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   589篇
  免费   30篇
林业   15篇
农学   5篇
  60篇
综合类   91篇
农作物   56篇
水产渔业   18篇
畜牧兽医   323篇
园艺   18篇
植物保护   33篇
  2020年   4篇
  2019年   13篇
  2018年   10篇
  2016年   9篇
  2015年   11篇
  2014年   10篇
  2013年   33篇
  2012年   25篇
  2011年   24篇
  2010年   19篇
  2009年   21篇
  2008年   22篇
  2007年   21篇
  2006年   15篇
  2005年   25篇
  2004年   12篇
  2003年   13篇
  2002年   15篇
  2001年   11篇
  2000年   19篇
  1999年   14篇
  1998年   12篇
  1997年   7篇
  1996年   4篇
  1995年   8篇
  1993年   9篇
  1992年   7篇
  1991年   11篇
  1990年   19篇
  1989年   8篇
  1988年   10篇
  1987年   9篇
  1985年   10篇
  1984年   12篇
  1983年   10篇
  1982年   4篇
  1981年   6篇
  1980年   10篇
  1979年   12篇
  1978年   7篇
  1977年   5篇
  1976年   5篇
  1975年   5篇
  1974年   6篇
  1973年   6篇
  1972年   9篇
  1971年   4篇
  1970年   6篇
  1967年   7篇
  1955年   4篇
排序方式: 共有619条查询结果,搜索用时 15 毫秒
591.
High-grade glioma is an aggressive cancer that occurs naturally in pet dogs. Canine high-grade glioma (cHGG) is treated with radiation, chemotherapy or surgery, but has no curative treatment. Within the past eight years, there have been advances in our imaging and histopathology standards as well as genetic charactereization of cHGG. However, there are only three cHGG cell lines publicly available, all of which were derived from astrocytoma and established using methods involving expansion of tumour cells in vitro on plastic dishes. In order to provide more clinically relevant cell lines for studying cHGG in vitro, the goal of this study was to establish cHGG patient-derived lines, whereby cancer cells are expanded in vivo by injecting cells into immunocompromized laboratory mice. The cells are then harvested from mice and used for in vitro studies. This method is the standard in the human field and has been shown to minimize the acquisition of genetic alterations and gene expression changes from the original tumour. Through a multi-institutional collaboration, we describe our methods for establishing two novel cHGG patient-derived lines, Boo-HA and Mo-HO, from a high-grade astrocytoma and a high-grade oligodendroglioma, respectively. We compare our novel lines to G06-A, J3T-Bg, and SDT-3G (traditional cHGG cell lines) in terms of proliferation and sensitivity to radiation. We also perform whole genome sequencing and identify an NF1 truncating mutation in Mo-HO. We report the characterization and availability of these novel patient-derived lines for use by the veterinary community.  相似文献   
592.
593.
New Forests - Afforestation of pasture sites results in a net reduction in atmospheric CO2. Coast redwood (Sequoia sempervirens) is well suited to carbon forestry due to its rapid growth and...  相似文献   
594.
Elemental abundances, so far obtained, derived from the analysis of Apollo 11 lunar material are reported. Similarities and differences exist between lunar material, the eucritic achondrites, and the augite achondrite Angra dos Reis, the analysis of which is also reported.  相似文献   
595.
The loss of yield resulting from infestation with potato leafhoppers,Empoasca fabae (Harris), was measured in a group of potato selections (Solatium tuberosum subsp.tuberosum). For 6 years in which 300 different clones were tested, the average loss of yield was 39%. The correlation over clones between level of infestation and percent hopperburn equaled 0.56; between level of infestation and loss of yield 0.36; and between percent hopperburn and loss of yield 0.41. Each coefficient is significant at P=0.01. Significant differences were found in levels of infestation and in percent hopperburn among the selections, but even those selections with the most resistant foliage had greatly reduced yields when infested. No selections were found that consistently produced a normal yield when infested with leafhoppers. The low level of infestation on the more resistant clones, however, may make chemical control at lower rates more effective.  相似文献   
596.
This study was conducted (1) to evaluate the effects of photoperiod (fixed vs. decreasing light), fish size (136 vs. 220 mm), dissolved ions (hardness and salinity) and diet (menhaden oil vs. coconut oil‐based) on the tolerance (survival) of Nile tilapia, Oreochromis niloticus, to low temperatures (decreased by approximately 0.5 °C per day) and (2) to evaluate the effect of dietary fatty acid composition on selected physiological characteristics of Nile tilapia exposed to decreasing temperatures. Size significantly affected mortality, with smaller fish being less tolerant to low temperatures than larger fish. Results were equivocal in the photoperiod, dissolved ion and dietary lipid experiments, and were dependent on the method of data analysis employed. Diet significantly affected plasma osmolarity, with higher values in fish fed the menhaden‐based diet. Haematocrit, serum glucose, sodium and cortisol concentrations, serum and splenic lysozyme activities, lymphocyte count and hepatosomatic index were not affected by diet. Haematocrit, white cell count and serum glucose and sodium concentrations were significantly affected by temperature, but serum and splenic lysozyme content, hepatosomatic index, and serum cortisol concentrations were not. The results of this series of experiments indicate that altering the environment or diet has little effect on the ability of Nile tilapia to survive low temperatures.  相似文献   
