全文获取类型
收费全文 | 1323篇 |
免费 | 70篇 |
国内免费 | 34篇 |
专业分类
1427篇 |
出版年
2023年 | 7篇 |
2022年 | 7篇 |
2021年 | 12篇 |
2020年 | 24篇 |
2019年 | 30篇 |
2018年 | 22篇 |
2017年 | 32篇 |
2016年 | 24篇 |
2015年 | 49篇 |
2014年 | 32篇 |
2013年 | 66篇 |
2012年 | 26篇 |
2011年 | 33篇 |
2010年 | 37篇 |
2009年 | 43篇 |
2008年 | 76篇 |
2007年 | 66篇 |
2006年 | 69篇 |
2005年 | 53篇 |
2004年 | 62篇 |
2003年 | 34篇 |
2002年 | 23篇 |
2001年 | 49篇 |
2000年 | 42篇 |
1999年 | 33篇 |
1998年 | 28篇 |
1997年 | 32篇 |
1996年 | 46篇 |
1995年 | 29篇 |
1994年 | 27篇 |
1993年 | 29篇 |
1992年 | 28篇 |
1991年 | 23篇 |
1990年 | 33篇 |
1989年 | 25篇 |
1988年 | 21篇 |
1987年 | 41篇 |
1986年 | 24篇 |
1985年 | 12篇 |
1984年 | 17篇 |
1983年 | 8篇 |
1982年 | 11篇 |
1981年 | 12篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1973年 | 3篇 |
1971年 | 3篇 |
排序方式: 共有1427条查询结果,搜索用时 0 毫秒
21.
22.
Mark Hamann Chu The Cuong Nguyen Duy Hong Pham Thuoc Bui Thi Thuhien 《Biodiversity and Conservation》2006,15(11):3703-3720
To establish baseline data on the distribution, abundance and threats to marine turtles in Viet Nam we conducted surveys with
local fishers, community members and provincial Ministry of Fisheries staff from 17 of Viet Nam’s 29 coastal provinces. These
data indicate that five species of marine turtle reside in Viet Nam’s waters (loggerhead, olive ridley, leatherback, green
and hawksbill turtles), and four species nest on Viet Nam’s beaches (all of the above except the loggerhead turtle). It is
evident from these data that significant declines have occurred in both foraging and nesting populations of all five marine
turtle species found in Viet Nam. The greatest current threats to marine turtle populations in Viet Nam are habitat degradation,
the accidental and opportunistic of turtles capture by fishers and the direct take of nesting females and their eggs. Successful
conservation efforts have been made in recent years through collaboration between international Non Government Organisations
and several Vietnamese Government Ministries. Continued success of these projects and the development and implementation of
marine conservation policy will depend upon building awareness among Government employees, fishers and the general public
about marine turtle biology, ecology, and the need to protect them. 相似文献
23.
Carrie V. Kappel Carlos M. Duarte Keith Brander Christopher J. Brown John F. Bruno Lauren Buckley Michael T. Burrows Benjamin S. Halpern Wolfgang Kiessling Pippa Moore John M. Pandolfi Camille Parmesan Elvira S. Poloczanska David S. Schoeman William J. Sydeman Anthony J. Richardson 《Global Ecology and Biogeography》2015,24(1):64-76
24.
Summary
Sedum wrightii is one of only a few species in the Crassulaceae for which there is evidence for a high degree of variability in the ratio of daytime to nighttime CO2 assimilation. There are both environmental and genetic components to this variability. S. wrightii grows over a wide altitudinal gradient. The purpose of this study was to compare low, intermediate, and high altitude populations with respect to the degree of CAM expression and the capability to tolerate limited water availability. We utilized clonallyreplicated genotypes of plants from each population in common environment greenhouse experiments. Genetic differences among the populations were found in long-term water use efficiency, in 24 hour CO2 exchange patterns, in biomass 13C values, in carbon allocation, and in water status and ultimately survival during prolonged drought. The differences among the populations appear to be closely related to differences in the native habitats. The low altitude, desert plants had the greatest ability to grow and survive under conditions of limited water availability and appear to have the greatest shift to nighttime CO2 uptake during periods without water, while the high altitude plants had the poorest performance under these conditions and appear to shut down net carbon uptake when severely water limited. 相似文献
25.
