全文获取类型
收费全文 | 725篇 |
免费 | 47篇 |
专业分类
林业 | 22篇 |
农学 | 18篇 |
基础科学 | 2篇 |
51篇 | |
综合类 | 164篇 |
农作物 | 19篇 |
水产渔业 | 38篇 |
畜牧兽医 | 434篇 |
园艺 | 8篇 |
植物保护 | 16篇 |
出版年
2021年 | 8篇 |
2020年 | 8篇 |
2019年 | 13篇 |
2018年 | 10篇 |
2017年 | 10篇 |
2016年 | 14篇 |
2015年 | 9篇 |
2014年 | 20篇 |
2013年 | 33篇 |
2012年 | 16篇 |
2011年 | 27篇 |
2010年 | 15篇 |
2009年 | 18篇 |
2008年 | 15篇 |
2007年 | 19篇 |
2006年 | 14篇 |
2005年 | 18篇 |
2004年 | 28篇 |
2003年 | 13篇 |
2002年 | 16篇 |
2001年 | 10篇 |
2000年 | 11篇 |
1999年 | 16篇 |
1998年 | 11篇 |
1996年 | 11篇 |
1995年 | 11篇 |
1993年 | 11篇 |
1992年 | 16篇 |
1991年 | 22篇 |
1990年 | 16篇 |
1989年 | 24篇 |
1988年 | 13篇 |
1987年 | 13篇 |
1986年 | 25篇 |
1985年 | 19篇 |
1984年 | 24篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1980年 | 7篇 |
1979年 | 18篇 |
1978年 | 6篇 |
1977年 | 13篇 |
1975年 | 6篇 |
1974年 | 10篇 |
1973年 | 13篇 |
1972年 | 8篇 |
1971年 | 12篇 |
1969年 | 8篇 |
1935年 | 5篇 |
1928年 | 5篇 |
排序方式: 共有772条查询结果,搜索用时 15 毫秒
41.
42.
Harvey EN 《Science (New York, N.Y.)》1916,44(1128):208-209
43.
The Marketplace Fairness Act (S. 743) recently passed by the U.S. Senate may portend a national move toward states imposing sales taxes for business‐to‐consumer e‐retail purchases. While much of the policy debate surrounding this question has focused on trade creation versus diversion, there are likely distinct compositional effects at the state level, which will affect both economic activity and tax revenue. Consumers are clearly hurt by an online sales tax. However, such a policy would seemingly benefit state tax coffers, as well as traditional brick‐and‐mortar retailers and their employees. This paper uses a computable general equilibrium model approach to get a better understanding of the state‐level income, employment, and tax revenue effects of such a policy shift, in particular the likely tradeoffs between these three traditional economic targets across reasonable ranges of price elasticities. 相似文献
44.
Batches of eggs (1 batch/female) from 17 rainbow trout in their first spawning season were stripped and reared separately, and the percentage which hatched and the percentage which reached the stage of first feeding were determined. The fertilised egg batches were analysed for egg wet weight, egg dry weight, chorion weight and levels in the eggs of free, bound and total lipid, precipitable protein, protein phosphorus, lipid phosphorus, calcium and iron. All determinations were made on single eggs, and six eggs from each batch were analysed for each variable. Results were expressed in absolute terms (as weight of component per egg) and as percentage of egg dry weight. Highly significant variations in all these aspects of egg composition were shown to occur between parent females. However, there was no significant correlation between the percentage of the original number of eggs which hatched and any one aspect of egg composition, except for a weak positive correlation (P<0.05) with the percentage of protein phosphorus in the eggs. Similarly, there was no significant correlation between the percentage of the original number of eggs which reached first feeding and any one aspect of egg composition except for weak positive correlations (P<0.05) with egg wet weight and with both the absolute level and the percentage of protein phosphorus in the egg. There was significant positive correlation (P<0.05) between the percentage of hatched eggs (alevins) surviving to first feeding and each of the following: egg wet weight, egg dry weight and absolute levels in the egg of bound lipid, precipitable protein and protein phosphorus. Egg batches with higher hatching percentage (>50%) differed significantly from those with zero hatching percentage in having (in absolute terms) higher egg weight, chorion weight, protein phosphorus (all P<0.001), egg dry weight, bound lipid and precipitable protein (all P<0.01) and (percentages) higher chorion weight (P<0.05) and protein phosphorus (P<0.001), and lower free and total lipid (both P<0.01) and iron (P<0.05). In a separate experiment to investigate the effects of allowing the eggs to be retained by the female within the abdominal cavity for increasing periods of time after ovulation, eggs were obtained from three females on three or four successive occasions 2–11 days apart. Although the above aspects of egg composition remained almost constant when the eggs were held in the female for up to 18 days after ovulation, the hatching percentage declined sharply within this period, in two females falling from over 90% to near zero. These results together indicate that the time of stripping of the eggs in relation to the date of ovulation is a much more significant determinant of egg quality than any of the chemical and physical aspects of egg composition which were investigated. 相似文献
45.
