全文获取类型
收费全文 | 430篇 |
免费 | 24篇 |
出版年
2015年 | 5篇 |
2014年 | 4篇 |
2013年 | 12篇 |
2012年 | 17篇 |
2011年 | 19篇 |
2010年 | 24篇 |
2009年 | 21篇 |
2008年 | 11篇 |
2007年 | 17篇 |
2006年 | 13篇 |
2005年 | 9篇 |
2004年 | 9篇 |
2003年 | 3篇 |
2001年 | 3篇 |
1999年 | 7篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 10篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 16篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1980年 | 8篇 |
1979年 | 3篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1973年 | 3篇 |
1971年 | 7篇 |
1959年 | 6篇 |
1958年 | 12篇 |
1957年 | 15篇 |
1956年 | 10篇 |
1955年 | 11篇 |
1954年 | 5篇 |
1953年 | 6篇 |
1952年 | 9篇 |
1951年 | 14篇 |
1950年 | 3篇 |
1948年 | 4篇 |
1912年 | 2篇 |
1911年 | 2篇 |
1907年 | 2篇 |
排序方式: 共有454条查询结果,搜索用时 16 毫秒
1.
A number of features of the two aberrant, sexually dimorphic, Asiatic crane-flies Holorusia carmichaeli and H.pluto are described for the first time. The possible adaptive significance of abdominal modifications in H.carmichaeli is discussed. 相似文献
2.
3.
4.
Debating the greening vs. browning of the North American boreal forest: differences between satellite datasets 总被引:1,自引:0,他引:1
DOMINGO ALCARAZ‐SEGURA EMILIO CHUVIECO HOWARD E. EPSTEIN ERIC S. KASISCHKE ALEXANDER TRISHCHENKO 《Global Change Biology》2010,16(2):760-770
A number of remote sensing studies have evaluated the temporal trends of the normalized difference vegetation index (NDVI or vegetation greenness) in the North American boreal forest during the last two decades, often getting quite different results. To examine the effect that the use of different datasets might be having on the estimated trends, we compared the temporal trends of recently burned and unburned sites of boreal forest in central Canada calculated from two datasets: the Global Inventory, Monitoring, and Modeling Studies (GIMMS), which is the most commonly used 8 km dataset, and a new 1 km dataset developed by the Canadian Centre for Remote Sensing (CCRS). We compared the NDVI trends of both datasets along a fire severity gradient in order to evaluate the variance in regeneration rates. Temporal trends were calculated using the seasonal Mann–Kendall trend test, a rank‐based, nonparametric test, which is robust against seasonality, nonnormality, heteroscedasticity, missing values, and serial dependence. The results showed contrasting NDVI trends between the CCRS and the GIMMS datasets. The CCRS dataset showed NDVI increases in all recently burned sites and in 50% of the unburned sites. Surprisingly, the GIMMS dataset did not capture the NDVI recovery in most burned sites and even showed NDVI declines in some burned sites one decade after fire. Between 50% and 75% of GIMMS pixels showed NDVI decreases in the unburned forest compared with <1% of CCRS pixels. Being the most broadly used dataset for monitoring ecosystem and carbon balance changes, the bias towards negative trends in the GIMMS dataset in the North American boreal forest has broad implications for the evaluation of vegetation and carbon dynamics in this region and globally. 相似文献
5.
6.
7.
B. ALEXANDER M. C. USMA H. CADENA B. L. QUESADA Y. SOLARTE W. ROA J. MONTOYA C. JARAMILLO B. L. TRAVI 《Medical and veterinary entomology》1995,9(3):273-278
Abstract. A survey was made of the phlebotomine sandfly fauna of La Guaira, a village with coffee plantations near Cali, Colombia, from which cases of American cutaneous leishmaniasis had been reported due to Leishmania (Viannia) panamensis and Le. ( V. ) braziliensis. Among six species of sandfly collected on human bait, Lutzomyia youngi was most important in terms of biting nuisance. Lu. columbiana, Lu. lichyi and Lu. scorzai as well as Lu. youngi adults occurred throughout the year. Sandfly man-biting activity occurred throughout the night and was highest within 2 h of sunset. Despite its abundance in nocturnal samples, Lu. youngi was rarely taken in diurnal resting site collections. In contrast, Lu. lichyi was collected on tree-trunks during the day in large numbers and was the only species biting in daylight. The implications of these and other findings for leishmaniasis control measures in La Guaira are considered. 相似文献
8.
Results are presented of a survey carried out by Rentokil Ltd on the distribution of the Oriental cockroach Blatta orientalis L. and the German cockroach Blattella germanica L. in the United Kingdom. The known ranges of both species are increased considerably by the findings of the survey, with several new vice-county records for Scotland (including the Western Isles), England and Wales. The frequency at which Blatta orientalis was observed in outdoor habitats may indicate that this species sometimes spreads to new areas without human assistance. 相似文献
9.
Porcine cDNAs clones encoding beta-casein were isolated and sequenced. The porcine beta-casein cDNA is 1100bp in length, excluding the poly(A) tail, and encodes a preprotein of 232 amino acids. 相似文献
10.
Abstract: Fossils of dyrosaurid crocodyliforms are limited in South America, with only three previously diagnosed taxa including the short‐snouted Cerrejonisuchus improcerus from the Paleocene Cerrejón Formation of north‐eastern Colombia. Here we describe a second dyrosaurid from the Cerrejón Formation, Acherontisuchus guajiraensis gen. et sp. nov., based on three partial mandibles, maxillary fragments, teeth, and referred postcrania. The mandible has a reduced seventh alveolus and laterally depressed retroarticular process, both diagnostic characteristics of Dyrosauridae. Acherontisuchus guajiraensis is distinct among known dyrosaurids in having a unique combination of craniomandibular characteristics, and postcranial morphology that suggests it may have occupied a more placid, fluvial habitat than most known Old‐World dyrosaurids. Results from a cladistic analysis of Dyrosauridae, using 82 primarily cranial and mandibular characters, support an unresolved relationship between A. guajiraensis and a combination of New‐ and Old‐World dyrosaurids including Hyposaurus rogersii, Congosaurus bequaerti, Atlantosuchus coupatezi, Guarinisuchus munizi, Rhabdognathus keiniensis and Rhabdognathus aslerensis. Our results are consistent with an African origin for Dyrosauridae with multiple dispersals into the New World during the Late Cretaceous and a transition from marine habitats in ancestral taxa to more fluvial habitats in more derived taxa. 相似文献