首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   609篇
  免费   51篇
林业   40篇
农学   6篇
基础科学   2篇
  90篇
综合类   36篇
农作物   22篇
水产渔业   38篇
畜牧兽医   343篇
园艺   6篇
植物保护   77篇
  2023年   6篇
  2022年   5篇
  2021年   24篇
  2020年   24篇
  2019年   20篇
  2018年   16篇
  2017年   18篇
  2016年   20篇
  2015年   26篇
  2014年   29篇
  2013年   34篇
  2012年   31篇
  2011年   36篇
  2010年   20篇
  2009年   36篇
  2008年   55篇
  2007年   35篇
  2006年   19篇
  2005年   17篇
  2004年   27篇
  2003年   32篇
  2002年   17篇
  2001年   17篇
  2000年   19篇
  1999年   3篇
  1997年   4篇
  1993年   2篇
  1992年   5篇
  1991年   4篇
  1990年   2篇
  1989年   3篇
  1987年   6篇
  1986年   5篇
  1985年   9篇
  1984年   3篇
  1983年   1篇
  1982年   2篇
  1980年   3篇
  1979年   1篇
  1978年   3篇
  1977年   3篇
  1976年   1篇
  1975年   2篇
  1974年   1篇
  1973年   1篇
  1972年   4篇
  1971年   1篇
  1970年   1篇
  1969年   3篇
  1968年   1篇
排序方式: 共有660条查询结果,搜索用时 31 毫秒
21.
22.
The aim of this retrospective study was to describe clinical features, treatment and outcome of 21 dogs with metastatic cancer of unknown primary (MCUP), a biopsy‐proven malignancy being diagnosed at a metastatic stage, in which the anatomical origin of the primary tumour cannot be detected. All dogs underwent total‐body computed tomography. Signalment, type and duration of clinical signs, metastasis site, pathology results, treatment and outcome were recorded. Carcinoma was the most common diagnosis (57.1%), followed by sarcoma, melanoma and mast cell tumour. The median number of disease sites per dog was 2, with bones, lymph nodes, lungs and spleen being the most frequent metastatic locations. The median survival for all dogs was 30 days. Overall, a primary site was not identified in 20 (95.2%) dogs. MCUP encompasses a variety of different pathologic entities and harbours a poor prognosis.  相似文献   
23.
PCR products obtained by selective amplification of transgenic DNA derived from food samples containing Roundup Ready soybean or Bt-176 maize have been analyzed by anion-exchange HPLC. Peptide nucleic acids (PNAs), oligonucleotide analogues known to bind to complementary single-stranded DNA with high affinity and specificity, have been used as specific probes in order to assess the identity of the peaks observed. Two different protocols were adopted in order to obtain single-stranded DNA: amplification with an excess of one primer or digestion of one DNA strand. The single-stranded DNA was mixed with the PNA probe, and the presence of a specific sequence was revealed through detection of the corresponding PNA:DNA peak with significantly different retention time. Advantages and limits of this approach are discussed. The method was tested with reference materials and subsequently applied to commercial samples.  相似文献   
24.
Feijoa sellowiana Berg. fruits and especially the acetonic extract have been shown to possess biological activities, although the responsible compounds have never been identified. The present study was designed to evaluate the anti-inflammatory activity of an acetonic extract from F. sellowiana Berg. fruits on the nitric oxide (NO) pathway, which plays an important role in inflammation. To this aim the J774 cell line, which expresses inducible nitric oxide synthase (iNOS) following stimulation with lipopolysaccharide (LPS), has been utilized, and the effects of this extract and its fractions on NO production, iNOS protein expression, and signal pathways involved in its regulation have been evaluated. This study demonstrates that at least some part of the anti-inflammatory activity of the acetonic extract is due to the suppression of NO production by flavone and stearic acid. The mechanism of this inhibition seems to be related to an action on the expression of the enzyme iNOS through the attenuation of nuclear factor kappaB (NF-kappaB) and/or mitogen-activated protein kinase (MAPK) activation.  相似文献   
25.
Contrast‐enhanced ultrasound with sulphur hexafluoride microbubbles was performed in seven healthy dogs without a history of reproductive pathology and with histologically confirmed normal testes and in 42 dogs with chronic scrotal anomalies. All dogs underwent orchiectomy and histological examination. Enhancement patterns and perfusion parameters (peak intensity and regional blood flow) of testes of healthy dogs and testes with chronic lesions were compared. Fourteen non‐pathologic and 60 pathologic testes were considered. Forty testes were neoplastic (24 interstitial cell tumours, 9 seminomas, 7 Sertoli cell tumours), 20 were non‐neoplastic (16 testicular degenerations, 2 chronic orchitis, 1 testicular atrophy, 1 interstitial cell hyperplasia). In healthy dogs, the contrast medium flow had a rapid homogeneous wash‐in and wash‐out, with a short peak phase. With contrast ultrasound, testes that were inhomogeneous with a hyperenhancing pattern were associated with neoplasia (sensitivity: 87.5%, specificity: 100%). Lesions with persistent inner vessels and a hypo‐to‐isoechoic background were significantly associated with seminomas (sensitivity: 77.8%, specificity: 100%). Testes with non‐neoplastic lesions were characterized by a scant/moderate homogeneous enhancement. Perfusion parameters were higher in neoplastic lesions. Contrast ultrasound was a feasible diagnostic tool in the assessment of testicular lesions, with hyperenhancement being an important feature in the diagnosis of malignancy.  相似文献   
26.

Context

The knowledge on cambial activity in water-limited environments, such as the Mediterranean, is still fragmentary. Dendrochronological studies have determined that spring precipitation plays an important part in determining tree-ring width and the properties of tracheids. However, the complex relation between cambial phenology and climate is still far from understood.

