全文获取类型
收费全文 | 3434篇 |
免费 | 347篇 |
国内免费 | 2篇 |
出版年
2022年 | 29篇 |
2021年 | 57篇 |
2020年 | 51篇 |
2019年 | 41篇 |
2018年 | 57篇 |
2017年 | 50篇 |
2016年 | 84篇 |
2015年 | 137篇 |
2014年 | 138篇 |
2013年 | 176篇 |
2012年 | 227篇 |
2011年 | 192篇 |
2010年 | 126篇 |
2009年 | 130篇 |
2008年 | 176篇 |
2007年 | 183篇 |
2006年 | 145篇 |
2005年 | 152篇 |
2004年 | 151篇 |
2003年 | 144篇 |
2002年 | 174篇 |
2001年 | 65篇 |
2000年 | 63篇 |
1999年 | 73篇 |
1998年 | 46篇 |
1997年 | 39篇 |
1996年 | 47篇 |
1995年 | 42篇 |
1994年 | 44篇 |
1993年 | 30篇 |
1992年 | 35篇 |
1991年 | 44篇 |
1990年 | 36篇 |
1989年 | 34篇 |
1988年 | 40篇 |
1987年 | 40篇 |
1986年 | 29篇 |
1985年 | 32篇 |
1984年 | 49篇 |
1983年 | 18篇 |
1982年 | 23篇 |
1981年 | 25篇 |
1980年 | 20篇 |
1979年 | 16篇 |
1978年 | 23篇 |
1977年 | 20篇 |
1976年 | 24篇 |
1973年 | 30篇 |
1972年 | 18篇 |
1971年 | 15篇 |
排序方式: 共有3783条查询结果,搜索用时 15 毫秒
1.
The Capricorn silvereye (Zosterops lateralis chlorocephalus) is ideally suited to investigating the genetic basis of body size evolution. We have isolated and characterized a set of microsatellite markers for this species. Seven out of 11 loci were polymorphic. The number of alleles detected ranged from two to five and observed heterozygosities between 0.12 and 0.67. One locus, ZL49, was found to be sex‐linked. This moderate level of diversity is consistent with that expected in an isolated, island population. 相似文献
2.
3.
4.
5.
Experiments are described which were designed to investigate the significance of individual differences in aggressiveness between polecats and the relation between the familiarity of the opponent and the pattern and outcome of fighting.
The behaviour of Mustela putorius, M. furo and hybrids between the two species was investigated in an indoor arena of 16 m2 .
Two kinds of fighting between male polecats were recorded, "companion fighting" between cage mates, in which biting was inhibited and neither of the opponents became intimidated and "uninhibited fighting" between unfamiliar individuals from which a winner and loser generally emerged and a rank order formed.
Individual differences in aggressiveness were assessed by means of a scoring system which statistical analysis showed, generally, to be consistent for any one individual. The rank of an animal, however, was found not to be invariably related to its aggression score.
A number of other factors influencing fighting were investigated, the earlier introduction of one individual into the arena increases its chances of winning a fight; when offered a choice, male polecats fight strangers in preference to cage mates; and the separation of a group of cage mates for as little as 48 hours induces them to behave towards one another like strangers with the result that a rank order is formed. Factors which had no apparent effect on fighting between males were the weight of the individual or the presence of females.
The attributes of familiar and unfamiliar opponents are discussed in the light of these findings. 相似文献
The behaviour of Mustela putorius, M. furo and hybrids between the two species was investigated in an indoor arena of 16 m
Two kinds of fighting between male polecats were recorded, "companion fighting" between cage mates, in which biting was inhibited and neither of the opponents became intimidated and "uninhibited fighting" between unfamiliar individuals from which a winner and loser generally emerged and a rank order formed.
Individual differences in aggressiveness were assessed by means of a scoring system which statistical analysis showed, generally, to be consistent for any one individual. The rank of an animal, however, was found not to be invariably related to its aggression score.
A number of other factors influencing fighting were investigated, the earlier introduction of one individual into the arena increases its chances of winning a fight; when offered a choice, male polecats fight strangers in preference to cage mates; and the separation of a group of cage mates for as little as 48 hours induces them to behave towards one another like strangers with the result that a rank order is formed. Factors which had no apparent effect on fighting between males were the weight of the individual or the presence of females.
The attributes of familiar and unfamiliar opponents are discussed in the light of these findings. 相似文献
6.
