全文获取类型
收费全文 | 180378篇 |
免费 | 12538篇 |
国内免费 | 484篇 |
出版年
2012年 | 21609篇 |
2011年 | 23494篇 |
2010年 | 3340篇 |
2009年 | 1577篇 |
2008年 | 17282篇 |
2007年 | 17605篇 |
2006年 | 16454篇 |
2005年 | 15201篇 |
2004年 | 14200篇 |
2003年 | 13250篇 |
2002年 | 11191篇 |
2001年 | 8827篇 |
2000年 | 11221篇 |
1999年 | 4368篇 |
1998年 | 576篇 |
1997年 | 352篇 |
1996年 | 247篇 |
1995年 | 278篇 |
1994年 | 231篇 |
1993年 | 240篇 |
1992年 | 228篇 |
1991年 | 200篇 |
1990年 | 196篇 |
1989年 | 194篇 |
1988年 | 186篇 |
1987年 | 165篇 |
1986年 | 155篇 |
1985年 | 134篇 |
1984年 | 104篇 |
1983年 | 147篇 |
1982年 | 108篇 |
1981年 | 84篇 |
1977年 | 63篇 |
1974年 | 78篇 |
1973年 | 70篇 |
1972年 | 105篇 |
1971年 | 96篇 |
1970年 | 72篇 |
1959年 | 430篇 |
1958年 | 912篇 |
1957年 | 1045篇 |
1956年 | 912篇 |
1955年 | 880篇 |
1954年 | 907篇 |
1953年 | 824篇 |
1952年 | 748篇 |
1951年 | 598篇 |
1950年 | 559篇 |
1949年 | 187篇 |
1948年 | 174篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Cutaneous leishmaniasis (CL) is one of the most important causes of chronic ulcerative skin lesions. The disease is endemic in many parts of the world, presenting a range of clinical forms - acute, chronic, recurrent and diffuse(1). Several species of Leishmania are involved, including L. major, L. tropica and L. aethiopica in the Old World, and several members of the L. braziliensis and L. mexicana complexes in the New World. Some forms of the disease produce only mild, self-limited lesions, while at the other extreme are the destructive mucocutaneous forms caused by L. braziliensis and L. panamensis(1-7). In all cases, chemotherapy tends to be difficult - often requiring prolonged parenteral administration of toxic drugs such as pentavalent antimonials or amphotericin B. Such drugs are also expensive and relatively inefficient in the sense that much of the active ingredient is excreted by the patient before reaching its target. Consequently, there is renewed interest in the development of active formulations suitable for topical application directly onto the lesions. 相似文献
112.
Beatty J 《Journal of the history of biology》1988,21(2):245-263
Conclusion: Scientists qua engineers Of all the scientists discussed by Mitman, Keller, and Taylor, Odum stands out most as the technocrat, the social engineer. But less obvious candidates, like Allee, also fancied themselves in this capacity: Our task as biologists and as citizens of a civilized country, is a practical engineering job. Allee had in mind the establishment of an international cooperative order based on his biological principles. He apparently did not recognize the extent to which his principles were themselves an engineering feat: he had already constructed a world in which eternal peace and order were possible.To an engineer in the traditional sense, the world is changeable, but not in all respects; there are constraints, and these constraints are taken very seriously. Scientists acting as engineers, in the traditional sense, must also pay attention to constraints. But scientists sometimes also take the option of engineering the very constraints, intellectually reconstructing the world so that it can (supposedly) be physically manipulated in the desired direction. There seems to be a lot of engineering, in the extended sense, going on in the very interesting stories that Mitman, Keller, and Taylor tell. 相似文献
113.
Metal Ion Interactions with Phosphoenolpyruvate Carboxylase from Crassula argentea and Zea mays 总被引:1,自引:1,他引:0
Metal ion interactions with phosphoenolpyruvate carboxylase from the CAM plant Crassula argentea and the C4 plant Zea mays were kinetically analyzed. Fe2+ and Cd2+ were found to be active metal cofactors along with the previously known active metals Mg2+, Mn2+, and Co2+. In studies with the Crassula enzyme, Mg2+ yielded the highest Vmax value but also generated the highest values of Km(metal) and Km(pep). For these five active metals lower Km(metal) values tended to be associated with lower Km(pep) values. PEP saturation curves showed more kinetic cooperativity than the corresponding metal saturation curves. The activating metal ions all have ionic radii in the range of 0.86 to 1.09 Å. Ca2+, Sr2+, Ba2+, and Ni2+ inhibited competitively with respect to Mg2+, whereas Be2+, Cu2+, Zn2+, and Pd2+ showed mixed-type inhibition. Vmax trends with the five active metals were similar for the C. argentea and Z. mays enzymes except that Cd2+ was less effective with the maize enzyme. Km(metal) values were 10- to 60-fold higher in the enzyme from Z. mays. 相似文献
114.
