全文获取类型
收费全文 | 7077篇 |
免费 | 60篇 |
国内免费 | 136篇 |
专业分类
系统科学 | 92篇 |
丛书文集 | 90篇 |
教育与普及 | 11篇 |
理论与方法论 | 41篇 |
现状及发展 | 2943篇 |
研究方法 | 305篇 |
综合类 | 3715篇 |
自然研究 | 76篇 |
出版年
2015年 | 56篇 |
2014年 | 58篇 |
2013年 | 58篇 |
2012年 | 114篇 |
2011年 | 200篇 |
2010年 | 65篇 |
2009年 | 58篇 |
2008年 | 148篇 |
2007年 | 173篇 |
2006年 | 163篇 |
2005年 | 157篇 |
2004年 | 186篇 |
2003年 | 252篇 |
2002年 | 405篇 |
2001年 | 353篇 |
2000年 | 297篇 |
1999年 | 186篇 |
1992年 | 105篇 |
1991年 | 83篇 |
1990年 | 77篇 |
1989年 | 71篇 |
1988年 | 61篇 |
1987年 | 78篇 |
1986年 | 78篇 |
1985年 | 140篇 |
1984年 | 88篇 |
1983年 | 74篇 |
1982年 | 70篇 |
1981年 | 69篇 |
1980年 | 79篇 |
1979年 | 148篇 |
1978年 | 142篇 |
1977年 | 150篇 |
1976年 | 139篇 |
1975年 | 145篇 |
1974年 | 166篇 |
1973年 | 118篇 |
1972年 | 117篇 |
1971年 | 165篇 |
1970年 | 254篇 |
1969年 | 171篇 |
1968年 | 152篇 |
1967年 | 177篇 |
1966年 | 141篇 |
1965年 | 98篇 |
1964年 | 57篇 |
1959年 | 68篇 |
1958年 | 86篇 |
1957年 | 85篇 |
1954年 | 55篇 |
排序方式: 共有7273条查询结果,搜索用时 15 毫秒
1.
Giulia Furfaro Sergio De Matteo Paolo Mariottini Salvatore Giacobbe 《Journal of Natural History》2018,52(11-12):645-657
The first record from Sicily of the introduced facelinid nudibranch Godiva quadricolor allowed the detection of trophic relationships with the polycerid Polycera hedgpethi, another non-native nudibranch, and with two bryozoan species, namely the naturalized Cheilostomatida Bugula neritina and the cryptogenic Ctenostomatida Amathia verticillata. The settlement of both nudibranchs was presumably promoted by a trophic shift of P. hedgpethi from the natural prey B. neritina towards the largely available and not exploited A. verticillata. This short food web, without evident links with native fauna and having G. quadricolor as the top predator, is described. A DNA barcoding approach was used to confirm the identity of this facelinid species and to explore the possible genetic divergence occurring among the samples analysed. 相似文献
2.
3.
复数,实数及实对称数据下Bruun FFT算法的实现 总被引:3,自引:0,他引:3
本文在复数、实数以及实对称数据下研究和实现了Bruun FFT算法。文章分析和评价了各种数据下BruunFFT的运算量和特点,提出了算法的实现结构和方法。最后给了TMS320C30上的运算时间,我们实现的算法比基2FFT速度提高35%。 相似文献
4.
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
5.
V.A.帕瑟根 《国外科技新书评介》2007,(2):7-7
分子间的范德华力是十分微弱的,但可以说普遍的存在着,在很多体系中起着巨大的作用,在物理学、化学、生物学等学科中得到了广泛的研究。但怎样计算范德华力?如何考虑各种条件对范德华力的影响? 相似文献
6.
7.
8.
Summary Acetone and alcohol extracts of the posterior salivary glands ofOctopus vulgaris, Eledone moschata andEledone Aldrovandi contain a principle which strongly reduces the diuresis in hydrated rats.The antidiuretic principle is absent in all otherOctopus tissues as well as in the salivary glands ofOctopus macropus.It shows a high termostability and is strongly adsorbed by animal charcoal at any reaction.The salivary antidiuretic substance is not identical with the posterior pituitary principle nor with the other active substances found in the salivary glands of Octopoda.The injection of salivary extracts satisfactorily reduces polyuria and polydipsia in human diabetes insipidus.
Le presenti ricerche sono state in parte sovvenzionate da un contributo straordinario del Consiglio Nazionale delle Ricerche. 相似文献
Le presenti ricerche sono state in parte sovvenzionate da un contributo straordinario del Consiglio Nazionale delle Ricerche. 相似文献
9.
M. Lazzari V. Franceschini G. Minelli F. Ciani 《Cellular and molecular life sciences : CMLS》1993,49(4):277-281
The corrosion cast technique provided for the first time an excellent three-dimensional visualization of the vascular pattern of the choroid and iris in the newt eye. The results show the presence of a single arterial afference to the choroidal and iris capillaries: the ophthalmic artery is the origin fo both ciliary arteries and the long posterior ciliary artery. Slightly behind the equatorial circumference of the eyeball the venous drainage consists of a single vessel on the dorsal side and two distinct vessels on the ventral one. It receives blood from both iris and choroid. The surface of the plastic endocasts shows some details of fine luminal structures of the endothelial cells. Shallow depressions may be regarded as imprints of endothelial cell nuclei, and they are distinctly different for arteries and capillaries. The angioarchitecture of the newt eye differs from that of brain in that hairpin-shaped capillary loops are not observed at all. 相似文献
10.
Low concentrations of inositol 1,4,5-trisphosphate (InsP3) evoke a very rapid mobilization of intracellular Ca2+ stores in many cell types, which can be followed by a further, much slower efflux. Two explanations have been suggested for this biphasic release. The first proposes that the Ca2+ stores vary in their sensitivity to InsP3, and each store releases either its entire contents or nothing (all-or-none release); the second proposes instead that the stores are uniformly sensitive to the effects of InsP3, but that they can release only a fraction of their Ca2+ before their sensitivity is somehow attenuated (steady-state release). Experiments using purified InsP3 receptor molecules reconstituted into lipid vesicles have shown heterogeneity of the receptors in their response to InsP3 under conditions in which the total Ca2+ level at both sides of the receptor is held constant. We now report that in permeabilized A7r5 smooth-muscle cells incubated in Ca(2+)-free medium, the amount of 45Ca2+ remaining in the stores after the rapid transient phase of release is independent of their initial Ca2+ levels, indicating that partially depleted stores are less sensitive to InsP3. Moreover, if the stores are reloaded with 40Ca2+ after the first stimulus, reapplication of the same low concentration of InsP3 will release further 45Ca2+. This recovery of InsP3 sensitivity is almost complete. Under these conditions, Ca2+ release must thus occur by a steady-state mechanism, in which the decreasing Ca2+ content of the stores slows down further release. 相似文献