全文获取类型
收费全文 | 77529篇 |
免费 | 4344篇 |
国内免费 | 1817篇 |
专业分类
林业 | 4791篇 |
农学 | 4482篇 |
基础科学 | 1512篇 |
9661篇 | |
综合类 | 14943篇 |
农作物 | 3193篇 |
水产渔业 | 3571篇 |
畜牧兽医 | 35190篇 |
园艺 | 1296篇 |
植物保护 | 5051篇 |
出版年
2021年 | 942篇 |
2020年 | 947篇 |
2019年 | 1049篇 |
2018年 | 1277篇 |
2017年 | 1368篇 |
2016年 | 1337篇 |
2015年 | 1476篇 |
2014年 | 1505篇 |
2013年 | 3443篇 |
2012年 | 2694篇 |
2011年 | 3177篇 |
2010年 | 2321篇 |
2009年 | 2250篇 |
2008年 | 2896篇 |
2007年 | 2831篇 |
2006年 | 2729篇 |
2005年 | 2490篇 |
2004年 | 2005篇 |
2003年 | 1925篇 |
2002年 | 1761篇 |
2001年 | 2137篇 |
2000年 | 2186篇 |
1999年 | 2029篇 |
1998年 | 1143篇 |
1997年 | 1064篇 |
1996年 | 917篇 |
1995年 | 1016篇 |
1994年 | 903篇 |
1993年 | 897篇 |
1992年 | 1482篇 |
1991年 | 1607篇 |
1990年 | 1433篇 |
1989年 | 1409篇 |
1988年 | 1297篇 |
1987年 | 1224篇 |
1986年 | 1241篇 |
1985年 | 1218篇 |
1984年 | 983篇 |
1983年 | 917篇 |
1979年 | 895篇 |
1978年 | 701篇 |
1977年 | 697篇 |
1976年 | 713篇 |
1975年 | 714篇 |
1974年 | 872篇 |
1973年 | 842篇 |
1972年 | 814篇 |
1971年 | 727篇 |
1970年 | 769篇 |
1969年 | 735篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
Abstract – The reproductive biology of three species of rainbowfish (Melanotaeniidae) in northeastern Australian rainforest streams was investigated. Two species, Melanotaenia eachamensis and Cairnsichthys rhombosomoides are endemic to the area, whereas the third, M. splendida splendida, is more widespread. The species were all highly fecund, producing many hundreds of eggs between 1.10 and 1.24 mm in diameter. Melanotaenia eachamensis was the most fecund, produced the largest eggs of the three species, and consequently exhibited the greatest maternal investment (as measured by gonadosomatic index). The majority of reproductive effort occurred during the dry season, although reproductively active fish were present year-round for each of the species, but particularly so for M. s. splendida and C. rhombosomoides. No evidence for a role by temperature or photoperiod as environmental cues for reproduction was found, and it was suggested that gonad development was strongly tied to somatic growth. The concentration of reproduction to the dry season ensures that larvae are produced during a period of relatively stable and benign physical conditions. Comparison of temporal changes in gonadosomatic index values suggest that the spawning season of M. eachamensis , which occurs in high-elevation streams, is more restricted and commences about 1 month earlier than either other species. A similar phenology was observed for the M. s. splendida population found at high elevation and highlights the potential for spatial differences in stream productivity to influence life history. Note 相似文献
52.
黄光明 《绿色中国(A版)》2001,(5):64
葛根又名粉葛、葛藤、甘葛 ,为豆科葛属多年生藤本植物的根块 ,形似红薯。在农村它是粮食、药材、饲料多用途的野生植物。葛根富含钾、磷、钙等微量元素、多种维生素、氨基酸和具有药物作用的化合成份 ,属卫生部公布的“药食兼用食品”。它全身都是宝 ,用途颇广 :一是食用价值 ,葛根块是优质蔬菜 ,它是广东向东南亚出口的传统蔬菜之一 ;葛根淀粉是理想的绿色保健食品 ,可作粮食食用 ,也可制成葛根粉丝、葛根晶、葛根冲剂等系列食品 ,在日本人均年食用量超过 3kg;二是药用价值 ,凡是中药店都少不了葛根片这味药 ,主要成分为异黄酮类化合物… 相似文献
53.
B. Götz 《Journal of pest science》1938,14(8):92-93
Ohne Zusammenfassung 相似文献
54.
农民收入构成变化特点与分析评价 总被引:1,自引:0,他引:1
近年来,农民收入增长缓慢已成为农业和农村经济发展中的突出问题,分析评价农民收入构成变化情况与特点,利于探索农民增收的新途径和对策措施.…… 相似文献
55.
Asymmetric illumination contrast: a method of image formation for video light microscopy 总被引:4,自引:0,他引:4
B Kachar 《Science (New York, N.Y.)》1985,227(4688):766-768
Images with high resolution and exceptionally broad gray scale can be obtained by the application of video contrast enhancement to an optimized procedure for imaging transparent objects with oblique rays of illumination. This technique is simple to set up. A conventional microscope with a light source whose position can be adjusted and a video camera with controls for gain and black level are the only essential components. Features such as high resolution, optical sectioning, control of contrast, and operation under low light intensity make this technique preferable, in several instances, to currently used video microscopy techniques. 相似文献
56.
Washed and unwashed red blood cells (RBC) from young calves, adult cattle, hamsters and humans were incubated with Leptospira interrogans serovars pomona and ballum. Washed cells suspended in saline were always haemolysed while unwashed cells and those which were washed and resuspended in plasma were never haemolysed, despite the presence of large numbers of organisms within the culture supernatant. Pomona produced greater haemolysis of cattle and human RBC than did ballum, but with hamster RBC ballum produced greater haemolysis than did pomona. A group of 6- to 9-month-old cattle infected with pomona showed no signs of clinical disease and RBC taken from them before infection and during the development of antibodies to pomona were haemolysed by pomona only after the cells were washed. Plasma therefore appears to have a protective function. This in vitro protective function of plasma even extended to plasma from young seronegative calves. 相似文献
57.
58.
59.
60.
B. Schaerffenberg 《Journal of pest science》1940,16(6):61-63
Ohne Zusammenfassung
Mit 7 Abbildungen 相似文献