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   73355篇
  免费   4137篇
  国内免费   32篇
林业   3176篇
农学   2398篇
基础科学   466篇
  8696篇
综合类   12020篇
农作物   2818篇
水产渔业   3531篇
畜牧兽医   38606篇
园艺   837篇
植物保护   4976篇
  2018年   968篇
  2017年   1073篇
  2016年   1001篇
  2015年   847篇
  2014年   1047篇
  2013年   2752篇
  2012年   1945篇
  2011年   2275篇
  2010年   1534篇
  2009年   1560篇
  2008年   2324篇
  2007年   2224篇
  2006年   2038篇
  2005年   1975篇
  2004年   1851篇
  2003年   1992篇
  2002年   1809篇
  2001年   2241篇
  2000年   2193篇
  1999年   1751篇
  1998年   714篇
  1997年   776篇
  1996年   687篇
  1995年   866篇
  1994年   734篇
  1993年   734篇
  1992年   1494篇
  1991年   1636篇
  1990年   1603篇
  1989年   1606篇
  1988年   1494篇
  1987年   1456篇
  1986年   1536篇
  1985年   1449篇
  1984年   1270篇
  1983年   1063篇
  1982年   748篇
  1979年   1171篇
  1978年   887篇
  1977年   840篇
  1976年   799篇
  1975年   829篇
  1974年   1030篇
  1973年   1070篇
  1972年   1018篇
  1971年   951篇
  1970年   928篇
  1969年   899篇
  1968年   755篇
  1967年   844篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
82.
The activity of succinate dehydrogenase (SDH) and lactage dehydrogenase (LDH) was studied in chick-embryo fibroblast cultures after inoculation of the virulent strain "A2" and the avirulent strain "MK" of herpesvirus suum. Strain "A2" reduced SDH activity, and so did strain MK, but here the decrease of enzyme activity was slower, and it did not become evident until the 24th hour. LDH activity fluctuated after "A2" infection but was generally increased, while there was no change in LDH activity, compared with uninfected control cells, after "MK" infection. When interaction of cell and virus took place in the presence of 5-iodo-2-desoxyuridine (IUDR), strain "A2" produced little change in the enzymes, but "MK" infection was accompanied by a definite fall in SDH and a slight increase in LDH. The presence of IUDR inhibited the proliferation of the virulent strain but had no apparent effect on proliferation of the attenuated strain "MK". Investigation of the enzyme activity of cells infected with Aujeszky's disease virus has revealed new biological properties of the virus, which might serve to distinguish between different strains of the virus.  相似文献   
83.
84.
A high incidence (86%) of potyvirus infection was noted in tobacco plants exhibiting a form of leaf curl in South Africa. Despite leaf curl being reported in the literature to be of geminiviral aetiology, no geminiviruses were detected. Furthermore, no other virus particles were detected by virus purification, TEM and serology. Twelve species of dsRNA were consistently isolated from these tobacco plants, but were absent from other forms of leaf curl-affected and healthy tobacco. Aphid and mechanical inoculation demonstrated that the purified potyvirus(es) did not cause leaf curl symptoms, but rather mild mottle and mosaic symptoms in tobacco. Partial characterization of the potyvirus preparation showed a possible relationship to a South African strain of potato virus Y. Because potyvirus-inoculated plants did not manifest leaf curl symptoms, and because leaf curl symptoms were noted in some plants not infected with a potyvirus, it was concluded that the potyvirus is not involved in the leaf curl aetiology, but causes a latent infection, the symptoms of which are masked. The pattern of the dsRNA banding, induction of enations and lack of mechanical and seed transmission are common to plant reoviruses. The possibility of a phytoreovirus involvement in this form of leaf curl is currently being investigated. The results from this study suggest that tobacco leaf curl disease worldwide, with regard to geminiviruses, be re-evaluated.  相似文献   
85.
Sunlight is known to reduce the incidence of blister blight disease of tea, caused by Exobasidium vexans . The effects of the ultraviolet-B (UV-B: 290–320 nm) component of solar radiation on the disease and the development of E. vexans were studied in the field in Sri Lanka using UV-screening filter materials held over a commercial crop. Exclusion of UV-B radiation by Polyester, which reduces fluxes in the UV-B region by approximately 75–85%, increased both the number of translucent spots (immature sites of infection) and number of sporulating blisters (mature sites of infection), but it had little or no effect on the sporulation of the pathogen. When basidiospores were artificially inoculated on leaves, and were exposed to full or filtered solar radiation, their survival and germination increased when UV-B wavelengths were removed; significant differences were found 62 h after inoculation. However, UV-B did not affect the extension of germ tubes. This study indicates that the UV-B component of solar radiation plays an important role in the natural regulation of blister blight disease in the field.  相似文献   
86.
Modelling crop:weed interactions in wheat with ALMANAC   总被引:1,自引:0,他引:1  
ALMANAC is a dynamic model for plant growth, water balance and soil nitrogen dynamics that can simulate on a daily basis two or more competing species. The simulation of competition for light is based on Beer's law, allowing a different extinction coefficient ( k ) for each species. Light is partitioned between species based on k -values, leaf area index and plant heights. Total hiomass is simulated with radiation use efficiency and grain yield with a harvest index approach, sensitive to water stress. The model simulates competition for water and nutrients based on each species current rooting zone and demand by each species. The effect of crop management on the competition issue can he simulated. The model was evaluated in Dijon (France) using 4 years of experimental data on wheat:oat mixtures, differing in oat ( Avena sativa L.) densities, the period of oat emergence, the date of weed suppression by herbicides and the wheat ( Triticun aestivum L.) genotype. Additional data on oilseed rape ( Brassica nupus L.) and vetch ( Vilcia sauiva L.) competition in spring wheat were also used. The wheat grain yield was reasonably simulated with a root mean square error (RMSE) of 0.10-0.35 t ha-1. Corresponding values for oats were 0.10-0.55 t ha-1. The competitiveness of oats, oilseed rape and vetch was correctly simulated. The model appears as a reasonable tool for estimating damage thresholds in integrated weed control programmes.  相似文献   
87.
The activity of the ovary-inhibiting hormone in the eyestalks of female prawns, Parapenaeopsis hardwickii, was determined during its ovarian developmental cycle. In the inactive (stage I) and post-spawned phases of ovarian development, the ovary-inhibiting hormone activity was found to be very high. During the phase of full vitellogenesis (stage IV), the inhibiting activity is almost negligible. After spawning (stage V) it is present, but at a lower level than during inactive and post-spawned phases. Hence it is inferred that ovary-inhibiting hormone appears to be responsible for the phase of ovarian quiescence and to control the rate of vitellogenesis.  相似文献   
88.
This paper describes the main features of the Sangay wilderness area in the Ecuadorian Andes. It is a region of great diversity with life zones ranging from tundra to subtropical rain forest. The conservation value of the area is assessed and the feasibility of protection determined. Boundaries based mainly on the limits of human interference are put forward with management proposals for its establishment as a National Park.  相似文献   
89.
90.
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