全文获取类型
收费全文 | 2593篇 |
免费 | 187篇 |
国内免费 | 305篇 |
专业分类
林业 | 293篇 |
农学 | 264篇 |
基础科学 | 266篇 |
416篇 | |
综合类 | 1141篇 |
农作物 | 60篇 |
水产渔业 | 160篇 |
畜牧兽医 | 284篇 |
园艺 | 66篇 |
植物保护 | 135篇 |
出版年
2024年 | 10篇 |
2023年 | 47篇 |
2022年 | 97篇 |
2021年 | 101篇 |
2020年 | 96篇 |
2019年 | 102篇 |
2018年 | 80篇 |
2017年 | 127篇 |
2016年 | 124篇 |
2015年 | 102篇 |
2014年 | 170篇 |
2013年 | 124篇 |
2012年 | 218篇 |
2011年 | 189篇 |
2010年 | 121篇 |
2009年 | 175篇 |
2008年 | 140篇 |
2007年 | 169篇 |
2006年 | 112篇 |
2005年 | 100篇 |
2004年 | 82篇 |
2003年 | 91篇 |
2002年 | 56篇 |
2001年 | 51篇 |
2000年 | 56篇 |
1999年 | 45篇 |
1998年 | 34篇 |
1997年 | 35篇 |
1996年 | 32篇 |
1995年 | 26篇 |
1994年 | 33篇 |
1993年 | 29篇 |
1992年 | 28篇 |
1991年 | 20篇 |
1990年 | 22篇 |
1989年 | 18篇 |
1988年 | 14篇 |
1987年 | 5篇 |
1986年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
排序方式: 共有3085条查询结果,搜索用时 265 毫秒
61.
在简化牛卵母细胞体外成熟的基础上,观察了保护剂、平衡温度、预平衡方法、解冻方法以及冷冻方法对牛卵泡卵母细胞冷冻的影响。结果表明:在体外成熟培养液中添加26.2 mmol/L的NaHCO3和对屠宰场卵巢进行选择更能促进牛卵母细胞的体外成熟;无论是程序冷冻还是玻璃化冷冻,乙二醇(EG)与甘油(GLY)相比更能促进牛卵泡卵母细胞的冷冻效果;在程序冷冻中,冷冻液中添加0.1 mol/L蔗糖比添加0.3mol/L的蔗糖更能促进牛卵泡卵母细胞的冷冻;在玻璃化冷冻中,37℃和25℃的平衡温度比4℃更适合牛卵泡卵母细胞的冷冻;在10%、5%、1%的预平衡浓度之间,5%的预平衡浓度即可达到预平衡的效果;在解冻时,多步脱除保护剂更能保护冷冻的卵母细胞;比较了几种最小样本量(minimum size sample,MSS)玻璃化冷冻方法,解冻成熟培养后的成熟率,拉细毛细玻璃管冷冻法为(41.67±3.19)%,极显著高于未拉细毛细玻璃管冷冻法(glassmicropipette,GMP)的(30.19±1.93)%和固体表面冷冻法(solid surface vitrification,SSV)的(28.33±2.89)%(P<0.01)。 相似文献
62.
63.
大黄鱼鱼苗耗氧率和窒息点的研究 总被引:2,自引:0,他引:2
为了给大黄鱼的生理学及大黄鱼养殖的放养密度、水质管理、饵料利用以及活鱼运输等提供技术参考依据,对大黄鱼基础代谢水平进行测定。在3种水温(14,20,25℃)条件下测定大黄鱼[Pseudosciaena crocea(Richardson)]鱼苗(体长7.8~9.1 cm,体重6.61~8.85 g)的耗氧状况,据此计算出大黄鱼鱼苗的耗氧率,及瞬时耗氧率与溶氧量及水温的相关关系。结果表明,大黄鱼鱼苗耐低氧能力差,其耗氧率和窒息点溶解氧水平都随水温的升高而增大,14℃时两者为2.833μg/(g.min)和1.42 mg/L;25℃时两者为4.677μg/(g.min)和2.27 mg/L。大黄鱼鱼苗的瞬时耗氧率在其呼吸生理的某个时段出现特殊现象:当水中溶解氧下降至其浮头点和昏迷临界点之间时,瞬时耗氧率会在一个较短的时间范围内呈现跳跃式提升,这种现象在已有的鱼类耗氧率研究中从未出现过。此后的瞬时耗氧率急速下降至最低点,其呼吸类型可归属于逆向型。 相似文献
64.
