for the Baudiere's Qe drought index, and lower numbers, 34 and 22, respectively, for the mean minimum coldest-month temperature and the daily temperature
range in the coldest month. Goodness-of-fit tests revealed a lower number of ecological profiles with statistically significant
deviations from equidistribution. We discuss the relative performance of the different methods and suggest that the combined
use of statistical tests and frequency analyses may improve estimation of the environmental requirements of species. We also
recommend using the species' responses to key environmental factors as reliable criteria in the definition of plant functional
types.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
869.
Habitat associations with topography and canopy structure of tree species in a tropical montane forest on Mount Kinabalu,Borneo 总被引:1,自引:0,他引:1
Habitat associations with topography and canopy structure of 42 abundant tree species were studied in a 2.74-ha plot of tropical
montane forest on Mount Kinabalu, Borneo. Many of these species belong to the same higher taxa including eight families and
four genera. Analysis of intraspecific spatial distributions for stems ≥ 10 cm diameter revealed that 28 species (including
all six species of Fagaceae) showed aggregated distributions at the 100-m2 and/or 400-m2 scales, and that 20 species showed habitat associations with topography by torus-translation tests; 17 species showed both
characteristics. Species' associations with the local canopy structure were characterized by crown position index (CPI), which
was defined relative to neighbour trees. The CPI differed greatly among individual stems at 10–40 cm diameter, and 19 species
showed significantly different frequencies of crowns exposed vertically versus those shaded beneath the canopy. Mean growth
rates at 10–40 cm diameter and size distributions of species were not related to topographic associations, but were explained
by the associations with canopy structure; species with more exposed crowns grew faster and had less positively skewed distributions.
Diversity in habitat associations was manifest between two genera (Syzygium and Tristaniopsis) in the family Myrtaceae and among species in these genera, but was less evident in other families and two genera (Garcinia and Lithocarpus).
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
870.
Expression of Arabidopsis APETALA1 in tomato reduces its vegetative cycle without affecting plant production 总被引:2,自引:0,他引:2
Ellul Philippe Angosto Trinidad García-Sogo Begoña García-Hurtado Noemí Martín-Trillo Mar Salinas María Moreno Vicente Lozano Rafael Martínez-Zapater José M. 《Molecular breeding : new strategies in plant improvement》2004,13(2):155-163
Important agronomic traits such as fruit quality, harvesting efficiency or production largely depend on flowering time. We have analysed the effect of the overexpression of the Arabidopsis APETALA1 MADS-box gene on vegetative and reproductive growth of tomato. Constitutive expression of APETALA1 in tomato plants has major effects on the length of their growth cycle as well as on their growth habit. Transgenic tomato plants initiated flowering after the production of 6 vegetative nodes as compared to 11 nodes for the wild type plants. Most of tomato 35S:AP1 plants also showed determinate growth habit, similar to the phenotype of self pruning tomato mutants, as well as an initial reduction of their axillary growth. Moreover, development and fertility of flowers were not affected in plants expressing AP1. Consequently, fruit formation in transgenic plants grown under greenhouse conditions occurred normally, which permitted a similar fruit yield compared to control plants. Since traits conferred by AP1 expression are dominant, its expression in tomato breeding lines could provide advantages for the development of new hybrid varieties with shorter generation time, determinate growth, and reduced pruning requirements. 相似文献