Elevated atmospheric CO2 is known to stimulate photosynthesis and growth of plants with the C3 pathway but less of plants with the C4 pathway. An increase in the CO2 concentration can therefore be expected to change the competitive interactions between C3 and C4 species. The effect of long term exposure to elevated CO2 (ambient CO2 concentration +340 µmol CO2 mol-1) on a salt marsh vegetation with both C3 and C4 species was investigated. Elevated CO2 increased the biomass of the C3 sedgeScirpus olneyi growing in a pure stand, while the biomass of the C4 grassSpartina patens in a monospecific community was not affected. In the mixed C3/C4 community the C3 sedge showed a very large relative increase in biomass in elevated CO2 while the biomass of the C4 species declined.The C4 grassSpartina patens dominated the higher areas of the salt marsh, while the C3 sedgeScirpus olneyi was most abundant at the lower elevations, and the mixed community occupied intermediate elevations.Scirpus growth may have been restricted by drought and salt stress at the higher elevations, whileSpartina growth at the lower elevations may be affected by the higher frequency of flooding. Elevated CO2 may affect the species distribution in the salt marsh if it allowsScirpus to grow at higher elevations where it in turn may affect the growth ofSpartina. 相似文献
26.
A relatively drought tolerant cultivar of maize ( Zea mays L. cv. Pioneer 3950) and a drought tolerant line of sorghum ( Sorghum bicolor [L.] Moench cv. ICSV 112) were grown hydroponically for 11 days. Treatments for non-ionic osmotic and salt stresses were started at the 8th day by addition of polyethylene glycol 6000 and NaCl, respectively, at 200 mOsm equivalent concentrations in the presence or absence of 0. 1 μ M abscisic acid. Relative growth rate was depressed by both stress factors, more severely for maize than sorghum. Abscisic acid increased the growth rate and reverted the negative effect of NaCl in maize, while sorghum was only slightly affected. In general, sorghum had higher levels of K+ and lower levels of Na+ and the K+ /Na+ ratio was further increased by abscisic acid treatment. From the pressure-volume curves, osmotic potential, the water potential at turgor loss point, bulk elastic modulus and the water saturation deficit at initial turgor loss were estimated. Most significantly, sorghum had a higher elastic modulus than maize and it decreased under osmotic treatment, while in maize it increased under NaCl stress. The results suggest that bulk tissue turgor was not limiting growth under these conditions and underscores the possible implications of changes in the elastic condition of the cell walls in stress responses. 相似文献
27.
1. One-year-old seedlings of shade tolerant Acer rubrum and intolerant Betula papyrifera were grown in ambient and twice ambient (elevated) CO2 , and in full sun and 80% shade for 90 days. The shaded seedlings received 30-min sun patches twice during the course of the day. Gas exchange and tissue–water relations were measured at midday in the sun plants and following 20 min of exposure to full sun in the shade plants to determine the effect of elevated CO2 on constraints to sun-patch utilization in these species.
2. Elevated CO2 had the largest stimulation of photosynthesis in B. papyrifera sun plants and A. rubrum shade plants.
3. Higher photosynthesis per unit leaf area in sun plants than in shade plants of B. papyrifera was largely owing to differences in leaf morphology. Acer rubrum exhibited sun/shade differences in photosynthesis per unit leaf mass consistent with biochemical acclimation to shade.
4. Betula papyrifera exhibited CO2 responses that would facilitate tolerance to leaf water deficits in large sun patches, including osmotic adjustment and higher transpiration and stomatal conductance at a given leaf-water potential, whereas A. rubrum exhibited large increases in photosynthetic nitrogen-use efficiency.
5. Results suggest that species of contrasting successional ranks respond differently to elevated CO2 , in ways that are consistent with the habitats in which they typically occur. 相似文献
2. Elevated CO
3. Higher photosynthesis per unit leaf area in sun plants than in shade plants of B. papyrifera was largely owing to differences in leaf morphology. Acer rubrum exhibited sun/shade differences in photosynthesis per unit leaf mass consistent with biochemical acclimation to shade.
4. Betula papyrifera exhibited CO
5. Results suggest that species of contrasting successional ranks respond differently to elevated CO
28.
