全文获取类型
收费全文 | 7076篇 |
免费 | 433篇 |
国内免费 | 487篇 |
出版年
2024年 | 13篇 |
2023年 | 119篇 |
2022年 | 107篇 |
2021年 | 172篇 |
2020年 | 216篇 |
2019年 | 245篇 |
2018年 | 188篇 |
2017年 | 192篇 |
2016年 | 217篇 |
2015年 | 268篇 |
2014年 | 294篇 |
2013年 | 407篇 |
2012年 | 235篇 |
2011年 | 294篇 |
2010年 | 234篇 |
2009年 | 388篇 |
2008年 | 408篇 |
2007年 | 402篇 |
2006年 | 383篇 |
2005年 | 302篇 |
2004年 | 289篇 |
2003年 | 314篇 |
2002年 | 206篇 |
2001年 | 175篇 |
2000年 | 178篇 |
1999年 | 151篇 |
1998年 | 168篇 |
1997年 | 163篇 |
1996年 | 112篇 |
1995年 | 109篇 |
1994年 | 113篇 |
1993年 | 118篇 |
1992年 | 119篇 |
1991年 | 86篇 |
1990年 | 86篇 |
1989年 | 75篇 |
1988年 | 61篇 |
1987年 | 51篇 |
1986年 | 59篇 |
1985年 | 68篇 |
1984年 | 36篇 |
1983年 | 23篇 |
1982年 | 37篇 |
1981年 | 35篇 |
1980年 | 18篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1972年 | 7篇 |
排序方式: 共有7996条查询结果,搜索用时 390 毫秒
181.
Anthers of different species of the genera Anemone, Clematis, Papaver and Nicotiana were cultured by floating on a liquid medium which overlay an agarified charcoal medium . This technique proved to be superior to conventional methods i.e. culture on either solid or liquid media. Cold treatment of Anemone anthers for 7 days after inoculation on the double layer medium gave about the same frequency of embryos per anther as corresponding cultures cold treated before inoculation. An elevation of the CO2 concentration to 2% stimulated embryogenesis in anther cultures of Anemone canadensis, Anemone vitifolia, Papaver setigerum and Papaver radicatum . Cold treatment of cultures of Anemone canadensis inhibited embryogenesis if the ensuing culture was performed in 2% CO2. On the other hand, cold treatment was stimulating, with an optimum of about 20 days, if the cultures were maintained in normal air. Chemical analysis of untreated anthers of Anemone canadensis showed the presence of abscisic acid (2.2 × 10−6 g/g anthers). Cold treatment reduced the concentration of abscisic acid to 0.6 × 10−6 g/g anthers. By use of assays with Lemna gibba as test organism, activated charcoal was shown to adsorb abscisic acid that was added to the medium. Medium treated with charcoal before inoculation of anthers of Anemone canadensis provided to inhibit embryo production. 相似文献
182.
A review of experimental studies of the effect of zinc nutrition on insulin metabolism is presented. In addition to a short
introduction to the synthesis, secretion, and action of insulin, the effects of zinc deficiency—specifically on glucose tolerance,
insulin secretion, insulin synthesis and storage, and on total insulin-like activity—are dealt with. The concentrations of
zinc and chromium in serum, pancreas, and liver are compared to those of zinc-deficient animals and pair-fed controls.
In contrast to pair-fed controls, zinc-deficient rats had unaltered proinsulin contents after glucose stimulation, but they
showed a diminished glucose tolerance, lowered serum insulin content, and an elevated total insulin-like activity. The serum
zinc concentration of the deficient animals was greatly reduced and did not change during glucose stimulation, whereas it
rose in the case of the pair-fed controls. The serum chromium concentration increased in both groups in response to glucose
stimulation. In the pancreas of the deficient animals, the zinc concentration was reduced 60% and it increased during the
glucose tolerance test. In the liver there were no significant differences. The chromium concentrations were elevated in both
the pancreas and liver of the zinc-deficient rats by 60 and 100%, respectively, and were not influenced by glucose injection.
These studies show clearly that nutritional zinc deficiency influences insulin metabolism and action. 相似文献
183.
目的: 未折叠蛋白质反应UPR是酵母最重要蛋白质质量控制机制之一,研究UPR响应规律有助于优化异源蛋白分泌途径合成和应对酸醇等胁迫因子的细胞自我保护。方法: 选择实验室菌株W303-1A和工业菌株An-a,以UPRE启动子控制下的Lac Z为报告基因,利用CRISPR/Cas9技术构建得到指示菌株W303-1A (leu 2::UPRE-lac Z)和An-a (leu 2::UPRE- lac Z),分别简称WZ和AZ。结果: 生长曲线测定显示WZ和AZ与亲本菌株的生长接近;添加下述试剂孵育4h后测定β-半乳糖苷酶酶活:1μg/ml衣霉素、8%(v/v)乙醇、0.3%(v/v)乙酸、5%(v/v)乙醇+0.1%(v/v)乙酸;菌株AZ的比酶活分别是对照值的8.2、26.4、1.1和7.9倍,而菌株WZ则分别为12.6、2.4、1.0和1.0倍;进一步以YEplac195为载体表达β-葡萄糖苷酶,AZ和WZ转化子在2%纤维二糖中生长24h的β-葡萄糖苷酶酶活值分别为0.35和6.12U/ml,相应的LacZ则分别为对照值的3.1和5.4倍。结论: 两个菌株显示了在抑制物和异源蛋白表达UPR响应和调控能力上的显著差异,为其改造利用提供了方向;研究也为分析抑制物耐受性和异源蛋白表达关键制约因素、优化酵母ER和UPR信号通路的调控奠定了初步方法基础。 相似文献
184.
