全文获取类型
收费全文 | 28994篇 |
免费 | 129篇 |
国内免费 | 263篇 |
专业分类
系统科学 | 462篇 |
丛书文集 | 557篇 |
教育与普及 | 106篇 |
理论与方法论 | 148篇 |
现状及发展 | 11845篇 |
研究方法 | 1200篇 |
综合类 | 14398篇 |
自然研究 | 670篇 |
出版年
2014年 | 195篇 |
2013年 | 403篇 |
2012年 | 666篇 |
2011年 | 1346篇 |
2010年 | 468篇 |
2009年 | 398篇 |
2008年 | 727篇 |
2007年 | 828篇 |
2006年 | 828篇 |
2005年 | 783篇 |
2004年 | 607篇 |
2003年 | 512篇 |
2002年 | 528篇 |
2001年 | 729篇 |
2000年 | 778篇 |
1999年 | 547篇 |
1992年 | 436篇 |
1991年 | 367篇 |
1990年 | 390篇 |
1989年 | 367篇 |
1988年 | 353篇 |
1987年 | 374篇 |
1986年 | 339篇 |
1985年 | 451篇 |
1984年 | 354篇 |
1983年 | 282篇 |
1982年 | 266篇 |
1981年 | 261篇 |
1980年 | 336篇 |
1979年 | 804篇 |
1978年 | 614篇 |
1977年 | 679篇 |
1976年 | 462篇 |
1975年 | 534篇 |
1974年 | 798篇 |
1973年 | 665篇 |
1972年 | 675篇 |
1971年 | 739篇 |
1970年 | 922篇 |
1969年 | 804篇 |
1968年 | 758篇 |
1967年 | 791篇 |
1966年 | 688篇 |
1965年 | 502篇 |
1959年 | 270篇 |
1958年 | 417篇 |
1957年 | 334篇 |
1956年 | 280篇 |
1955年 | 276篇 |
1954年 | 258篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Russell L. Burke Beverly Clendening Alexandra Kanonik 《Journal of Natural History》2018,52(25-26):1723-1732
Clutch size (CS) is a well-studied life history characteristic, and optimal egg size theory predicts that CS should correlate positively with reproductive investment. Turtles are good subjects for studies of reproductive strategies because few turtle species exhibit parental care; therefore quantifying their short-term reproductive investment is mostly limited to measuring egg size and number. Clutch size is usually reported as an average value for each turtle population, and where CS variation is noted, it is usually attributable to variation in adult body size. In two long-term studies of ecologically dissimilar species we detected a dramatic increase in CS in a common snapping turtle (Chelydra serpentina) population and in a diamondback terrapin (Malaclemys terrapin) population. It is unknown whether these changes are due to either variation in adult body size or resource availability; but the temporal patterns we observed have apparently never been reported previously. These trends remain unexplained and should be explored further. 相似文献
2.
刘伟霞 《科技情报开发与经济》2004,14(6):147-148
对矿井索车优化设计中的几个关键性问题进行了探讨,包括驱动轮和尾轮的设计、钢丝绳直径的选择计算、托绳轮间距的确定、绳卡的设计、张紧力和张紧行程的确定、互锁问题的解决等。 相似文献
3.
西文期刊回溯建库中几个重要项目的著录 总被引:4,自引:0,他引:4
在实践的基础上,从期刊的出版日期、出版周期、卷、期、年、月及其他标识、ISSN、增刊、出版者以及期刊的历史发展等方面对西文期刊机读目录的标准化著录问题进行了探讨。 相似文献
4.
关于21世纪图书馆员继续教育问题的思考 总被引:4,自引:1,他引:3
罗雪晴 《科技情报开发与经济》2004,14(6):14-15
论述了21世纪对图书馆员进行继续教育的必要性,介绍了图书馆员继续教育的内容,针对我国图书馆员继续教育的状况,提出了应采取加大投入、进行针对性培训、运用激励机制等措施来加强对图书馆员的继续教育。 相似文献
5.
F. Mulè 《Cellular and molecular life sciences : CMLS》1948,4(3):115-117
Summary A study was undertaken on the variations of the redox potential level produced by streptomycinin vitro andin vivo. We have been able to show that, owing to an oxidative effect, streptomycin produces an increase of the redox potential level. This oxidative effect varies in degree according to the condition of the patient.We also found that in the blood and in the spinal fluid of patients suffering from tubercular meningitis factors are present which inhibit the action of streptomycin.The results of our findings lead to the conclusion that the dose of streptomycin must be varied according to the condition of the patient if the constant level required for an efficient therapy is to be maintained in the blood and in the spinal fluid. 相似文献
6.
7.
F. Weygand A. Wacker V. Schmied-Kowarzik 《Cellular and molecular life sciences : CMLS》1948,4(11):427-428
Summary By condensing 2:4:5-triamino-6-hydroxy-pyrimidine with dihydroxyacetone (diacetate), diaminoacetone or acetone-1,3-di (p-formylaminobenzoic acid) not the expected 8- or 9-oxymethyl resp. -aminomethyl-pteridines but 8-or 9-methyl-pteridines were obtained. With p-tolyl-d-isoglucosamine not a tetrahydroxybutyl-pteridine but a trihydroxybutyl-pteridine was formed. For an explanation of these results it is supposed that from the dihydro-pteridines formed at first by intramolecular splitting off of H2O or R·NH2 aromatization takes place. 相似文献
8.
9.
NAFTALIN L 《Nature》1948,161(4098):763
10.
Summary (1) By precipitation with ammonium sulfate, streptomycin, or calcium salts, we obtained from the pulp of tomatoes a substance containing carotinoids. The behaviour of this substance was analogous to that of chloroplst-substance and to that of animal cytoplasmatic nucleoproteins. Like these it contains proteins, lipids, and very probably nucleic acids. We regard this substance as achromoplastine.(2) Experiments with slices of green tomatoes show that the changing over of the chlorophyll content into the carotinoid content is inhibited by the presence of streptomycin.(3) Streptomycin inhibits the formation of chlorophyll in etiolated separated cabbage leafs, just as this drug inhibits the formation of chlorophyll in growing seeds.(4) The development of anthocyanides is not influenced by streptomycin. 相似文献