Fog in the California redwood forest: ecosystem inputs and use by plants 总被引:25,自引:0,他引:25
T. E. Dawson 《Oecologia》1998,117(4):476-485
Fog has been viewed as an important source of moisture in many coastal ecosystems, yet its importance for the plants which inhabit these ecosystems is virtually unknown. Here, I report the results of a 3-year investigation of fog inputs and the use of fog water by plants inhabiting the heavily fog inundated coastal redwood (Sequoia sempervirens) forests of northern California. During the study period, 34%, on average, of the annual hydrologic input was from fog drip off the redwood trees themselves (interception input). When trees were absent, the average annual input from fog was only 17%, demonstrating that the trees significantly influence the magnitude of fog water input to the ecosystem. Stable hydrogen and oxygen isotope analyses of water from fog, rain, soil water, and xylem water extracted from the dominant plant species were used to characterize the water sources used by the plants. An isotopic mixing model was employed to then quantify how much fog water each plant used each month during the 3-year study. In summer, when fog was most frequent, ~19% of the water within S.sempervirens, and ~66% of the water within the understory plants came from fog after it had dripped from tree foliage into the soil; for S.sempervirens, this fog water input comprised 13–45% of its annual transpiration. For all plants, there was a significant reliance on fog as a water source, especially in summer when rainfall was absent. Dependence on fog as a moisture source was highest in the year when rainfall was lowest but fog inputs normal. Interestingly, during the mild El Niño year of 1993, when the ratio of rainfall to fog water input was significantly higher and fog inputs were lower, both the proportion and coefficient of variation in how much fog water was used by plants increased. An explanation for this is that while fog inputs were lower than normal in this El Niño year, they came at a time when plant demand for water was highest (summer). Therefore, proportional use of fog water by plants increased. The results presented suggest that fog, as a meteorological factor, plays an important role in the water relations of the plants and in the hydrology of the forest. These results demonstrate the importance of understanding the impacts of climatic factors and their oscillations on the biota. The results have important implications for ecologists, hydrologists, and forest managers interested in fog-inundated ecosystems and the plants which inhabit them. 相似文献
29.
1. To test whether differences in response to drought can help to explain the differing distributions of Briza media and Koeleria macrantha , changes in their leaf growth and water relations during soil drying were measured. After droughting, plants were re-watered and recovery recorded.
2. Leaf growth of the two species showed a similar sensitivity to drought with respect to duration of soil drying and soil moisture content.
3. In both species tiller base relative water content (RWC) was maintained at similar levels to controls until soil moisture content had fallen to less than 9%. This may aid survival in habitats subject to periodic droughting by preventing damage to the meristems.
4. Briza media did not respond to re-watering. However, plants of K. macrantha re-grew after periods of over 20 days of drought, when tiller base RWCs had fallen as low as 13%. This ability to resurrect may explain the occurrence of K. macrantha in xeric calcicolous grasslands subject to episodes of severe droughting, from which B. media is absent. 相似文献
2. Leaf growth of the two species showed a similar sensitivity to drought with respect to duration of soil drying and soil moisture content.
3. In both species tiller base relative water content (RWC) was maintained at similar levels to controls until soil moisture content had fallen to less than 9%. This may aid survival in habitats subject to periodic droughting by preventing damage to the meristems.
4. Briza media did not respond to re-watering. However, plants of K. macrantha re-grew after periods of over 20 days of drought, when tiller base RWCs had fallen as low as 13%. This ability to resurrect may explain the occurrence of K. macrantha in xeric calcicolous grasslands subject to episodes of severe droughting, from which B. media is absent. 相似文献
30.
Michael L. Blakey 《American journal of physical anthropology》1988,75(4):493-502
The Nanticoke-Moors of Delaware are an ethnic group of Native American, Afro-American, and Euro-American descent. Their physiognomic and ethnic marginality has subjected them to a limited range of social and economic options under the influence of American racial policies. This article concerns their ethnic formation in the colonial period and the demographic effects (changes in fertility, mortality, and structure) of their 19th- and 20th-century social history. The demographic sample consists of 406 headstones from three community cemeteries. Each cemetery represents a socially and economically distinct unit, including a group that identifies with its traditional Indian heritage, an Afro-American acculturated group, and a migrant community of marginal ethnic affiliation. Variation and change in life expectancy is shown. Relationships between the political and economic processes affecting Nanticoke-Moor social affiliation, and those affecting color caste-class formation among mainstream Afro-Americans, are discussed. 相似文献