Herbicide Resistance in Datura innoxia: Cross-Resistance of Sulfonylurea-Resistant Cell Lines to Imidazolinones 总被引:3,自引:3,他引:0
Cells resistant to the sulfonylurea herbicides chlorsulfuron and sulfometuron methyl were isolated from a predominantly haploid cell suspension culture of Datura innoxia P. Mill. Exponentially growing cell colonies (aggregates of about 40 cells) were mutagenized with ethyl methane sulfonate, subcultured for 10 days to allow growth recovery and plated on a medium containing either chlorsulfuron or sulfometuron methyl at a concentration (10−8 molar) which killed wild type cells. Surviving clones were picked up after 3 to 4 weeks, further proliferated as callus or cell suspension cultures, and tested for their resistance to both the sulfonylureas and imidazolinones, a chemically different class of herbicides. The variants were stable and showed high (100- to 1000-fold) resistance to the sulfonylureas. While some also exhibited cross resistance to imidazolinones, others showed no cross-resistance at all or, as in one case, greater sensitivity than wild type cells to the imidazolinones. Both classes of herbicides tested inhibited acetolactate synthase activity isolated from wild type cells. The acetolactate synthase of the resistant variants, however, was found to be resistant to the sulfonylureas and also to the imidazolinone(s) in those cells showing cross-resistance to the latter. The lack of cross-resistance observed in some cases provides evidence that the two groups of herbicides have slightly different sites on the acetolactate synthase molecule. 相似文献
115.
Hydrogen-1 nuclear magnetic resonance spectroscopy was used to study water allocation in cell compartments of sun and shade leaves. NMR spectra of Acer platanoides were resolved into two peaks that were assigned to chloroplast and nonchloroplast water. Sun leaves contained 1.7 times more water per unit area of surface than shade leaves, and the water was allocated differently. Chloroplasts in sun leaves contained 17% of the total leaf water versus 47% in shade leaves. Comparing equal leaf surface areas, the chloroplasts in shade leaves contained 60% more water than those in sun leaves. 相似文献
116.
Analysis of [3H]-(fructosyl)-sucrose translocation in tomato (Lycopersicon esculentum Mill.) indicates that phloem unloading in the fruit occurs, at least in part, to the apoplast followed by extracellular hydrolysis. Apoplastic sucrose, glucose, and fructose concentrations were estimated as 1 to 7, 12 to 49, and 8 to 63 millimolar, respectively in the tomato fruit pericarp tissue. Hexose concentrations were at least four-fold greater than sucrose at all developmental stages. Short-term uptake of [14C]sucrose, -glucose, and -fructose in tomato pericarp disks showed first order kinetics over the physiologically relevant concentration range. The uptake rate of [14C]-(glucosyl)-1′-fluorosucrose was identical to the rate of [14C]sucrose uptake, suggesting sucrose may be taken up directly without prior extracellular hydrolysis. Short-term uptake of all three sugars was insensitive to 10 micromolar carbonyl cyanide m-chlorophenylhydrazone and to 10 micromolar p-chloromercuribenzene sulfonic acid. However, long-term accumulation of glucose was sensitive to carbonyl cyanide m-chlorophenylhydrazone. Together these results suggest that although sucrose is at least partially hydrolyzed in the apoplast, sucrose may enter the metabolic carbohydrate pool directly. In addition, sugar uptake across the plasma membrane does not appear to be energy dependent, suggesting that sugar accumulation in the tomato fruit is driven by subsequent intracellular metabolism and/or active uptake at the tonoplast. 相似文献
117.
