全文获取类型
收费全文 | 253699篇 |
免费 | 15255篇 |
国内免费 | 651篇 |
专业分类
林业 | 23400篇 |
农学 | 13983篇 |
基础科学 | 2946篇 |
42635篇 | |
综合类 | 25315篇 |
农作物 | 17182篇 |
水产渔业 | 16700篇 |
畜牧兽医 | 97298篇 |
园艺 | 6304篇 |
植物保护 | 23842篇 |
出版年
2021年 | 2478篇 |
2020年 | 2859篇 |
2019年 | 3657篇 |
2018年 | 3942篇 |
2017年 | 4338篇 |
2016年 | 4773篇 |
2015年 | 4293篇 |
2014年 | 5656篇 |
2013年 | 16223篇 |
2012年 | 6271篇 |
2011年 | 8185篇 |
2010年 | 7262篇 |
2009年 | 7870篇 |
2008年 | 7558篇 |
2007年 | 6614篇 |
2006年 | 7271篇 |
2005年 | 6490篇 |
2004年 | 6329篇 |
2003年 | 6127篇 |
2002年 | 5454篇 |
2001年 | 6137篇 |
2000年 | 5734篇 |
1999年 | 5402篇 |
1998年 | 4143篇 |
1997年 | 4190篇 |
1996年 | 3916篇 |
1995年 | 4507篇 |
1994年 | 3888篇 |
1993年 | 3630篇 |
1992年 | 4347篇 |
1991年 | 4408篇 |
1990年 | 4142篇 |
1989年 | 4190篇 |
1988年 | 3658篇 |
1987年 | 3757篇 |
1986年 | 3717篇 |
1985年 | 3994篇 |
1984年 | 3668篇 |
1983年 | 3480篇 |
1982年 | 2884篇 |
1981年 | 2812篇 |
1980年 | 2768篇 |
1979年 | 3096篇 |
1978年 | 2818篇 |
1977年 | 2632篇 |
1976年 | 2460篇 |
1975年 | 2307篇 |
1974年 | 2492篇 |
1973年 | 2470篇 |
1972年 | 2205篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
182.
The Chilean Intermediate Depression to the north of Santiago has experienced a physiognomical transformation from a Prosopis chilensis woodland to an Acacia caven savanna. Today P. chilensis trees are scarce and belong mostly to the larger size classes. By contrast A. caven seems to reproduce frequently and its populations consist of individuals of all size classes. In this paper we document these changes and report the results of tests aimed at determining the causes of these physiognomical changes. We found that livestock, leporids, introduced Mediterranean forbs and agriculture account for differences in seed dispersal and survival of A. caven and P. chilensis, which can explain the documented changes in the Chilean landscape. 相似文献
183.
M L van der Leek J B Dame C L Adams K D Gillis R C Littell 《American journal of veterinary research》1992,53(6):877-882
Experimental and field trials were conducted to evaluate an ELISA for its ability to detect Trichinella-infected domestic swine and to compare ELISA results with muscle-digestion test results. The ELISA used was a commercial double-antibody kit, containing an excretory-secretory antigen, and was evaluated principally for epidemiologic use. Experimentally induced infection in swine (4 groups of 3 pigs each; inoculated with 0, 50, 500 or 5,000 larvae) was detected as early as postinoculation week 4, with seroconversion of all inoculated swine by postinoculation week 8. The rate of seroconversion appeared to be affected by initial larval dose, time after inoculation, and immunocompetence of the individual host. Determination of antibody kinetics generally revealed rapidly increasing antibody titer, followed by its steady decrease in most pigs. Once seropositive, however, all pigs remained seropositive for the duration of the 10-week study. Presence of muscle larvae was confirmed in all infected pigs at termination of the study. We recognize that the experimental conditions may not be truly representative of those under which natural infection develops in pigs; however, the ELISA detected an infected pig with muscle larval density of 0.87 larvae/g of tissue. Results of a field trial (n = 310) indicated no muscle digestion test-positive pigs (35 g of diaphragm muscle digested/pig), but 3 samples tested positive by ELISA for specificity of 99.0%. 相似文献
184.
