全文获取类型
收费全文 | 48972篇 |
免费 | 2688篇 |
国内免费 | 33篇 |
专业分类
林业 | 1701篇 |
农学 | 1661篇 |
基础科学 | 251篇 |
5835篇 | |
综合类 | 8196篇 |
农作物 | 1545篇 |
水产渔业 | 2717篇 |
畜牧兽医 | 26328篇 |
园艺 | 574篇 |
植物保护 | 2885篇 |
出版年
2019年 | 416篇 |
2018年 | 777篇 |
2017年 | 823篇 |
2016年 | 743篇 |
2015年 | 607篇 |
2014年 | 750篇 |
2013年 | 1736篇 |
2012年 | 1347篇 |
2011年 | 1690篇 |
2010年 | 1154篇 |
2009年 | 1096篇 |
2008年 | 1682篇 |
2007年 | 1593篇 |
2006年 | 1497篇 |
2005年 | 1380篇 |
2004年 | 1394篇 |
2003年 | 1371篇 |
2002年 | 1184篇 |
2001年 | 1913篇 |
2000年 | 1984篇 |
1999年 | 1498篇 |
1998年 | 569篇 |
1997年 | 499篇 |
1996年 | 449篇 |
1995年 | 612篇 |
1994年 | 524篇 |
1993年 | 459篇 |
1992年 | 1151篇 |
1991年 | 1211篇 |
1990年 | 1178篇 |
1989年 | 1097篇 |
1988年 | 1056篇 |
1987年 | 1081篇 |
1986年 | 1034篇 |
1985年 | 966篇 |
1984年 | 803篇 |
1983年 | 683篇 |
1982年 | 421篇 |
1981年 | 382篇 |
1979年 | 635篇 |
1978年 | 500篇 |
1977年 | 441篇 |
1976年 | 408篇 |
1975年 | 451篇 |
1974年 | 523篇 |
1973年 | 507篇 |
1972年 | 531篇 |
1971年 | 449篇 |
1970年 | 424篇 |
1969年 | 464篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
891.
T.A. Coffelt F.S. Nakayama D.T. Ray K. Cornish C.M. McMahan 《Industrial Crops and Products》2009,29(2-3):326-335
Guayule is a new crop being commercialized for hypoallergenic latex production. Because natural processes that occur in the plant following harvest, notably dehydration, result in rapid loss of latex and immediate processing of guayule shrub for latex on a commercial scale is not feasible, storage conditions that maintain latex concentration and yield need to be established. The objective of this study was to determine the effects of different storage conditions on the extractable latex, total rubber, resin, and guayulin A and B contents, and extractable latex, total rubber, and resin yields in harvested guayule shrub. The experiment was established using plants transplanted into the field at the University of Arizona Maricopa Agricultural Center, Maricopa, AZ, USA, on 22 March 2001. A randomized complete block design with four replications was used. Two germplasm lines (11591 and AZ-2) were used for this experiment. Twenty plants of each line were harvested six times (November 2002, March 2003, July 2003, November 2003, March 2004, and July 2004) from each field plot. Two plants of each line were randomly assigned to each of 10 storage treatment combinations reflecting wet, dry, or wet alternated with dry conditions prior to chipping for latex extraction. Extractable latex content, total rubber content, resin content, and guayulin A and B contents were determined after storage and compared with freshly harvested shrub. Plant biomass, latex yield, rubber yield, and resin yield were also determined and compared with fresh harvested shrub. AZ-2 was significantly lower in latex, rubber, and guayulin A content than 11591, and significantly higher in biomass, latex yield, rubber yield, resin content, resin yield, and guayulin B content. The results from this study show that moist storage of harvested shrub prior to dry chipping allows a higher yield of latex. Storing the shrub under moist conditions may allow more flexible harvesting and processing schedules, by limiting post-harvest latex losses and increasing the time interval between harvesting and processing. 相似文献
892.
This study focuses on the determination of structure change of surface area, morphology, crystallinity and pyrolysis characteristics of cellulosic feedstock treated by hot acid solution. The microscopic structure change of acid-treated samples was examined by using BET, SEM/EDX, XRD and pyrolysis–GC–MS under reasonable condition. Results showed that both surface area and pore volume of treated sample were conspicuous increased partial due to dissolution of silica and organic polar-components. Morphological change of plant cell wall surface was viewed as damage such as exfoliation, cave and fracture appeared on epidermis, vascular bundles. The crystallinity index of residual cellulose microcrystalline was widely enhanced because amorphous hemicellulose was removed off while crystalline was changeless. Under hot acid solution reaction, amorphous hemicellulose was hydrolyzed and leaving larger perfect cellulose thus increasing of crystalline size. Those available cellulose was obvious exposed after acid treatment and gave intensive characteristic peak on pyrogram analyzed by pyrolysis–GC–MS. 相似文献
893.
M. KOUTSOVITI-PAPADOPOULOU T. A. PSARRA &G. C. BATZIAS 《Journal of veterinary pharmacology and therapeutics》2009,32(2):177-181
This study demonstrates that the inotropic agent milrinone and the bronchodilator drug theophylline exert a relaxing effect on the rabbit lower oesophageal sphincter in vitro . The relaxing effect of milrinone and theophylline, which is concentration-dependent, involves a second messenger 3',5'-cyclic adenosine monophosphate pathway and most probably it is accomplished through inhibition of phosphodiesterase (PDE) type III, as according to the obtained results it is not significantly modified either by nicotinic acid, an inhibitor of adenylate cyclase, or by the inhibitor of nitric oxide-synthetase N ω -nitro- l -arginine methylester and the purinergic antagonist suramin; moreover, it persists under non-adrenergic non-cholinergic conditions and it is both hexamethonium- and tetrodotoxin-insensitive. Both milrinone and theophylline display equal efficacy, comparable to that of the calcium blocker verapamil and the non-selective PDE inhibitor papaverine, but milrinone appears 50 times more potent than theophylline and three times less potent than verapamil, as, according to the pIC50 values the potency rank of order is found to be verapamil (5.56) > milrinone (5.12) > theophylline (3.42). The here obtained pharmacodynamic profiles of the drugs suggest that both milrinone and theophylline may be considered as potent relaxing agents of the lower oesophageal sphincter. 相似文献
894.
