全文获取类型
收费全文 | 2591篇 |
免费 | 118篇 |
国内免费 | 179篇 |
专业分类
2888篇 |
出版年
2023年 | 31篇 |
2022年 | 39篇 |
2021年 | 52篇 |
2020年 | 55篇 |
2019年 | 59篇 |
2018年 | 53篇 |
2017年 | 34篇 |
2016年 | 54篇 |
2015年 | 59篇 |
2014年 | 77篇 |
2013年 | 132篇 |
2012年 | 82篇 |
2011年 | 118篇 |
2010年 | 80篇 |
2009年 | 109篇 |
2008年 | 117篇 |
2007年 | 134篇 |
2006年 | 140篇 |
2005年 | 114篇 |
2004年 | 104篇 |
2003年 | 95篇 |
2002年 | 103篇 |
2001年 | 70篇 |
2000年 | 68篇 |
1999年 | 59篇 |
1998年 | 53篇 |
1997年 | 70篇 |
1996年 | 48篇 |
1995年 | 39篇 |
1994年 | 63篇 |
1993年 | 53篇 |
1992年 | 35篇 |
1991年 | 41篇 |
1990年 | 35篇 |
1989年 | 20篇 |
1988年 | 22篇 |
1987年 | 30篇 |
1986年 | 19篇 |
1985年 | 39篇 |
1984年 | 49篇 |
1983年 | 32篇 |
1982年 | 22篇 |
1981年 | 35篇 |
1980年 | 26篇 |
1979年 | 25篇 |
1978年 | 17篇 |
1977年 | 29篇 |
1976年 | 17篇 |
1975年 | 14篇 |
1973年 | 6篇 |
排序方式: 共有2888条查询结果,搜索用时 8 毫秒
121.
Greater size and strength are common attributes of contest winners. Even in social insects with high cooperation, the right to reproduce falls to the well-fed queens rather than to poorly fed workers. In Dictyostelium discoideum, formerly solitary amoebae aggregate when faced with starvation, and some cells die to form a stalk which others ride up to reach a better location to sporulate. The first cells to starve have lower energy reserves than those that starve later, and previous studies have shown that the better-fed cells in a mix tend to form disproportionately more reproductive spores. Therefore, one might expect that the first cells to starve and initiate the social stage should act altruistically and form disproportionately more of the sterile stalk, thereby enticing other better-fed cells into joining the aggregate. This would resemble caste determination in social insects, where altruistic workers are typically fed less than reproductive queens. However, we show that the opposite result holds: the first cells to starve become reproductive spores, presumably by gearing up for competition and outcompeting late starvers to become prespore first. These findings pose the interesting question of why others would join selfish organizers. 相似文献
122.
Cvak L Jegorov A Sedmera P Císarová I Cejka J Kratochvíl B Pakhomova S 《Amino acids》2005,29(2):145-150
Summary. A novel natural peptide ergot alkaloid γ-ergokryptinine containing norleucine has been isolated from ergot sclerotia of the field-growing parasitic fungus Claviceps purpurea CCM 8059. Its structure was deduced from the NMR and mass spectral data. The final structural proof was provided by the crystal structure determination, which is the first X-ray structure of a natural Nle-containing secondary metabolite. The conformations of three ergopeptinines: γ-ergokryptinine, ergoladinine, and α-ergokryptinine were compared. 相似文献
123.
Fourteen phenolic derivatives including two new compounds (1, 2), one new natural product (10) and eleven known ones (3 - 9 and 11–14) were obtained from the roots of Fallopia multiflora var. ciliinervis. The structures were determined using IR, MS, 1D and 2D NMR spectroscopy and the absolute configuration of the new structure (2) was deduced by ECD experiments. All of the isolated compounds were assayed for their cytotoxic activities against three tumor cell lines (A549, HCT116 and SW620) and the results showed that compounds 12-14 showed cytotoxicities against A549 cells while compounds 10 and 11 showed cytotoxicities against SW620 cells. 相似文献
124.
Sex genotypes in mature androgenetic and control rainbow trout Oncorhynchus mykiss were determined using primed in situ labelling (PRINS) detection of 5S rDNA sequences on the X chromosomes. Three sex genotypes, corresponding to phenotypic sex, were revealed: female XX, male XY and supermale YY. 相似文献
125.
Chagot B Escoubas P Villegas E Bernard C Ferrat G Corzo G Lazdunski M Darbon H 《Protein science : a publication of the Protein Society》2004,13(5):1197-1208
Animal toxins block voltage-dependent potassium channels (Kv) either by occluding the conduction pore (pore blockers) or by modifying the channel gating properties (gating modifiers). Gating modifiers of Kv channels bind to four equivalent extracellular sites near the S3 and S4 segments, close to the voltage sensor. Phrixotoxins are gating modifiers that bind preferentially to the closed state of the channel and fold into the Inhibitory Cystine Knot structural motif. We have solved the solution structure of Phrixotoxin 1, a gating modifier of Kv4 potassium channels. Analysis of the molecular surface and the electrostatic anisotropy of Phrixotoxin 1 and of other toxins acting on voltage-dependent potassium channels allowed us to propose a toxin interacting surface that encompasses both the surface from which the dipole moment emerges and a neighboring hydrophobic surface rich in aromatic residues. 相似文献
126.