Erich Kessler 《Archives of microbiology》1977,113(1-2):139-141
Five physiological and biochemical characters, which had proved to be valuable for the taxonomy of the genus Chlorella, were studied in the genus Prototheca. There is no hydrogenase activity and no liquefaction of gelatin. Most strains are very acidtolerant (limit of growth at pH 2.0 or 2.5) and very salt-tolerant (limit of growth at 4 or 5% NaCl). Two strains grow well at 38°C. The 16 strains, which were previously assigned to seven taxa, fall into four different groups. Our results tend to support the assumption that Prototheca might be related to Chlorella protothecoides. 相似文献
185.
Why the evolution of resistance to anthropogenic toxins normally involves major gene changes: the limits to natural selection 总被引:1,自引:1,他引:0
M. R. Macnair 《Genetica》1991,84(3):213-219
Standard population genetic theory suggests that adaptation should normally be achieved by the spread of many genes each of small effect (polygenes), and that adaptation by major genes should be unusual. Such models depend on consideration of the rates of acquisition of adaptation. In practice, adaptation to pollutants and anthropogenic toxins has most frequently been achieved by the spread of major genes. A simple model is developed to explain this discrepancy, in which the determining factor is not the rate of spread, but the maximum response achievable under the two contrasting models of polygenic or major gene inheritance. In the short term, for a given mean and genetic variance, characters in which the additive genetic variance is produced by the segregation of many genes of small effect at intermediate gene frequencies are unable to produce as large a response to directional selection as characters in which the variance is caused by genes of large effect at low frequency. If the target for selection is a long way from the mean prior to selection (as it may well be for adaptation to novel anthropogenic stresses) then adaptation can only be achieved by species possessing major genes. The model is discussed with reference to the example of heavy metal tolerance in plants. 相似文献
186.
Most green gemmules of Spongilla lacustris survived enclosure in ice at –20 °C for up to 30 days; however, their rate of germination at 20 °C was less rapid than that of control gemmules. The length of time spent at low temperature had little effect on gemmule survival. In contrast, repeated cooling to –20 °C and warming to 4 °C led to a progressive decline in gemmule viability. These results indicate that cold injury occurs primarily during transitions between high and low temperatures. 相似文献
187.
Salinity tolerance and morphology of the osmoregulation organs in Cladocera with special reference to Cladocera from the Aral sea 总被引:6,自引:2,他引:4
Nick V. Aladin 《Hydrobiologia》1991,225(1):291-299
The hyperosmotic regulation of adult Cladocera is determined mainly by the amount of salts consumed with the food and by reabsorption of salts in cells of the nuchal (neck) organ. The hypoosmotic regulation both in adults and embryos is determined mainly by excretion of salts in special epipodite cells or in cells of the nuchal (neck) organ. The salinity of the Aral sea for the last 30 years increased from 8–10 to 26–28, which led to changes in the Cladocera fauna. At present only 4 species of Cladocera inhabit the Aral sea instead of 14 species that were previously found. These changes are in agreement with osmoregulation capacities of Cladocera.
Note added in proof. Since this paper was accepted for publication, all Cladocera have disappeared from the Aral Sea. This happened when salinity reached 30–32. This disappearance was predicted by and agrees with earlier laboratory experiments with Aral Sea Cladocera (Aladin, 1982b). 相似文献
188.
189.
190.
L. E. Francois 《Plant and Soil》1992,147(2):163-170
The boron tolerance of two summer squash cultivars (Cucurbita pepo L. Aristocrat Zucchini and Peter Pan Scallop) and one winter squash cultivar (Cucurbita moschata Poir. Butter boy) was determined in large, outdoor sand cultures. Boron treatments were imposed by irrigation with culture solutions that contained 1.0, 3.0, 6.0, 9.0, 12.0, or 15.0 mg B L-1. Relative fruit yields of Zucchini, Scallop, and Butter boy were reduced 5.2%, 9.8%, and 4.3% with each unit (mg L-1) increase in soil solution B (Bsw)>2.7, 4.9, and 1.0 mg B L-1, respectively. Reduced yields of all cultivars were attributed to a reduction in fruit number and not fruit size. Boron concentrations in leaves and fruit were directly correlated to Bsw. 相似文献