Aims

We studied the influence of climate, especially water stress, on maritime pine wood formation with the aim of determining the influence of drought on cambial activity.

Methods

A plantation of maritime pine (Pinus pinaster) was selected in the west coast of Portugal, to monitor cambial activity and wood formation using anatomical observations and band dendrometers. The trees were monitored weekly over 2 years (2010 and 2011).

Results

Xylem differentiation started earlier when warmer late winter temperatures were observed. Water stress triggered an earlier stop of wood formation and also the formation of tracheids with smaller lumen area. In both years a bimodal pattern of stem radial increment was registered by band dendrometers with two periods of increment: one in spring and another in autumn. The xylem anatomy study suggests that the autumnal increment period corresponded mostly to stem rehydration, since the differentiation of new xylem cells by the cambium was not observed.

Conclusion

Maritime pine cambial activity appears to be under a double climatic control: temperature influences cambial onset and water availability growth cessation.  相似文献   
27.
Little is known about the potentially disrupting effects of wind farms on the habitat connectivity of flying vertebrates at the landscape scale. We developed a regional-scale model to assess the wind farm impact on bat migration and commuting routes. The model was implemented for the bat Nyctalus leisleri in a region of central Italy currently undergoing considerable wind farm development. A Species Distribution Model (SDM) for N. leisleri was generated using the MaxEnt algorithm based on 47 presence records (reduced to 19 after the autocorrelation procedure) and 10 environmental variables derived from topographic and land cover maps. We used the SDM to create a map of connectivity using the software UNICOR to identify potential commuting corridors (PCCs). The incidence of each wind farm on bat flight corridors was assessed by overlaying the existing (380) and planned (195) turbine locations onto the PCCs. The SDM was statistically robust (AUC > 0.8). Most of the corridors were concentrated in the western part of the region, which hosts the largest suitable areas for the species; most of the existing (54 %) and planned (72 %) wind farms interfered with important corridors connecting the western and the eastern parts of the region. Our results provide key information on the impact of the wind farm industry on biodiversity on a regional scale. The novel approach adopted, based on SDM and connectivity analysis, could be easily extended to other flying vertebrates and landscapes and constitutes a promising planning tool necessary for harmonizing the development of renewable energy infrastructures with issues of biodiversity conservation.  相似文献   
28.
This study was designed to determine if the activity of paraoxonase (PON1), an antioxidant enzyme that works as a negative acute phase reactant, is a better predictor for the clinical recovery of leishmaniotic dogs receiving standard treatments compared with inflammatory markers such as C reactive protein (CRP) and electrophoretic fractions. For this purpose we tested 20 healthy dogs (controls) and 39 leishmaniotic dogs classified as sick (group A, n = 23) or severely sick (group B, n = 16) and tested at admission and after 3, 7, 14, 21, 28, 35 and 42 days.At admission, CRP and electrophoresis were altered in both groups, while PON1 activity was abnormal only in group B. There were no differences related to the outcome (mortality, complications or time of recovery). PON1 activity normalized in about 2 weeks in dogs that had abnormal values at admission and a final positive outcome; CRP normalized in 4–6 weeks and electrophoretic fractions were still altered after 6 weeks. The results show that, at admission, inflammatory markers did not predict the outcome of leishmaniasis. PON1 activity decreased only in some dogs with systemic inflammation but not in those with mild leishmaniasis: when decreased, PON1 normalized earlier than other markers in dogs that responded to treatment. This finding most likely depends on the rapid decrease in oxidative phenomena. PON1 activity should therefore be tested on admission: if low values are recorded, severe inflammation may be suspected and PON1 measurement may be repeated during treatment to early identify responsive dogs.  相似文献   
29.
Plant Foods for Human Nutrition - Silver linden (Tilia tomentosa Moench, TtM) flowers possess several health-promoting properties, especially at the neurological level, such as intestinal...  相似文献   
30.
Background: In routine canine medicine, anticoagulated blood is often the only sample sent to laboratories for diagnostic purposes. This hampers the interpretation of protein electrophoretic tracings because plasma contains fibrinogen, which migrates in the β–γ region. In human medicine, fibrinogen can be precipitated from plasma using ethanol. Objectives: The purpose of this study was to assess ethanol precipitation as a method for removing fibrinogen from canine plasma so as to facilitate the interpretation of electorphoresis results. Methods: Blood samples collected from 40 dogs were divided into plain tubes and tubes containing EDTA (n=20) or lithium–heparin (n=20). An aliquot of plasma from each sample was incubated with ethanol at a final concentration of 100 mL/L. Cellulose acetate electrophoresis was then performed on serum, plasma, and plasma treated with ethanol. To verify the efficiency of ethanol treatment, fibrinogen was added to 5 canine serum samples at final concentrations of 2.5, 5.0, and 10.0 g/L, and electrophoresis was performed before and after ethanol treatment. Results: Visual analysis of electrophoretograms from ethanol‐treated samples confirmed the disappearance of the fibrinogen peak from the β2‐globulin region. Treatment with ethanol caused a significant decrease in the percentage of β2‐globulins and a significant increase in the percentage of α2‐globulins. Absolute values of most electrophoretic fractions were significantly decreased in ethanol‐treated plasma compared with serum. Conclusions: Ethanol treatment successfully removed fibrinogen from canine plasma and normalized electrophoretic profiles, but probably also precipitated proteins other than fibrinogen. Ethanol treatment is recommended to facilitate visual identification of abnormal monoclonal peaks, but not for determining absolute protein concentrations in electrophoretic fractions.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号