We examined regrowth architecture of 4 species of savanna shrubs following 4 levels of defoliation. Defoliation had little effect on the regrowth architecture of honey mesquite ( Prosopis glandulosa ), which is rarely browsed by mammalian herbivores. The 3 acacia species ( Acacia berlandieri , A. greggii , A. schaffneri ) responded to defoliation by increasing leaf and spine density on the regrowth branches, but spine length and branching architecture remained unchanged. Only A. greggii , which is a preferred food plant of many browsers, exhibited an increase in the number and length of current annual growth branches in response to defoliation. The changes in plant architecture due to defoliation had little effect on the subsequent feeding behavior of captive white-tailed deer ( Odocoileus virginianus ). Food intake rate of the deer was most strongly related to internode distance, a parameter not significantly altered by defoliation. This suggests that the architectural responses of these shrubs to defoliation may not provide increased defense against browsing by co-evolved mammals. 相似文献
7.
The relationship between thef-ratio [NO3 uptake/(NO3+ NH4+) uptake] and ambient nitrate concentration was evaluatedfor eight data sets from coastal waters. The f-ratio increasedasymptotically with increase in nitrate concentration in mostdata sets. However, the rate at which f-ratio increased at lownitrate concentration (slope = m) and the maximum attained f-ratio(fmax) varied among regions; the initial slope varied most withvalues ranging in excess of an order of magnitude. The datawere analyzed in relation to environmental factors and methodologicalconsiderations known to influence the f-ratio. Ambient ammoniumconcentration was important in accounting for regional differencesin the f versus NO3 relationship. A further analysisof the data, relating f-ratio to the ratio of NO3/(NO3+ NH4+) concentrations yielded a much more regionally consistentand approximately linear relationship; slopes varied by lessthan a factor of two in the extreme cases. Inclusion of knownalternative (aside from NH4+) sources of reduced-N (e.g. urea)and correction for methodological/computational errors (isotopedilution) systematically reduce f-ratio estimates. Other factors,e.g. reduced-N uptake by microheterotrophs, may systematicallyincrease the f-ratio. 相似文献
8.
Microinjection of plant protoplasts and cells has been recently reported, however a system that combines simplicity of design, harmless immobilization, high resolution visibility and ability to monitor individual target cells is lacking. This report describes a system which combines these features. It consists of a microinjection-microculture dish containing immobilized protoplasts and a simple chamber that maintains sterility and humidity during injection. Highly purified protoplast preparations are plated at low population density as a thin monolayer of widely separated cells embedded in agarose layered over a thicker (0.2 mm at center to 1 mm at edge) layer of agarose-solidified medium. This physical arrangement allows for rapid location, mapping and injection of the immobilized protoplasts and also their subsequent location for growth monitoring. The double layers of agarose provide adequate nutrition for culturing injected cells to the microcalli stage. In addition to protoplast injection, this system was also used to inject 3- to 4-day old nonspherical cells derived from protoplasts. Colony formation rates from injected protoplasts and cells with regenerated walls were equivalent to those of uninjected controls. Furthermore, tobacco protoplasts stored at 4°C in liquid medium for up to two weeks remained fully competent for plating and injection. These cold-stored protoplasts, when injected, formed colonies at rates similar to those from fresh preparations. The ability to store protoplasts without loss of viability considerably increases the ease and convenience of cell injection experiments.Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the USDA, and does not imply its approval to the exclusion of the other products that may also be suitable. 相似文献
9.
Excised cotyledons of Pinus radiata D. Don cultured under shoot-forming(plus benzyladenine) and non shoot-forming (minus benzyladenine)conditions for 10 and 21 days were fed U-[14C]-glucose for 3h in the light followed by a 3 h chase period. The labellingof individual metabolites as well as 14C incorporation intoprotein was assessed. It was found that the general metabolicpatterns were qualitatively the same in shoot-forming and nonshoot-forming conditions, however, metabolism leading to respirationas well as to the synthesis of some amino acids and proteinsynthesis was enhanced in the shoot-forming cultures. (Received February 16, 1987; Accepted July 8, 1987) 相似文献
10.
W E Owens S C Nickerson 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1989,190(1):79-86
An anti-inflammatory factor isolated from milk of hyperimmunized cows was analyzed in vitro and in vivo. Macrophages collected from lacteal secretions of a unimmunized nonlactating cow showed increased ability to kill phagocytosed Staphylococcus aureus when incubated with the anti-inflammatory factor. Mice injected intraperitoneally with 10 mg/kg of anti-inflammatory factor demonstrated an increased LD50 to S. aureus when challenged intraperitoneally. Injected mice also demonstrated significantly (P less than 0.05) less mammary inflammation and involution and increased clearance of S. aureus when challenged intramammarily. Quantitative histologic analysis of mammary tissues from mice injected with anti-inflammatory factor demonstrated significantly (P less than 0.05) more lumen, less interalveolar connective tissue, and less leukocytic infiltration compared with control mice. Mammary glands of mice injected with anti-inflammatory factor and challenged with S. aureus also contained fewer colony-forming units than control mice. The product appeared to exert its effect on the nonspecific defense system via modulation of leukocyte function. 相似文献