Reduced Cytosolic Fructose-1,6-Bisphosphatase Activity Leads to Loss of O(2) Sensitivity in a Flaveria linearis Mutant 总被引:4,自引:4,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The mutant plant of Flaveria linearis characterized by Brown et al. (Plant Physiol. 81: 212-215) was studied to determine the cause of the reduced sensitivity to O2. Analysis of CO2 assimilation metabolites of freeze clamped leaves revealed that both 3-phosphoglycerate and ribulose 1,5-bisphosphate were high in the mutant plant relative to F. linearis with normal O2 sensitivity. The kcat of ribulose-1,5-bisphosphate carboxylase (RuBPCase) was equal in all plant material tested (range 18-22 s−1) indicating that no tight binding inhibitor was present. The degree of RuBPCase carbamylation was reduced in the mutant plant relative to the wild-type plant. Since 3-phosphoglycerate was high in the mutant plant and photosynthesis did not exhibit properties associated with RuBPCase limitations, we believe that the decarbamylation of RuBPCase was a consequence of another lesion in photosynthesis. Fructose 1,6-bisphosphate and its precursors, such as the triose phosphates, were in high concentration in the mutant plant relative to the wild type. The concentrations of the product of the fructose 1,6-bisphosphatase reaction, fructose 6-phosphate, and its isomer, glucose 6-phosphate, were the same in both plants. We found that the mutant plant had up to 75% less cytosolic fructose 1,6-bisphosphatase activity than the wild type but comparable levels of stromal fructose 1,6-bisphosphatase. We conclude that the reduced fructose-1,6-bisphosphatase activity restricts the mutant plant's capacity for sucrose synthesis and this leads to reduced or reversed O2 sensitivity. 相似文献
118.
DNA Methylation Occurred around Lowly Expressed Genes of Plastid DNA during Tomato Fruit Development 总被引:5,自引:1,他引:4
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We have analyzed DNA methylation of plastid DNA from fully ripened red fruits, green mature fruits, and green leaves of tomato (Lycopersicon esculentum var. Firstmore). Essentially identical restriction profiles were obtained between chromoplast and chloroplast DNAs by EcoRI digestion. BstNI/EcoRII and HpaII/MspI are pairs of isoschizomers that can discriminate between methylated and unmethylated DNAs. These endonucleases produced different restriction patterns of plastid DNAs from tomato fruits compared to tomato leaves. Moreover, we have found from Southern blots that methylation was not detected in DNA fragments containing certain genes that are actively expressed in chromoplasts, whereas DNA fragments bearing genes that are barely transcribed in chromoplasts are methylated. 相似文献
119.
Antitranspirant associated abscisic Acid effects on the water relations and yield of transplanted bell peppers 总被引:4,自引:0,他引:4
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Greenhouse and field experiments were performed to determine if increased leaf resistance induced by exogenous application of abscisic acid (ABA) could enhance the water status of transplanted bell pepper seedlings. Seedling survival and yield were also monitored in the field experiment. When seedlings were transplanted into either wet or dry potting mix in the greenhouse, ABA increased leaf resistance and leaf water potential. In the field, plots were irrigated either immediately after, or 1 day after transplanting. Under both treatments, ABA application resulted in increased leaf resistance and water potential, but seedling survival and yield were enhanced due to ABA only in plots which were irrigated 1 day after transplanting. It is concluded that antitranspirant application can reduce transplant shock and increase yield of bell pepper. 相似文献
120.
From a variety of undifferentiated plant cell suspensions, 2,4-dichlorophenoxyacetic acid-dependent cells of sunflower (Helianthus annuus L. Spanners Allzweck) produced large quantities of ethylene. The maximum rate was about 1 nanomole × gram fresh weight−1 × hour−1 during the exponential growth phase. The action of various compounds known to interfere with ethylene formation in plant tissue was studied in sunflower cell suspensions. The influence on ethylene, 1-aminocyclopropanecarboxylic acid (ACC), and N-malonyl-ACC (MACC) levels suggested that the final steps in ethylene synthesis resemble those of other plant systems. This makes sunflower cells suitable for analyzing the effects of biologically active compounds on cellular ethylene biosynthesis. In particular, plant growth retardants of the norbornenodiazetine and triazole type inhibited ethylene production of sunflower cells. On the other hand, the ACC level was considerably elevated while that of MACC did not change significantly. It is assumed that the conversion of ACC to ethylene catalyzed by the ethylene-forming enzyme was influenced. 相似文献