全文获取类型
收费全文 | 1890篇 |
免费 | 128篇 |
国内免费 | 166篇 |
出版年
2024年 | 11篇 |
2023年 | 14篇 |
2022年 | 21篇 |
2021年 | 25篇 |
2020年 | 19篇 |
2019年 | 26篇 |
2018年 | 33篇 |
2017年 | 40篇 |
2016年 | 26篇 |
2015年 | 41篇 |
2014年 | 54篇 |
2013年 | 48篇 |
2012年 | 43篇 |
2011年 | 48篇 |
2010年 | 39篇 |
2009年 | 117篇 |
2008年 | 107篇 |
2007年 | 112篇 |
2006年 | 113篇 |
2005年 | 103篇 |
2004年 | 73篇 |
2003年 | 57篇 |
2002年 | 49篇 |
2001年 | 39篇 |
2000年 | 59篇 |
1999年 | 67篇 |
1998年 | 64篇 |
1997年 | 60篇 |
1996年 | 47篇 |
1995年 | 43篇 |
1994年 | 54篇 |
1993年 | 52篇 |
1992年 | 49篇 |
1991年 | 53篇 |
1990年 | 48篇 |
1989年 | 37篇 |
1988年 | 43篇 |
1987年 | 37篇 |
1986年 | 29篇 |
1985年 | 33篇 |
1984年 | 33篇 |
1983年 | 10篇 |
1982年 | 23篇 |
1981年 | 29篇 |
1980年 | 20篇 |
1979年 | 15篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有2184条查询结果,搜索用时 296 毫秒
221.
The current study presents phenology data for Rhizophora mangle from two equatorial mangrove stands with different salinity regimes in Brazil. Observations based on litter fall and individual shoot development were compared and related to environmental factors. Patterns observed in litter fall were consistent with results of direct monitoring. While both reproductive organs and leaves were produced throughout the year, rates of formation followed seasonal trends. Distinct differences in propagule production between low and high salinity sites and between years of observation were detected; main propagule release was, however, restricted to the wet season which offers enhanced conditions for propagule establishment. Emergence of flowers was linked to leaf production. While there was no obvious single peak in leaf production, it was reduced towards the end of the dry season at both high and low salinity sites. Time series analysis revealed an independent pattern of leaf development superimposed on this annual seasonal trend, indicating slower development of leaf primordia during periods of low light availability in the wet season. No significant difference in age structure was detected between sun and shade leaves; maximum leaf life-time was approximately 1 year. 相似文献
222.
Cornelissen JH Quested HM van Logtestijn RS Pérez-Harguindeguy N Gwynn-Jones D Díaz S Callaghan TV Press MC Aerts R 《Oecologia》2006,147(2):315-326
Plant traits have become popular as predictors of interspecific variation in important ecosystem properties and processes.
Here we introduce foliar pH as a possible new plant trait, and tested whether (1) green leaf pH or leaf litter pH correlates
with biochemical and structural foliar traits that are linked to biogeochemical cycling; (2) there is consistent variation
in green leaf pH or leaf litter pH among plant types as defined by nutrient uptake mode and higher taxonomy; (3) green leaf
pH can predict a significant proportion of variation in leaf digestibility among plant species and types; (4) leaf litter
pH can predict a significant proportion of variation in leaf litter decomposability among plant species and types. We found
some evidence in support of all four hypotheses for a wide range of species in a subarctic flora, although cryptogams (fern
allies and a moss) tended to weaken the patterns by showing relatively poor leaf digestibility or litter decomposability at
a given pH. Among seed plant species, green leaf pH itself explained only up to a third of the interspecific variation in
leaf digestibility and leaf litter up to a quarter of the interspecific variation in leaf litter decomposability. However,
foliar pH substantially improved the power of foliar lignin and/or cellulose concentrations as predictors of these processes
when added to regression models as a second variable. When species were aggregated into plant types as defined by higher taxonomy
and nutrient uptake mode, green-specific leaf area was a more powerful predictor of digestibility or decomposability than
any of the biochemical traits including pH. The usefulness of foliar pH as a new predictive trait, whether or not in combination
with other traits, remains to be tested across more plant species, types and biomes, and also in relation to other plant or
ecosystem traits and processes. 相似文献
223.