E. DUVEILLER 《EPPO Bulletin》1994,24(1):135-157
The literature related to bacterial leaf streak of cereals, due to various pathovars of Xanthomonas campestris , is confusing and dispersed. This review presents a synthesis of the knowledge that has been accumulated on the disease in cereals since the early reports on barley and wheat. The aim is to present the current status on geographical distribution of the disease, to explain possible confusions in symptoms, and to show the limited quantitative information available on losses and on host/pathogen interactions. Disease etiology and means of control are also reviewed. The objective is to define the present situation of black chaff in wheat, barley and triticale, and to delineate the major research needs in these crops. Difficulties in controlling bacterial leaf streak arise as a result of limited epidemiological knowledge and of the absence of satisfactory seed treatment permitting the eradication of the bacterium. There is an urgent need for a seed certification scheme and for more information on genetic resistance, with a view to limiting outbreaks of this sporadic but devastating disease. 相似文献
185.
利用自制的应力-应变试验台,对草莓果实的应力-应变特性和蠕变特性进行了测定与研究,建立了草莓果实应力-应变的数学模型,指出了草莓果实的极限屈服点变动范围。同时对草莓在不同贮藏条件下流变特性的变化规律进行了初步的研究,发现草莓的成熟度、贮藏温度及贮藏时间对果实的硬度、屈服应力均有较显著的影响,建立了屈服应力随时间变化的不同阶段的回归方程,可用于预测在相应贮藏条件下的草莓果实的屈服应力。 相似文献
186.
This paper reviews the radiological features of 50 canine incisive or maxillary tumours and discusses the value of radiological assessment in the diagnosis and staging of these tumours. The 50 tumours examined included 21 sarcomas, 15 carcinomas, three melanomas and an assortment of benign tumours of the oral cavity. There was not any site specificity for the different histological tumour types within the upper dental arcade, although fibrosarcomas had a tendency to be maxillary whereas the squamous cell carcinomas were equally distributed between the incisive and maxillary regions. Seventy-eight per cent of fibrosarcomas, 82 per cent of squamous cell carcinoma and all three melanomas examined showed radiological evidence of bone involvement. Radiographic changes were also seen in the benign tumours. The pattern of growth of tumours correlated with the radiological changes observed. Malignant tumours showed a tendency to irregular or aggressive bone loss whereas bone production predominated in the benign tumours. 相似文献
187.
An account is given of the methodology for fractionation of cock spermatozoa into head and tail fractions by ultrasonication, followed by sucrose density gradient centrifugation. Quantitative estimates of DNA attested to 89.4% purity of the head fraction and low contamination of tails with heads. Recovery of protein and malic dehydrogenase (MDH) activity, following sperm fractionation, averaged 94.3% and 95.7%, respectively. Contamination of the head fraction with tails, as assessed by MDH assay, was only 4.65%, and the purity of the tail fraction was 91%. Intensive succinic dehydrogenase (SDH) activity was histochemically localised in the separated tail fraction and in the tail portion of intact spermatozoa. However, SDH activity was discernible neither in the head fraction nor in the head of intact spermatozoa. 相似文献
188.
The relationship between the levels of CO and airborne Pb was evaluated against conditions of traffic flow, ambient air temperature and the ventilation rates in 3 underground car parks. During the study period significant variations were noted in the levels of the pollutants in each of the car parks which frequently exceeded the proposed Malaysian Air Quality Standard of 0.7 μg m?3Pb and 9 μg L?1 (8 hr average) CO for indoor environs. High correlation coefficient values were obtained between the levels of CO and traffic flow and Pb and traffic flow. High traffic flow also increased the mean air temperature inside the car parks. Reductions in the ventilation rates in the car parks caused an almost two-fold increase in the level of CO and vice versa. A simple mathematical model depicting the CO concentrations in relation to other parameters was formulated. 相似文献
189.
Lumbosacral CSF pressure was measured in 6 horses via a catheter inserted through the lumbosacral space. Heart rate, facial artery pressure, central venous pressure, and CSF pressure were measured before IV injection of a saline solution control, for 15 minutes after saline solution injection, and for 60 minutes after the IV injection of 1.1 mg of xylazine/kg of body weight. Arterial pH and blood gases were analyzed before saline solution injection, 15 minutes after saline solution injection, and at 15, 30, and 60 minutes after xylazine injection. Constant craniocervical posture was maintained during sedation. Lumbosacral CSF pressure was significantly decreased for 15 minutes after xylazine injection. Diastolic arterial pressure was significantly increased 4 minutes after xylazine administration and diastolic and mean arterial pressure were increased at 6 and 8 minutes after xylazine administration. Small increases in systolic arterial blood pressure and central venous pressure, and a small decrease in heart rate were observed. There were no significant differences in the arterial blood gas values. It was concluded that IV injection of xylazine causes a decrease in intracranial pressure in healthy conscious horses. The effects may be different in horses with neurologic disease or cerebral trauma. 相似文献
190.