Prognostic factors for feline mammary tumors 总被引:1,自引:0,他引:1
E G MacEwen A A Hayes H J Harvey A K Patnaik S Mooney S Passe 《Journal of the American Veterinary Medical Association》1984,185(2):201-204
The case records of 100 cats with malignant mammary tumors were reviewed. All cats were staged clinically according to the staging system of the World Health Organization. The following information was obtained from the medical records: age at time of diagnosis, breed, tumor size, date of surgery, type of surgical procedure performed, histologic type of tumor, disease-free interval, survival time, and cause of death. Factors of no prognostic value were age (less than or equal to 10 years vs greater than 10 years) and breed. Tumor size was the most significant prognostic factor (P less than 0.0001). Cats with small tumors (1 cm3 to 8 cm3) had the best prognosis. The type of surgery, conservative vs radical, was significantly (P less than 0.01) related to disease-free interval, but was of no significance in prolonging survival time. 相似文献
46.
47.
Lake Kivu, an African rift lake, contains about 50 cubic kilometers of methane (at standard temperature and pressure) in its deep water. Data resulting from two recent expeditions to the lake and a reevaluation of earlier data suggest that most of the methane was formed by bacteria from abiogenetic carbon dioxide and hydrogen, rather than being of volcanic origin or having formed from decomposing organic matter. 相似文献
48.
Kendal E Harr Kathryn Allison Robert K Bonde David Murphy John W Harvey 《Journal of zoo and wildlife medicine》2008,39(2):180-187
Muscle injury is common in Florida manatees (Trichechus manatus latirostris). Plasma aspartate aminotransferase (AST) is frequently used to assess muscular damage in capture myopathy and traumatic injury. Therefore, accurate measurement of AST and alanine aminotransferase (ALT) is important in managed, free-ranging animals, as well as in those rehabilitating from injury. Activities of these enzymes, however, are usually not increased in manatees with either acute or chronic muscle damage, despite marked increases in plasma creatine kinase activity. It is hypothesized that this absence of response is due to apoenzymes in the blood not detected by commonly used veterinary assays. Addition of coenzyme pyridoxal-5-phosphate (P5P or vitamin B6) should, therefore, result in higher measured enzyme activities. The objective of this study was to determine the most accurate, precise, and diagnostically useful method for aminotransferase measurement in manatees that can be used in veterinary practices and diagnostic laboratories. Additionally, appropriate collection and storage techniques were assessed. The use of an optimized commercial wet chemical assay with 100 micromol P5P resulted in a positive bias of measured enzyme activities in a healthy population of animals. However, AST and ALT were still much lower than that typically observed in domestic animals and should not be used alone in the assessment of capture myopathy and muscular trauma. Additionally, the dry chemistry analyzer, typically used in clinics, reported significantly higher and less precise AST and ALT activities with poor correlation to those measured with wet chemical methods found in diagnostic laboratories. Therefore, these results cannot be clinically compared. Overall, the optimized wet chemical method was the most precise and diagnostically useful measurement of aminotransferase in samples. Additionally, there was a statistically significant difference between paired serum and plasma measurement, indicating that separate reference intervals should be established for serum and plasma. Finally, storage of these enzymes at -70 degrees C for 1 mo resulted in up to a 25% decrease in enzymatic activity in manatee plasma. 相似文献
49.
Effects of mineral nutrition on susceptibility to cavitation were examined in four hybrid poplar clones. Two drought-sensitive and two drought-resistant hybrid clones of black cottonwood (Populus trichocarpa Torr. & Gray) and eastern cottonwood (P. deltoides Bartr.) were grown at three concentrations of nitrogen (N) applied factorially with two concentrations of phosphorus (P) in a greenhouse, and subjected to varying degrees of drought stress before measurement of cavitation and of anatomical features that might affect cavitation. Mean vessel pit pore diameters were 0.132 micro m at low P, and 0.074 micro m at high P, but no other significant effects of mineral nutrition on vessel dimensions were observed. Vessel diameter and specific conductivity were greater in the drought-resistant clones than in the drought-susceptible clones. Drought-resistant clones did not reach such low water potentials as drought-sensitive clones during the cavitation induction experiments, suggesting better stomatal and cuticular control of water loss. Scanning electron microscope observations showed less damage to pit membranes, also suggesting greater membrane strength in drought-resistant clones than in drought-sensitive clones. High concentrations of N increased cavitation, whereas high concentrations of P decreased cavitation as measured by both hydraulic flow apparatus and dye perfusion techniques. For one test, cavitation was 48% at high N and low P, but only 28% at high N and high P. We consider that N fertilization may make poplars more susceptible to cavitation on dry sites, but P fertilization may reduce this effect. 相似文献
50.
Abstract: Research on the economic performance of small states has concentrated on the implications of small size and thin local markets. An interesting feature of many of the world's smallest states, of which the Pacific region has many, is that they face additional challenges than just small size. Many are remote from global markets for their products. In addition, a large number are also islands, many of which are also mountainous. Many of the smaller states are not just islands, but are also archipelagos. Hence in addition to small size, many of the world's small states also exhibit four other characteristics that may affect their economic performance: insularity, remoteness, being archipelagos and being highly mountainous entities. This paper examines the nature of the challenges posed by these four characteristics and seeks to produce empirical evidence of how difficult it has been to overcome these challenges. The paper draws on empirical evidence for 126 very small global states, dependent territories and highly autonomous regions. The paper then turns to the position of the Pacific small states and dependent territories. 相似文献