597.
Fish are cultured in ponds, recirculating systems, raceways, and cages. Turbulence is associated with one or more of mechanisms to facilitate food accessibility, maintain adequate levels of oxygen, remove carbon dioxide, urinary and fecal wastes, as well as from locomotion of fishes themselves. Turbulence has been shown to have positive and negative effects on fish swimming, feeding, and energetics, usually with negative impacts at very low and at high levels, and least effects and sometimes positive effects at intermediate levels. Differences in responses of fishes with varying levels of turbulence are related to the size of eddies relative to the size of a fish (larvae, juveniles, and adults). Impacts on locomotor functions are associated with eddy diameters of the order of 0.5–1L, where L is the total length of a fish. Negative locomotor impacts of turbulence are associated with eddies challenging stability, while positive effects promote drafting and station holding with reduced locomotor motions. Deployment of control surfaces increases with the level of turbulence up to a threshold where control is overwhelmed. The design of culture facilities is expected to affect levels of turbulence and may be engineered to provide optimal levels facilitating high growth.  相似文献   
598.
Over the last two decades, there have been significant advances in the use of assisted reproductive technology for genetic and reproductive management of captive dolphin populations, including evaluation of sperm DNA quality. This study validated a customized sperm chromatin dispersion test (SCDt) for the bottlenose dolphin (Tursiops truncatus) as a means of assessing sperm DNA damage both in the field and in the laboratory. After performing the SCDt, two different sperm morphotypes were identified: (i) sperm with fragmented DNA showed large haloes of dispersed DNA fragments emerging from a compact sperm nucleoid core and (ii) sperm containing non‐fragmented DNA displayed small compact haloes surrounded by a dense core of non‐dispersed DNA and protein complex. Estimates of sperm DNA fragmentation by means of SCDt were directly comparable to results obtained following a two‐tailed comet assay and showed a significant degree of correlation (= 0.961; p < 0.001). This investigation also revealed that the SCDt, with minor modifications to the standard protocol, can be successfully conducted in the field using a LED florescence microscopy obtaining a high correlation (= 0.993; p = 0.01) between the data obtained in the laboratory and in the field.  相似文献   
599.
600.
Seed of the Potentate variety of tomato was sown at weekly intervals for 56 weeks and weekly measurements of the diameters of every fruit were made to provide estimates of fruit volume production. Development of the plants could suitably be divided into the following periods : (a) from germination to the start of fruiting, (b) an initial lag in fruit volume production, (c) rapidly increasing production, (d) declining production and (e) arrested development, when fruit swelling virtually ceased.

The duration of the period from germination to the start of fruiting was related to day-length.

The condition of arrested fruit development occurred from approximately the shortest day to the vernal equinox in mature plants that had already attained a maximal rate of fruit volume production ; in juvenile plants it occurred, and was maintained, from approximately the autumnal equinox to the vernal equinox, i.e. when the duration of darkness exceeded that of the light. In both juvenile and mature plants the condition of arrested fruit development ceased when the duration of the light exceeded that of darkness, and a period of increasing fruit volume production ensued.

The rate of fruit volume production during this period of rapid increase was related to day-length and the relation was curvilinear. It continued to increase as long as the days continued to lengthen. When the day-length began to decline so also did the rate of production and the relation with day- length was linear.  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