全文获取类型
收费全文 | 3544篇 |
免费 | 250篇 |
国内免费 | 295篇 |
专业分类
林业 | 268篇 |
农学 | 272篇 |
基础科学 | 165篇 |
793篇 | |
综合类 | 859篇 |
农作物 | 189篇 |
水产渔业 | 493篇 |
畜牧兽医 | 759篇 |
园艺 | 132篇 |
植物保护 | 159篇 |
出版年
2024年 | 17篇 |
2023年 | 96篇 |
2022年 | 115篇 |
2021年 | 153篇 |
2020年 | 163篇 |
2019年 | 161篇 |
2018年 | 124篇 |
2017年 | 185篇 |
2016年 | 181篇 |
2015年 | 160篇 |
2014年 | 180篇 |
2013年 | 254篇 |
2012年 | 240篇 |
2011年 | 264篇 |
2010年 | 202篇 |
2009年 | 200篇 |
2008年 | 168篇 |
2007年 | 179篇 |
2006年 | 147篇 |
2005年 | 85篇 |
2004年 | 102篇 |
2003年 | 95篇 |
2002年 | 71篇 |
2001年 | 80篇 |
2000年 | 72篇 |
1999年 | 50篇 |
1998年 | 36篇 |
1997年 | 39篇 |
1996年 | 42篇 |
1995年 | 38篇 |
1994年 | 32篇 |
1993年 | 34篇 |
1992年 | 22篇 |
1991年 | 16篇 |
1990年 | 18篇 |
1989年 | 17篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1979年 | 2篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 1篇 |
1973年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有4089条查询结果,搜索用时 154 毫秒
901.
Otolith size differences during early life of dwarf and cannibal Arctic char (Salvelinus alpinus)
下载免费PDF全文
![点击此处可从《Ecology of Freshwater Fish》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Bjørn Florø‐Larsen Anders G. Finstad Ole K. Berg Per H. Olsen 《Ecology of Freshwater Fish》2016,25(2):203-210
In Arctic and alpine lakes, Arctic char [Salvelinus alpinus (L.)] often form two distinct morphs: invertebrate feeders (‘dwarfs’) and piscivores (‘giant or cannibals’). Here, we test for early life history growth variation in dimorphic Arctic char as a proximate explanation for the observed life history variation between the two forms. Char were sampled in 11 alpine and Arctic Norwegian lakes. Dwarfs (defined as sexually mature char less than 15 cm long; N = 304) had a mean total length of 105 mm, whereas the typical cannibal (body length above 20 cm; N = 153) was 388 mm long. A positive correlation between egg size and otolith hatching ring were ascertained in a separate hatching experiment with brown trout (Salmo trutta) and it is assumed that this relationship also is valid for Arctic char, and otolith size was used as a proxy for length. Otolith hatching ring from Arctic char cannibals was larger (mean ± SD; 187 ± 24 μm) than those from dwarfs (mean ± SD; 164 ± 23 μm). There were only minor size differences between dwarfs and cannibals during the next three years, after which dwarfs usually matured. Two mutually nonexclusive, proximate explanations for the differentiation into separate morphs (dwarfs and cannibals) are therefore maternal effects and/or genetic based differentiation. The high catchability of large piscivorous char and low production in alpine and Arctic lake ecosystems may make these stocks particularly vulnerable to overexploitation. 相似文献
902.
与“以学生为中心”的教学理念一致,美国大学的课程教学呈现多样化的特点。论文分析了我国目前大学课程教学的现状后发现,为了给科研分配出足够的时间,教师投入教学的有效时间不能与不断上升的大学扩招规模保持同步增加。由此引发出在课堂规模扩大的情况下怎样保证课程教学质量的思考。结合在美国伊利诺伊大学香槟分校(UIUC)的访学经历,提出配套相应技术支撑体系、教学与人文结合以及采取课程组授课与“大班授课,小组讨论”等方式方法有助于提高“大班”课程授课的教学效果。 相似文献
903.
