全文获取类型
收费全文 | 732篇 |
免费 | 17篇 |
国内免费 | 15篇 |
出版年
2023年 | 9篇 |
2022年 | 3篇 |
2021年 | 5篇 |
2020年 | 10篇 |
2019年 | 34篇 |
2018年 | 26篇 |
2017年 | 27篇 |
2016年 | 16篇 |
2015年 | 18篇 |
2014年 | 17篇 |
2013年 | 93篇 |
2012年 | 12篇 |
2011年 | 19篇 |
2010年 | 23篇 |
2009年 | 33篇 |
2008年 | 32篇 |
2007年 | 47篇 |
2006年 | 30篇 |
2005年 | 28篇 |
2004年 | 17篇 |
2003年 | 19篇 |
2002年 | 12篇 |
2001年 | 8篇 |
2000年 | 13篇 |
1999年 | 12篇 |
1998年 | 16篇 |
1997年 | 15篇 |
1996年 | 9篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1992年 | 8篇 |
1991年 | 5篇 |
1990年 | 10篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 13篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 13篇 |
1980年 | 11篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有764条查询结果,搜索用时 140 毫秒
1.
2.
Abstract Ultrastructure of the apical cell of Halopteris scoparia (L.) Sauvageau (Phaeophyceae, Sphacelariales). - The ultrastructure of resting apical cells of Halopteris scoparia (L.) Sauvageau from material collected in December is described. The cytoplasm is higly vacuolated with lipids, poliphenolic substances and polisaccharides occurring inside the vacuoles (the classic « physodes »). Two cell organelles are prominently active at this stage: conspicuosly hypertrophic dictyosomes and the budding endoplasmic reticulum. Both light and electron microscope observations show that the cell wall has an outer stratification and inner discontinuous thickenings, the constituent material of which is uniformerly dispersed. The above observations point out that the apical cell of Halopteris scoparia at this stage of its life cycle is engaged in an elaboration of vacuolar and parietal substances which will be therefore readly available at the outset of the growing season. 相似文献
3.
Distinct effects of climate warming on populations of silver fir (Abies alba) across Europe 下载免费PDF全文
4.
W. Rossi B. Corrias P. Arduino R. Cianchi L. Bullini 《Plant Systematics and Evolution》1992,179(1-2):43-58
Data are presented on genetic variation at 27 enzyme loci of the Green-Winged orchid,Orchis morio, in 18 population samples from Italy. The existence in Italy of two subspecies, i.e. subspp.morio andpicta, is not supported by allozyme data. No genetic heterogeneity was found betweenmorio-like andpicta-like samples and specimens. Moreover, morphological transition between the two forms was observed in different Italian populations. The parameters of genetic variability estimated forO. morio populations are consistent with those found among monocotyledon plants, and among those outcrossing, animal-pollinated and with wind-dispersed seeds. Genetic diversity of ItalianO. morio is mostly within populations. Correspondingly, low values of interpopulational genetic distance were found. This appears to be due to high levels of gene flow, which were estimated with different methods. The lack ofO. longicornu from Italian samples, as well as of any hybrid withO. morio (F1, backcrossed or recombinant individuals) is demonstrated on the basis of genetic data. It is concluded that recurrent reports ofO. longicornu from Italy are due to confusion withO. morio or with otherOrchis species. 相似文献
5.
The Castel di Guido site, with an estimated age of approximately 300,000 years, has yielded abundant animal remains, Acheulian stone and bone bifaces, and small tools. On the surface of the original deposit, turned over by agricultural activities, fragments of human remains were discovered between 1980 and 1986, including a temporal bone described here. 相似文献
6.
The heterocyclic moiety of 17 beta-(2-aminooxazol-4-yl) steroids is sensitive to the oxidizing action of hydrogen peroxide and yields products mainly from the opening of the amino-oxazole ring. Unlike simple 2-aminooxazoles, it does not rearrange to 2-imidazolone and the expected steroidal hydroperoxyimidazolidinones were not detected. Among the substances we isolated, N-(aminocarbonyl)-17 alpha-hydroxy-17-carboxamides (2a) and (3a) undergo spontaneous cyclization, in the reaction conditions, giving steroid-17-spirooxazolidinediones (2d) and (3d). Spirane (2d) was synthesized in high yields from (2a) in strongly alkaline medium. 相似文献
7.
A careful analysis by 1H and 13C FT-NMR on the Cu(II) (L-histidine)2 complex was carried out which allows delineation of structure and dynamics in solution. A mixture of complexes was shown such that 24% of the Cu(II) (L-histidine)2 complex contains both histidines bound in the histaminelike way, while the remaining 76% contains one L-His molecule bound in the histaminelike way and the other L-His molecule bound in the glycinelike way. The motional correlation time and relevant features of the exchange process were also delineated. 相似文献
8.
Dithionite causes the depletion of dioxygen from suspensions of erythrocytes by reduction of the external dioxygen and not by diffusion into the cell. The molar enthalpy for the reduction shows a small difference with respect to the values found for free hemoglobin; and the normal stoichiometry of 2 moles dithionite/mole dioxygen found there is not observed with erythrocytes. At low hematocrit, the stoichiometry is 2.6:1 and decreases to 1.5:1 at high hematocrit. The change is not due to differences in the hemoglobin saturation or to an inability of dithionite to reduce all dioxygen present at the higher hematocrit. Neither catalase nor peroxidase added to the extracellular volume significantly alters the stoichiometry or the enthalpy of dioxygen reduction by dithionite. Addition of superoxide dismutase, however, restores the normal stoichiometry at high hematocrit and further increases the stoichiometry at low hematocrit. The calorimetrical signal of hydrogen peroxide, clearly seen with free dioxygen, is not present with erythrocytes. In all these cases the total heat evolved is the same. 相似文献
9.
Multiple discriminant analysis is a useful multivariate technique in vegetation studies that can be employed for several purposes, even if the underlying statistical assumptions are not satisfied. An example of application of this method is discussed: multiple discriminant analysis was successfully used for evaluating the predictivity of forest types defined by numerical classification of vegetation data with respect to soil variables.Species nomenclature follows Pignatti (1982) for vascular plants and Augier (1966) for mosses.The research has been supported by the IDROSER (Bologna, Italy) and by the Italian C. N. R. (Grant no. 83.02548.04, resp. Prof. A. Pirola). We wish to thank Prof. E. Feoli, Prof. E. van der Maarel, Prof. L. Orlóci and Prof. A. Pirola for suggestions. We are also indebted to Dr N. Filippi who analyzed the soil samples. 相似文献
10.
Using [3H] diazepam as ligand, it is possible to distinguish neuronal binding sites from those present on glial elements and in peripheral tissues (non-neuronal). The function of the "non-neuronal" binding sites is still obscure. Preliminary data showed a distribution of [3H] diazepam binding sites in kidney that could suggest a localization along the renal tubules. This is the site at which a renal peptide, arginine-vasopressin (AVP) is supposed to act. In an attempt to examine the function of these "non-neuronal" sites, we studied the [3H] diazepam binding in kidney of Brattleboro rats which lack AVP and present the symptoms of diabetes insipidus. The homozygous Brattleboro rats showed an increase in the apparent number of benzodiazepine binding sites (Bmax) compared to Long-Evans control rats. Replacement of AVP in these animals results in a reversal of the electrolyte alterations of diabetes insipidus and in an increase of the affinity of the [3H] diazepam binding. These findings may indicate a possible relationship between benzodiazepine binding sites and vasopressin action in kidney and may support receptor function of these "non-neuronal" binding sites. 相似文献