The interaction between landscape structure and spatial patterns of plant invasion has been little addressed by ecologists
despite the new insights it can provide. Because of their spatial configuration as highly connected networks, linear wetlands
such as roadside or agricultural ditches, can serve as corridors facilitating invasion at the landscape scale, but species
dynamics in these important habitats are not well known. We conducted a landscape scale analysis of Phragmites australis invasion patterns (1985–2002 and 1987–2002) in two periurban areas of southern Québec (Canada) focusing on the interaction
between the network of linear wetlands and the adjacent land-uses. Results show that, at the beginning of the reference period,
the two landscapes were relatively non-invaded and populations occurred mostly in roadside habitats which then served as invasion
foci into other parts of the landscape. The intrinsic rates of increase of P. australis populations in linear anthropogenic habitats were generally higher than those reported for natural wetlands. Riparian habitats
along streams and rivers were little invaded compared to anthropogenic linear wetlands, except when they intersected transportation
rights-of-way. Bivariate spatial point pattern analysis of colonization events using both Euclidian and network distances
generally showed spatial dependence (association) to source populations. An autologistic regression model that included landscape
and edaphic variables selected transportation rights-of-way as the best predictor of P. australis occurrence patterns in one of the landscapes. Given the high invasion rates observed, managers of linear wetlands should
carefully monitor expansion patterns especially when roads intersect landscapes of conservation or economic value. 相似文献
65.
输油管道添加降凝剂是改善原油代温流变性的一种物理化学方法,从降凝剂的筛选和复配,处理条件,应用方式,配套设施、应用效果等方面系统地介绍了含蜡原油添加降凝剂输送工艺的研究及应用现状,指出了降凝剂在研究和应用中存在的问题,对添加降凝剂的输油技术进行了客观评价,展望了降凝输油技术的发展方向。 相似文献
66.
67.
阐述水库溢洪道闸门防冰冻的设备结构、安装和运行维护防法。延边河龙水库地处吉林省东部严寒地区,水库溢流坝闸门的冬季防冰冻措施尤为重要,该水库采用潜水泵直接射流方式,防止闸前冰冻,取得很好的效果。 相似文献
68.
蒋月评 《湖南农业大学学报(自然科学版)》1999,26(4)
研究了4 种高维 Mobius 群的初等子群定义之间的关系⒚得到了高维 Mobius 群是初等群的充要条件;在离散的条件下,给出了这种初等子群的结构性定理⒚ 相似文献
69.
《中国林业科技(英文版)》2009,(2)
After freezing snow disaster,stands of Pinus massoniana,Cunninghamia lanceolata. Liquidambar formosana,Cinnamomum camphora and Phyllostachys pubescens and new burned areas in Hunan Province were surveyed by setting field plots.Moisture content and fuel load were measured by sampling and fire behavior(such as rate of fire spread) was calculated.The results showed that fuel load got doubled instantly,fuel thickness enhanced,fuel continuity swelled and water content of fuel decreased as shrubs and herbs die... 相似文献
70.
Photosynthetic Apparatus Responses to Short‐term low‐temperature Flooding May Contribute to Freezing Tolerance Changes in Forage Grasses 下载免费PDF全文
Winter conditions are subjected to rapid climate changes. Increased precipitation and snow melting during warmer winters may result in low‐temperature flooding. These factors probably affect plant overwintering strategies. This study investigated the relationships between the photosynthetic adjustment and freezing tolerance following a short‐term low‐temperature flooding in two forage grasses (Lolium perenne and Festuca pratensis). The effect of flooding on the photosynthetic apparatus acclimation to cold was determined using chlorophyll a fluorescence measurements. Freezing tolerance was estimated using an electrolyte leakage test. It has been shown that genotypes activating non‐photochemical mechanisms of photosynthetic acclimation to cold during low‐temperature flooding may show increased freezing tolerance. Freezing tolerance reduction observed in Festuca pratensis was probably connected with decreased photochemical activity and photoinhibition of photosynthesis. It can be concluded that photosynthetic apparatus response to low‐temperature flooding may contribute to changes in the freezing tolerance. The direction of the changes is associated with different photosynthetic apparatus performance. 相似文献