I. F. López P. D. Kemp J. Dörner C. A. Descalzi O. A. Balocchi S. García 《Journal of Agronomy and Crop Science》2013,199(6):449-459
Often perennial pastures have to tolerate soil water restriction during summer, which can affect the relative abundance of the most desired species. In the south of Chile, Bromus valdivianus and Lolium perenne are preferred species. The competitiveness of B. valdivianus and L. perenne was evaluated when sown in pots in a glasshouse as monocultures or a mix (50/50 %) with restricted water availability. The water restriction treatments were as follows: 80–85 % field capacity (FC), 45–50 % FC and 20–25 % FC, maintained for 1196 growing degree days. For both species, dry matter (DM) accumulated per plant and per pot decreased similarly with increasing water restriction, but at tiller level, resource allocation differed, for B. valdivianus root growth was accentuated over aerial development, but this was less so for L. perenne. The foliage mass per tiller for B. valdivianus decreased relatively more than that for L. perenne with increasing water restriction. As monocultures, B. valdivianus produced larger tillers than L. perenne, such that B. valdivianus tillers had 2.2 times greater lamina mass, 3.6 times more leaf area and 2.5 times greater root mass than those of L. perenne. However, L. perenne produced a larger number of smaller tillers that enabled foliage, root and total mass at plant level, to be similar to that of B. valdivianus. Within the mixed pasture, L. perenne tiller density increased compared with when grown as a monoculture, but not for B. valdivianus. The results of the study suggested that L. perenne and B. valdivianus have differences in growth strategies that allow them to survive under environmental stress and competition. Competitiveness increased for L. perenne aboveground without water restriction and that for B. valdivianus increased belowground as water restriction was increased. 相似文献
904.
Abinash Padhi 《Ecology of Freshwater Fish》2011,20(2):263-269
Abstract – Using the Bayesian coalescent methodology and mitochondrial control region sequence data, the present study reconstructs the dynamics of effective population size of a temperature‐driven seasonal migratory North American freshwater fish, Ictalurus furcatus, during the last glacial cycle. The trend in effective population size is directly associated with the trend in surface air temperature; it is inversely associated with the accumulation of ice sheets in North America during the contemporary period. The long‐term decrease in effective population size during the cold period followed by rebound during the warmer period could be explained by this species’ preference for relatively warmer temperatures for its optimal growth and survival. 相似文献
905.
906.
907.
908.
对云南省宁蒗县珍贵药用植物三尖杉种子大小性状变异的研究表明 :种子的长度、宽度、厚度、长度与宽度比、长度与厚度比、宽度与厚度比和重量等性状变异均为连续分布的数量性状 ,它们的平均值分别为 18 2 9± 2 4 5mm、 9 4 2± 0 6 5mm、 7 71± 0 5 6mm、 1 94± 0 2 4、 2 38± 0 34、 1 2 2± 0 0 6和 0 4 6 3± 0 10 6g ;变异系数分别为 13 37%、 6 88%、 7 2 2 %、 12 2 2 %、 14 16 %、 4 84 %和 2 3 0 2 %。 相似文献
909.
Piao RenzhuInstitute of Wildlife of Heilongjiang Province Sun Sen An Dongdong Northeast Forestry University 《林业研究》1991,2(2):83-88
Coordination of the aenal servey,the author carried ground census on the popula-tion size of Red-erowned crane(Grus japonensis)in Khanka Lakes District from 1984 to 1986.Ac-cording to the census in spring of 1984,the total number of Red-crowncd crane is 72 individualsand 20 nests.So that this pound census has recorded 59 indivlduals more than the aerial survey of13 individuals,and the percentage of nesting individuals in his flock is 55.6%,which is obviouslyhigher than 36.9% of the average ration in China.After this,in fall of 1984 and in spring of 1985 and1986,we repeated the same censuses three times in the same districts,the results indicated that theflock of the crane in Khanka Lakes District is stable basiuclly,but some individuals interchangewith opposite flock of USSR because of gegraphical adjoin So,We blieve this flock in China andthat lock in USSR belong to the same population and suggest two countries should act in concertto studing on this population in future. Meanwhile,the aerial servey 相似文献
910.
Supplemental mass pollination (SMP) success in a grafted lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) seed orchard in southern British Columbia was studied by employing four chloroplast DNA (cpDNA) markers to determine
fertilization success of 10 pollen parents. SMP was conducted operationally with a bulked pollen mix twice during peak receptivity
in the seed orchard. Fertilization success of the 10 SMP parents averaged 16% greater than for wind-pollinated controls in
a different section of the orchard. SMP also increased the uniformity of the male contribution in treated seeds. 相似文献