L. R. THUESEN M. F. BERTELSEN L. BRIMER & M. T. SKAANILD 《Journal of veterinary pharmacology and therapeutics》2009,32(6):613-617
The third generation cephalosporin cefovecin has been shown to have an exceptionally long elimination half-life in dogs and cats, making it suitable for antibacterial treatment with a 14-day dosing interval in these species. Pharmacokinetic parameters for cefovecin were investigated in juvenile hens and green iguanas, following subcutaneous injections with 10 mg cefovecin/kg bodyweight. Preliminary studies in eight additional species of birds and reptiles were performed and results were compared with the parameters found in hens and green iguanas. The kinetics were characterized by rapid absorption with peak plasma concentration of 6 ± 2 μg/mL in hens and 35 ± 12 μg/mL in green iguanas. The mean plasma half-life for cefovecin was 0.9 ± 0.3 h for hens and 3.9 h in green iguanas. Volume of distribution was 1.6 ± 0.5 L/kg for hens and 0.3 L/kg for green iguanas and clearance was 1252 ± 185 mL·h/kg for hens and 53 mL·h/kg for green iguanas. Results from preliminary studies did not differ notably from those seen in hens and green iguanas. Cefovecin is not suitable for the treatment of bacterial infections with a 14-day dosing interval in hens or green iguanas and seems not to be in a number of other bird and retile species either. 相似文献
895.
A puppy was evaluated for rapid growth and large stature. Extensive diagnostic evaluation suggested a growth hormone independent disorder. As the sole detected abnormality was elevated leptin concentration, an obesity syndrome causing leptin resistance was speculated to explain the puppy's condition but was not confirmed. Except for large body size, the puppy remained clinically normal. 相似文献
896.
Zanoni RG Bugnon P Deranleau E Nguyen TM Brügger D 《Schweizer Archiv für Tierheilkunde》2010,152(12):561-568
Data of 13'469 blood samples from 10'999 dogs and 2'470 cats tested for rabies neutralizing antibodies within the framework of pet travel schemes were analysed for single and combined factors influencing antibody titres and failures. The time span between vaccination and drawing the blood sample was confirmed as a major source of failure in dogs with a proportion of 23 % at 4 months after primary vaccination (single dose). Failures in dogs and cats (titre < 0.5 IU) were significantly reduced after double primary vaccination (2 doses within 7 - 10 days), although failures reached comparable levels in dogs as early as 6 months after vaccination. In contrast, failure after vaccination was generally below 5 % in dogs and absent in cats after a booster applied at earliest 12 months after single primary vaccination. Statistically significant differences between the failures of the vaccine brands ?Rabisin? (1.5 %), ?Defensor? (6.7 %), ?Nobivac Rabies? (11.0 %) and ?Rabdomun? (18.2 %) were found in dogs but also between the titres induced in cats. Significant differences were found between different dog breeds with some small breeds showing a significantly higher responsiveness. Taken together, a new regimen for rabies vaccination consisting of double primary vaccination with a short interval of 7 - 10 days and a one-year booster appears to be highly recommended for dogs and cats. 相似文献
897.
Tyrosinase inhibition: its role in suntanning and in albinism 总被引:5,自引:0,他引:5
Tyrosinase inhibitor (molecular weight less than 5000; extracted from various melanomas) fully inhibits soluble tyrosinase but only partially inhibits tyrosinase "aggregated" into melanosomes; the inhibitor can be inactivated by ultraviolet light. S91 Albinotyrosinase Type B apparently cannot "aggregate" into melanosomes because its protein carrier is genetically altered. Therefore, albinotyrosinase remains vulnerable to its inhibitor and cannot produce melanin, even though the enzyme has a functioning active center. 相似文献
898.
Ciguatoxin: isolation and chemical nature 总被引:5,自引:0,他引:5
Ciguatoxin, the agent responsible for ciguatera, a disease produced in humans from ingestion of certain fishes, has been isolated from specimens of the moray eel, Gymnothorax javanicus. The toxin is apparently a lipid containing quaternary nitrogen, hydroxyl, and carbonyl functions. 相似文献
899.
Transport and phosphorylation as factors in the antitumor action of cytosine arabinoside 总被引:1,自引:0,他引:1
Survival of mice bearing different transplantable leukemias and treated with cytosine arabinoside was compared with uptake and subsequent phosphorylation of the drug in vitro. Capacity for nucleotide formation was correlated with response and is apparently an important determinant of drug sensitivity. Drug uptake, although apparently mediated, was similar in all cell lines. 相似文献
900.
Cognitive capacity of very young children 总被引:1,自引:0,他引:1
Children between 2 years, 6 months old and 3 years, 2 months old correctly discriminate the relative number of objects in two rows; between 3 years, 2 months and 4 years, 6 months they indicate a longer row with fewer objects to have "more"; after 4 years, 6 months they again discriminate correctly. The discriminative ability of the younger children shows that the logical capacity for cognitive operations exists earlier than previously acknowledged. 相似文献