Hydrogen isotopic compositions of n-alkanes from terrestrial plants correlate with their ecological life forms 总被引:1,自引:0,他引:1
Stable hydrogen isotopic compositions (δD) of compound-specific biomarkers, such as n-alkanes from plant leaf waxes, can be used as a proxy for paleoclimatic change. However, the relationship between hydrogen isotopes of plant leaf wax and plant ecological life forms is not well understood. Here, we report the δD of n-alkanes from 34 modern terrestrial plants, including twenty-one C3 plants and thirteen C4 plants from northwestern China, determined using gas chromatography/thermal conversion/isotope ratio mass spectrometry. Our data show that the stable hydrogen isotopes are poorly correlated with the plant photosynthetic pathway (C3 vs. C4) and that they do not give clear regional precipitation signals. Together with a comparative analysis of published δD values from plant leaf waxes in other regions, we believe that the stable hydrogen isotope of plant leaf waxes is more closely related to ecological life forms of these terrestrial plants (i.e. tree, shrub, and grass). In general, the grasses have more negative δD values than the co-occurring trees and shrubs. Our findings suggest that the δD values of sedimentary leaf waxes from higher plants may record changes of a plant ecosystem under the influence of environmental alteration and imply that reconstruction of the paleoclimate using δD values from plant n-alkanes should be based upon specific plant taxa, and comparison should be made among plants with similar ecological life forms. 相似文献
224.
The major component (35-65%) of the volatile oil obtained by steam distillation of the leaves of Melaleuca triumphalis has been identified as (rel)-1beta-pentyl-1alpha,6alpha-dihydroxy-3,3,5,5-tetramethylcyclohexa-2,4-dione (trivial name triumphalone). Relative stereochemistry was established by nuclear Overhauser experiments and X-ray studies on the 2-(3,5-dinitrobenzoic acid) derivative. The remainder of the oil was composed of mono- and sesquiterpene hydrocarbons and alcohols. On prolonged standing the presence of a rearrangement product of triumphalone was observed which was characterized as (rel)-1beta-pentyl-1alpha,3alpha-dihydroxy-4,4,6,6-tetramethylcyclohexa-2,5-dione (trivial name isotriumphalone), presumably arising from an acid catalyzed shift of the pentyl group from C-1 to C-2. 相似文献
225.
In cuticular waxes from leaves of Pisum sativum, 19 secondary alcohols, 10 primary/secondary alkanediols and three secondary/secondary alkanediols were identified by various chemical transformations with product assignment employing GC-MS. The homologous series of C29-C33 secondary alcohols (1.1 microg/cm2) was dominated by hentriacontanol isomers (94%). Only octacosanediols and trace amounts of hexacosanediols (< 1%) were detected in the primary/secondary alkanediol faction (0.7 microg/cm2). The secondary/secondary alkanediols (0.12 microg/cm2) contained a single homologue with chain length C31. All three compound classes showed characteristic isomer distributions with secondary functional groups predominantly located between C-14 and C-16. Based on the isomer compositions, the sequence of biosynthetic steps introducing the hydroxyl functions is discussed. 相似文献
226.
227.
228.
Tracheid analysis was carried out on the veinlets and minor veins of the coleus (Solenostemon scutellarioides [L.] Codd) leaf. Third- to fifth-order, or minor, veins average 3.4 tracheids in tandem and they bipartition islets when these enclosed islets reach a critical size; both these features of vein length and islet size contribute to a self-similar process of vein pattern generation. An areole was calculated to be initially comprised of about ten cells making the patterning event for vein formation requiring only a few cells. An algorithmic model developed here for minor vein formation includes five production rules, and this computer model explains the 3–4 tracheids per minor vein, presence of isolated tracheids, the structure of veinlets, and the elaborate branching patterns of veinlets in coleus and other plants. 相似文献
229.
230.