全文获取类型
收费全文 | 882篇 |
免费 | 70篇 |
国内免费 | 80篇 |
出版年
2024年 | 3篇 |
2023年 | 14篇 |
2022年 | 7篇 |
2021年 | 28篇 |
2020年 | 26篇 |
2019年 | 40篇 |
2018年 | 36篇 |
2017年 | 30篇 |
2016年 | 25篇 |
2015年 | 37篇 |
2014年 | 42篇 |
2013年 | 61篇 |
2012年 | 36篇 |
2011年 | 40篇 |
2010年 | 41篇 |
2009年 | 53篇 |
2008年 | 40篇 |
2007年 | 36篇 |
2006年 | 35篇 |
2005年 | 29篇 |
2004年 | 32篇 |
2003年 | 31篇 |
2002年 | 29篇 |
2001年 | 20篇 |
2000年 | 28篇 |
1999年 | 22篇 |
1998年 | 12篇 |
1997年 | 17篇 |
1996年 | 7篇 |
1995年 | 13篇 |
1994年 | 17篇 |
1993年 | 16篇 |
1992年 | 15篇 |
1991年 | 7篇 |
1990年 | 9篇 |
1989年 | 16篇 |
1988年 | 13篇 |
1987年 | 3篇 |
1986年 | 9篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1973年 | 4篇 |
排序方式: 共有1032条查询结果,搜索用时 562 毫秒
1.
2.
3.
Håkan Tegelström 《Biochemical genetics》1987,25(1-2):95-110
Genetic variation of mitochondrial DNA (mtDNA) in 18 great tits (Parus major) from three neighboring localities in Sweden was investigated with eight tetranucleotide restriction endonucleases. The 18 individuals could be separated into 13 different maternal lineages. The high number of female lineages present in this regional population contrasts with a low level of sequence divergence between the different mtDNA clones, with a mean of 0.19% sequence divergence between all individuals. There was no obvious spatial structuring of mtDNA clones among the three localities. The presence of a high number of different clones with a low degree of sequence divergence could be explained by the effects of a large long-term effective population size, with the mtDNA clones having diverged about 25,000–200,000 years ago.This study was supported by the Swedish Natural Science Research Council, the Erik Philip-Sörensen Foundation, and the Nilsson-Ehle Foundation. 相似文献
4.
5.
6.
Robert G. Sheath 《Hydrobiologia》1986,138(1):75-83
Thermokarst ponds are the most abundant type of water body in the arctic tundra, with millions occurring in the coastal plains of Alaska, Northwest Territories and Siberia. Because ice covers of at least 2 m in thickness are formed at these latitudes, tundra ponds freeze solid every winter As a result, the growing season is shortened to a range of 60 to 100 days, during which time the photoperiod is altered to a prolonged light phase. Tundra ponds are generally close to neutral in pH and low in ions, contain dissolved gases near saturation and are nutrient poor. In low arctic ponds there are two phytoplankton biomass and primary production peaks, whereas they may be only one in the high arctic. Nanoplanktonic flagellates of the Chrysophyceae and Cryptophyceae dominate the maxima. The mid-summer decline in phytoplankton in the low arctic can be attributed to a combination of phosphorus limitation and heavy grazing pressure. The cryptomonad Rhodomonas minuta Skuja is one of the most widespread phytoplankters in tundra ponds. Because of the altered photoperiods, many species do not form resting spores prior to ice formation but survive freezing in the vegetative state. 相似文献
7.
In the Biomphalaria glabrata-Schistosoma mansoni association, variations in cercarial production, in cercarial infectivity, and in the growth of infected snails are rhythmic. These chronobiological aspects are correlated with the dynamics of the intramolluscan larval stages of the parasite during the course of the infection. Rhythmic variations in the growth kinetics of infected snails are interpreted in terms of host-parasite metabolic exchanges. 相似文献
8.
To evaluate the influence of overwintering individuals of zooplankton on spring zooplankton communities, the dynamics of zooplankton
communities with or without overwintering individuals were observed in experimental ponds from fall to spring. An insecticide,
carbaryl, was used to regulate the overwintering individuals. In ponds which received insecticide applications in November
or January, all cladoceran and rotiferan species were eliminated by the treatments and did not reappear until late March or
early April, even when the chemical disappeared rapidly. The low water temperature may delayed the establishment of the populations
from resting eggs. In these ponds, populations of various cladoceran and rotiferan species, which seemed to be originated
from resting eggs, were built up in the spring. In control ponds,Daphnia ambigua orD. longispina overwintered as juveniles and adults and established a large spring population earlier than other cladocerans and rotifers
overwintering as resting eggs. The latter zooplankters did not increase in the spring probably because their growth was suppressed
by the precedingDaphnia species through competition. In nature, even if the number of overwintering individuals is small, they may have a potential
to build up a large population earlier than the individuals hatching from resting eggs. As a result, the species which have
overwintered as individuals seem to predominate in the spring and have a large influence on the spring zooplankton community. 相似文献
9.
Physella heterostropha size distributions and densities were measured in Pen Branch delta, a thermally altered swamp of the Savannah River, South Carolina. During cessation of thermal input, measurements were taken in a highly impacted area and more natural area. Of the three substrates sampled, logs, small woody material and benthos, snail density was highest on small pieces of submerged wood and < 20% of the snails on logs were out of the water. Following resumption of thermal input, snail density was four times higher on logs than previously measured. Snails responded to elevated water temperatures by congregating in a narrow band slightly above the water. Snail size did not appear to affect their response. However, the thermal history of the snails influenced their behaviour and survival rates. Logs may be a potential refuge for snails when water temperature are greater greater than 38°C. However, our results indicate that long-term survival in this manner may be impossible. 相似文献
10.