全文获取类型
收费全文 | 17750篇 |
免费 | 1288篇 |
国内免费 | 3篇 |
出版年
2021年 | 111篇 |
2020年 | 102篇 |
2019年 | 99篇 |
2018年 | 284篇 |
2017年 | 247篇 |
2016年 | 428篇 |
2015年 | 761篇 |
2014年 | 689篇 |
2013年 | 1022篇 |
2012年 | 1236篇 |
2011年 | 1199篇 |
2010年 | 707篇 |
2009年 | 545篇 |
2008年 | 1079篇 |
2007年 | 1071篇 |
2006年 | 1034篇 |
2005年 | 986篇 |
2004年 | 894篇 |
2003年 | 846篇 |
2002年 | 781篇 |
2001年 | 401篇 |
2000年 | 425篇 |
1999年 | 356篇 |
1998年 | 145篇 |
1997年 | 128篇 |
1996年 | 110篇 |
1995年 | 126篇 |
1994年 | 110篇 |
1993年 | 85篇 |
1992年 | 215篇 |
1991年 | 219篇 |
1990年 | 195篇 |
1989年 | 153篇 |
1988年 | 169篇 |
1987年 | 140篇 |
1986年 | 135篇 |
1985年 | 126篇 |
1984年 | 111篇 |
1983年 | 92篇 |
1982年 | 85篇 |
1981年 | 106篇 |
1980年 | 74篇 |
1979年 | 107篇 |
1978年 | 102篇 |
1977年 | 81篇 |
1976年 | 93篇 |
1975年 | 80篇 |
1974年 | 91篇 |
1973年 | 68篇 |
1971年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
Four main molecular forms of acetylcholinesterase (AChE) can be solubilized from newborn rat superior cervical ganglia (SCG), homogenized in the presence of a high-ionic-strength, detergent-containing medium. These forms, respectively referred to as 16, 10, 6.5, and 4 S, are characterized by their sedimentation coefficients. Their relative proportions in SCG are notably different in vivo during postnatal maturation, and in culture. The 16-S AChE appears to be mainly neuronal in origin, is maintained in culture independently of original presynaptic in vivo elements, and its cellular pool is not depleted in the presence of tetrodotoxin (TTX). 相似文献
32.
Administration of choline chloride i.p. to rats causes a dose-dependent increase in the brain concentration of the neurotransmitter, acetylcholine (ACh). This increase is maximal (22% after a 60-mg/kg dose) 40 minutes after injection. These observations suggest that precursor availability may influence brain ACh synthesis, just as brain tryptophan and tyrosine levels have previously been shown to control the synthesis of brain serotonin and catecholamines. 相似文献
33.
34.
35.
36.
37.
P Navarro-Rosinés C Roux A Bellemère 《Comptes rendus de l'Académie des sciences. Série III, Sciences de la vie》2001,324(5):443-451
An ultrastructural study of Weddellomyces epicallopisma (ascomata wall, asci, ascospores and vegetative hyphae), the first done on the family Dacampiaceae, confirms most of the observations made in light microscopy. Moreover it shows that ascospores are provided with an endospore (not visible in light microscope) and that the structure of the ascospore septum is more complex. The similarity of the wall structure between the ascospore and the hyphoid appendages, developed on the upper part of the ascoma, is emphasized. 相似文献
38.
39.
P. lanceolata andP. major were grown in culture solutions with nitrate or ammonium as the nitrogen source. Dry matter accumulation in the shoot was
faster with nitrate than with ammonium, whilst that of the roots was not affected by the nitrogen source. As a consequence,
the shoot-to-root ratio was lower with ammonium than with nitrate. InP. lanceolata, dry matter percentage of shoot and root tissue was lower with nitrate nutrition, suggesting better elongation growth than
with ammonium. However, in shoot tissue ofP. major the opposite was found. The rate of root respiration declined with time, and this was almost completely due to a declining
activity of the alternative path, which amounted to about 30–60% of total root respiration. Respiration via the cytochrome
path was for a part of time slightly increased by ammonium, whereas the activity of the alternative path was strongly enhanced.
The concentration of ethanol-soluble carbohydrates (SC) in the roots of both species was higher when nitrate was used, but
no difference in the concentration of starch was found. When the plants were transferred from one nitrogen source to the other,
many parameters, including the concentration of nitrate and chloride, and the shoot to root ratio, adjusted to the new situation
in both species.
Grassland Species Research Group, Publication no. 116. 相似文献
40.
Growth of 2-month-old nonnodulatedHippophaë rhamnoides seedlings supplied with combined N was compared with that of nodulated seedlings grown on zero N. Plant growth was significantly better with combined N than with N2 fixation and, although not statistically significant for individual harvests, tended to be highest in the presence of NH 4 + , a mixture of NH 4 + and NO 3 ? producing the highest yields. Growth was severely reduced when solely dependent on N2 fixation and, unlike the combined-N plants, shoot to root ratios had only slightly increased after an initial decrease. An apparently insufficient nodule mass (nodule weight ratio <5 per cent) during the greater part of the experimental period is suggested as the main cause of the growth reduction in N2-fixing plants. Thein vivo nitrate reductase activity (NRA) of NO 3 ? dependent plants was almost entirely located in the roots. However, when grown with a combination of NO 3 ? and NH 4 + , root NRA was decreased by approximately 85 per cent.H. rhamnoides demonstrated in the mixed supply a strong preference for uptake of N as NH 4 + , NO 3 ? contributing only for approximately 20 per cent to the total N assimilation. Specific rates of N acquisition and ion uptake were generally highest in NO 3 ? +NH 4 + plants. The generation of organic anions per unit total plant dry weight was approximately 40 per cent less in the NH 4 + plants than in the NO 3 ? plants. Measured extrusions of H+ or OH? (HCO 3 ? ) were generally in good agreement with calculated values on the basis of plant composition, and the acidity generated with N2 fixation amounted to 0.45–0.55 meq H+. (mmol Norg)?1. Without acidity control and in the presence of NH 4 + , specific rates of ion uptake and carboxylate generation were strongly depressed and growth was reduced by 30–35 per cent. Growth of nonnodulatedH. rhamnoides plants ceased at the lower pH limit of 3.1–3.2 and deterioration set in; in the case of N2-fixing plants the nutrient solution pH stabilized at a value of 3.8–3.9 without any apparent adverse effects upon plant performance. The chemical composition of experimental and field-growing plants is being compared and some comments are made on the nitrogen supply characteristics of their natural sites. 相似文献