全文获取类型
收费全文 | 9901篇 |
免费 | 982篇 |
国内免费 | 1542篇 |
专业分类
12425篇 |
出版年
2024年 | 36篇 |
2023年 | 158篇 |
2022年 | 152篇 |
2021年 | 252篇 |
2020年 | 391篇 |
2019年 | 390篇 |
2018年 | 334篇 |
2017年 | 387篇 |
2016年 | 373篇 |
2015年 | 437篇 |
2014年 | 468篇 |
2013年 | 719篇 |
2012年 | 462篇 |
2011年 | 413篇 |
2010年 | 411篇 |
2009年 | 499篇 |
2008年 | 496篇 |
2007年 | 534篇 |
2006年 | 476篇 |
2005年 | 407篇 |
2004年 | 364篇 |
2003年 | 320篇 |
2002年 | 299篇 |
2001年 | 296篇 |
2000年 | 280篇 |
1999年 | 239篇 |
1998年 | 203篇 |
1997年 | 208篇 |
1996年 | 193篇 |
1995年 | 189篇 |
1994年 | 177篇 |
1993年 | 193篇 |
1992年 | 150篇 |
1991年 | 134篇 |
1990年 | 109篇 |
1989年 | 137篇 |
1988年 | 104篇 |
1987年 | 102篇 |
1986年 | 103篇 |
1985年 | 131篇 |
1984年 | 116篇 |
1983年 | 94篇 |
1982年 | 85篇 |
1981年 | 80篇 |
1980年 | 90篇 |
1979年 | 58篇 |
1978年 | 50篇 |
1977年 | 39篇 |
1976年 | 32篇 |
1973年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
41.
ANDERS FORSMAN RICHARD SHINE 《Biological journal of the Linnean Society. Linnean Society of London》1995,55(4):273-291
Females of Lampropholis delicata are dimorphic for colour pattern, the difference between morphs being the presence or absence of a distinct white mid-lateral stripe. A less distinct striped morph occurs also in males. We evaluated alternative hypotheses for the maintenance of this polymorphism by examining temporal and spatial variation in morph frequency, testing for differential selection among morphs using data on body size and reproductive traits from preserved specimens, and experimentally manipulating colour pattern in free-ranging lizards of both sexes, to assess the influence of the lateral stripe on survival rates. We found that the relative frequency of striped individuals varied among populations and decreased from north to south in both sexes, coincident with an increasing incidence of regenerated tails. Morph frequencies did not change through time within a population. Striped gravid females appeared to survive better and produced larger clutches than did non-striped females. In our experimental study, the relationship between survival and colour morph differed between the two sexes; males painted with a white lateral stripe had lower survival than control (brown stripe) males, but survival did not differ between striped and control females. The different response in the two sexes may be due partly to differences in temperature and microhabitat selection. We propose that the white lateral stripe decreases susceptibility to predators in gravid females but increases risk of predation in males, especially in combination with low temperatures. The polymorphism might be maintained by: (1) opposing fitness consequences of the stripe in males and females; (2) sex-specific habitat selection; and (3) gene flow in combination with spatial variation in relative fitness of the two morphs. 相似文献
42.
Two intertidal snails, Littorina saxatilis (Olivi, 1972) (upper eulittoral fringe/maritime zone) and Littorina obtusata (Linnaeus, 1758) (lower eulittoral) were collected from a boulder shore on Nobska Point, Cape Cod, Massachusetts, in July
and acclimated for 15–20 days at 4 ° or 21 °C. Oxygen consumption rate (Vo2) was determined for 11–15 subsamples of individuals at 4 °, 11 ° and 21 °C with silver/platinum oxygen electrodes. Multiple
factor analysis of variance (MFANOVA) of lo10 transformed values of whole animal Vo2 with log10 dry tissue weight (DTW) as a covariant revealed that increased test temperature induced a significant increase in Vo2 in both species (P<0.00001). In contrast, MFANOVA revealed that temperature acclimation did not affect Vo2 in either L. saxatilis (P= 0.35) or L. obtusata (P= 0.095). Thus, neither species displayed a capacity for the typical metabolic temperature compensation marked by an increase
in Vo2 at any one test temperature in individuals acclimated to a lower temperature that is characteristic of most ectothermic animals.
Lack of capacity for metabolic temperature acclimation has also been reported in other littorinid snail species, and may be
characteristic of the group as a whole. Lack of capacity for respiratory temperature acclimation in these two species and
other littorinids may reflect the extensive semi-diurnal temperature variation that they are exposed to in their eulittoral
and eulittoral fringe/maritime zone habitats. In these habitats, any metabolic benefits derived from longer-term temperature
compensation of metabolic rates are negated by extreme daily temperature fluctuations. Instead, littorinid species appear
to have evolved mechanisms for immediate metabolic regulation which, in L. saxatilis and L. obtusata and other littorinids, appear to centre on a unique ability for near instantaneous suppression of metabolic rate and entrance
into short-term metabolic diapause at temperatures above 20–35 °C, making typical seasonal respiratory compensation mechanisms
characteristic of most ectotherms of little adaptive value to littorinid species. 相似文献
43.
低温预处理过程中大麦花药内源激素的变化 总被引:9,自引:0,他引:9
很多文章曾报道低温预处理可以明显提高大麦花药反应频率。但对于低温预处理的机理,至今研究报道甚少。我们应用ELISA方法测定了大麦花药低温预处理过程中内源激素IAA、iPA和ABA含量。根据实验结果,推测低温预处理改变了花药内源IAA和iPA含量,阻断了花粉原来的发育方向,使其由配子体的发育途径转向孢子体的发育途径。 相似文献
44.
Shigeru Ando 《Ecological Research》1995,10(3):321-325
Body temperatures of 11 bird species, including cuckoos, were measured in an artificial meteorological room. Ratios of change
in body temperature to that in air temperature were thereby obtained for each species. Cuckoos demonstrate a remarkably high
value, indicating a particularly low ability to regulate body temperature. Viewed in this light, the cuckoo's parasitic behavior
is very likely an adaptation to overcome a physiological disadvantage. This in turn might be expected to reinforce delay in
evolution of temperature homeostasis. 相似文献
45.
46.
Expression of calf prochymosin in Saccharomyces cerevisiae 总被引:18,自引:0,他引:18
47.
48.
Crayfish (Cambarus bartoni) were tested individually in an electronic shuttlebox thermoregulatory device which allowed them to control water temperature, and thereby their body temperature, by their movements.Crayfish which initially selected 22.1°C water increased their preferred temperature by 1.8°C following injection of killed bacteria (Aeromonas hydrophila) into the gill cavity.This behavioral fever appears similar to that of vertebrates, and may enhance host defense reactions against pathogens. 相似文献
49.
Edward G. Durbin 《Journal of phycology》1977,13(2):150-155
The effect of cell size on growth rates and some cellular contents of Thalassiosira nordenskioeldii Cleve has been measured at 0 and 10 C. At 0 C the growth rate did not vary with cell size. The 2 smallest clones at this temperature had reduced growth rates because of the induction of sexuality in that size range. The clones grown at 10 C showed a significant negative relationship between growth rate and valve diameter with the cell surface area/volume ratio positively related to growth rate. At both temperatures the smaller cells had proportionately more carbon and nitrogen/unit cell volume. The amount of chlorophyll a and silica/unit cell surface area increased with increasing cell surface area at both 0 and 10 C. Both the C/N and C/chl a ratios showed no significant change with cell size at either temperature but there was a significant increase in the C/chl a ratio at 0 C. The C/Si ratio decreased with increasing cell size at both 0 and 10 C. 相似文献
50.