Large‐scale sampling of spontaneous rainbow trout Oncorhynchus mykiss intersexes indicated a strong asymmetry of gonad differentiation in XX females; the right gonad was more sensitive to the mutation‐induced masculinization than the left one. 相似文献
127.
不同株型芝麻种质湿害后产量性状研究及耐湿性评价 总被引:4,自引:0,他引:4
于盛花期对66份种质进行湿害处理,结果表明:湿害对单秆型和分枝型种质的产量性状影响差异明显,对单秆型芝麻产量性状的影响(湿害指数值)大小依次为单株种子干重(69.18%)蒴果数(67.48%)有效果节数(49.10%)有效果轴长度(45.69%)株高(16.40%),对分枝性芝麻的影响依次为分枝有效果节数(65.96%)分枝蒴果数(64.73%)总蒴果数(52.01%)单株种子干重(49.92%)主茎蒴果数(41.66%)主茎有效果轴长度(37.57%)有效分枝数(34.21%)主茎有效果节数(20.12%)株高(15.43%);湿害对三蒴型芝麻的侧位蒴果影响较大,对单秆三蒴型芝麻的影响为侧位蒴果数(92.25%)中位蒴果数(50.25%),对分枝三蒴型芝麻的影响为分枝侧位蒴果数(92.86%)主茎侧位蒴果数(69.14%)分枝中位蒴果数(44.17%)主茎中位蒴果数(32.97%)。根据相对湿害产量可以将供试种质聚为耐湿与不耐湿二大类,不耐湿类型种质61份,占92.42%;耐湿类型种质5份,占7.58%,为竹山白芝麻、西平二郎花、阜南芝麻、嘉兴紧口黑和麻城黑芝麻,可作为耐湿种质加以利用。 相似文献
128.
129.
In many gonochoristic taxa, sex is influenced by developmental environment, a system that can lead to temporal fluctuations in offspring sex ratio. Demographic models suggest that only short‐lived species with environmental sex determination (ESD) are negatively impacted by sex‐ratio fluctuations, yet these models fail to account for the potential mutation load associated with reductions in genetically effective population sizes. In this study, we developed a series of individual‐based simulation models that explore the fixation rates of mildly deleterious alleles under different sex‐determining systems and examine the impacts of variation in lifespan and offspring sex ratio. Populations with ESD exhibited increases in fixation rates in both short‐ and long‐lived populations, but substantial increases were limited to populations characterized by a combination of high sex‐ratio variation and short lifespan. Fixation rates were negatively associated with effective population size, indicating that purifying selection operates less efficiently under ESD relative to genotypic sex determination. Reductions in effective population size could be attributed to both intragenerational forces (unequal sex ratio) and intergenerational forces (variable census population sizes). Levels of temporal sex‐ratio variation calculated from wild populations of ESD species were capable of yielding large increases in fixation rates, although this relationship was strongly mediated by lifespan. Our results may help to explain the limited phylogenetic distribution of ESD in short‐lived taxa. 相似文献
130.
Andrea M. Quiroga Federico J. Berli Daniela Moreno Juan B. Cavagnaro Rubén Bottini 《Journal of Plant Growth Regulation》2009,28(1):28-35
In many cultivars of Vitis vinifera periods of mild water stress during ripening are thought to increase grape quality for winemaking, even though yields may
be negatively affected. Because abscisic acid (ABA) is involved in the signaling of water stress in plants, we examine the
effects of the ABA signal being given without the concomitant water stress. ABA at 250 mg l−1 was sprayed weekly or biweekly from bud-burst until harvest onto the leaves of vineyard-grown plants of cv. Cabernet Sauvignon.
For ABA-treated plants berry yield per bunch and per plant was significantly increased (1.5- to 2.0-fold) across three consecutive
harvests (2005 through 2007). Number of berries per bunch and per plant was the primary basis for the significant crop increases,
although bunches per plant also tended to increase (1.1- to 1.3-fold) across all three harvests. Other parameters assessed
included number of internodes, shoot length, leaf area, leaf water potential at midday, photosynthesis, and stomatal conductance.
These parameters showed no significant change with ABA treatment, although shoot length tended to be reduced, as was leaf
area relative to control plants. The significantly increased fruit yields were thus accomplished without accompanying increases
in leaf photosynthesis and leaf areas. Juice at harvest had equal levels of sugars (Brix) and somewhat higher levels of anthocyanins
and total polyphenols relative to control values. The two latter trends continued for the resultant wine across two vintage
years. In conclusion, three seasons of experimental trials have demonstrated that ABA application can significantly enhance
yield per plant in the field-grown grape (cv. Cabernet Sauvignon) by favoring increased berry set without diminishing the
quality of the fruit for winemaking use. 相似文献