全文获取类型
收费全文 | 43104篇 |
免费 | 3870篇 |
国内免费 | 33篇 |
专业分类
47007篇 |
出版年
2023年 | 166篇 |
2022年 | 309篇 |
2021年 | 599篇 |
2020年 | 395篇 |
2019年 | 490篇 |
2018年 | 641篇 |
2017年 | 555篇 |
2016年 | 1044篇 |
2015年 | 1758篇 |
2014年 | 1880篇 |
2013年 | 2456篇 |
2012年 | 2979篇 |
2011年 | 3057篇 |
2010年 | 1945篇 |
2009年 | 1657篇 |
2008年 | 2517篇 |
2007年 | 2507篇 |
2006年 | 2273篇 |
2005年 | 2338篇 |
2004年 | 2290篇 |
2003年 | 2181篇 |
2002年 | 2130篇 |
2001年 | 505篇 |
2000年 | 406篇 |
1999年 | 510篇 |
1998年 | 606篇 |
1997年 | 462篇 |
1996年 | 411篇 |
1995年 | 393篇 |
1994年 | 355篇 |
1993年 | 345篇 |
1992年 | 385篇 |
1991年 | 327篇 |
1990年 | 270篇 |
1989年 | 272篇 |
1988年 | 274篇 |
1987年 | 251篇 |
1986年 | 234篇 |
1985年 | 302篇 |
1984年 | 321篇 |
1983年 | 257篇 |
1982年 | 352篇 |
1981年 | 275篇 |
1980年 | 255篇 |
1979年 | 190篇 |
1978年 | 222篇 |
1977年 | 208篇 |
1976年 | 178篇 |
1974年 | 188篇 |
1973年 | 194篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
92.
The geographical variation in the calls of the Manx shearwater Puffinus puffinus and British storm petrel Hydrobales pelagicus was investigated, as well as its temporal stability in the shearwater. Within-island call variation was not apparent in Puffinus , although male calls were relatively more distinct locally than female calls. This is discussed in relation to the differential dispersal of the sexes. Between-island call variation was apparent in both Puffinus and Hydrobates , but the variation was continuous, and not discrete. Call divergence is unrelated to inter-island distance, and may reflect the influence of coastlines on each species'dispersal opportunities. The calls of a shearwater population changed over a period of six to seven years, apparently due to the appearance of new birds rather than surviving birds altering their calls. Male calls changed more than female calls, which, taken with dispersal factors, suggests that calls are transmitted across generations at least in part by cultural copying. The vocal'drift'over time therefore probably results from errors in this copying process. 相似文献
93.
94.
When the picrosirius red technique was applied to cardiac muscle sections, intense yellow myocyte staining sometimes obscured thin collagenous septa. The picrosirius red technique was modified to include treatment of the sections in 0.2% (w/v) aqueous phosphomolybdic acid prior to staining. With 1-5 min treatment, cytoplasmic staining was eradicated; diminution of collagen staining occurred only with long treatments at much higher concentrations of phosphomolybdic acid. Using this phosphomolybdic acid-picrosirius red technique, collagenous septa as thin as 0.2-0.5 /im and fine collagen fibers making up the septa were clearly discernible. The technique also worked well on sections stained by other techniques and then destained. The phosphomolybdic acid-picrosirius red technique should be useful in experiments designed to investigate the effects of collagen distribution on the electrical and mechanical behavior of cardiac muscle. 相似文献
95.
Paul M. Catling 《Brittonia》1983,35(2):120-125
Examination of flowers from herbarium specimens ofSpiranthes ovalis revealed two kinds of column structure. Some plants, including the type, have flowers with a normally developed rostellum and viscidium; other plants that have flowers in which these structures are entirely lacking are here described asSpiranthes ovalis var.erostellata. Self-pollination in the new variety was proven by pollinia removal and observation of plants in insect-proof cages. The var.erostellata has a somewhat smaller perianth with sepals 3.5–5 instead of 4–6.1 mm long and is more common and more widespread than the typical variety. 相似文献
96.
97.
The different types of recrystallization which can be observed in the Triassic involutinids should be considered in the taxonomic treatment of this foraminiferal family. If the diagenetic modifications of the test are neglected the same form could be placed into different suborders. Our inclusion of the genusTriasina Majzon in the Involutinidae proposed earlier on purely morphologic grounds, seems to be supported by the fact that the same types of recrystallization exist inTrocholina, Involutina undTriasina. 相似文献
98.
During two years of passage, several myxamoebal strains of Physarum polycephalum changed with regard to their properties in crosses and plaque morphology. These changes have been correlated with increased nuclear deoxyribonucleic acid content. 相似文献
99.
Selective esterification reactions of 1,6-anhydro-3-deoxy-β-D-xylo-hexopyranose(1), 1,6-anhydro-β-D-glucopyranose (7), and several derivatives of 7, were conducted with an acid chloride or acid anhydride in pyridine. Reaction of 1 with p-toluenesulfonyl chloride and with benzoyl chloride gave 70 and 63%, respectively, of the 2-esters. The 2-methyl and 2-benzyl ethers of 7, both having strongly hydrogen-bonded C-4 hydroxyl group, reacted with p-toluenesulfonyl chloride to yield the 4-monosulfonates (71 and 74%, respectively). Esterification of the 2-methyl ether and 2-p-toluenesulfonate of 7 with p-toluenesulfonic anhydride instead of with p-toluenesulfonyl chloride led to increased yields of the 4-p-toluenesulfonates after a shorter reaction-time. 相似文献
100.