全文获取类型
收费全文 | 36529篇 |
免费 | 5289篇 |
国内免费 | 21193篇 |
出版年
2024年 | 486篇 |
2023年 | 1349篇 |
2022年 | 2163篇 |
2021年 | 2263篇 |
2020年 | 2231篇 |
2019年 | 2601篇 |
2018年 | 1722篇 |
2017年 | 1645篇 |
2016年 | 1736篇 |
2015年 | 2289篇 |
2014年 | 3232篇 |
2013年 | 2861篇 |
2012年 | 3928篇 |
2011年 | 3955篇 |
2010年 | 3236篇 |
2009年 | 3311篇 |
2008年 | 3510篇 |
2007年 | 3284篇 |
2006年 | 3112篇 |
2005年 | 2586篇 |
2004年 | 1978篇 |
2003年 | 1673篇 |
2002年 | 1476篇 |
2001年 | 1474篇 |
2000年 | 1337篇 |
1999年 | 833篇 |
1998年 | 461篇 |
1997年 | 296篇 |
1996年 | 236篇 |
1995年 | 201篇 |
1994年 | 146篇 |
1993年 | 150篇 |
1992年 | 143篇 |
1991年 | 130篇 |
1990年 | 100篇 |
1989年 | 78篇 |
1988年 | 106篇 |
1987年 | 81篇 |
1986年 | 74篇 |
1985年 | 84篇 |
1984年 | 58篇 |
1983年 | 44篇 |
1982年 | 80篇 |
1981年 | 36篇 |
1979年 | 15篇 |
1958年 | 12篇 |
1957年 | 22篇 |
1954年 | 12篇 |
1953年 | 13篇 |
1950年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Ting Zhang Bai‐Ge Miao Bo Wang Yan‐Qiong Peng Clive T. Darwell 《Ecological Entomology》2019,44(6):844-848
1. Species interactions in tightly bound ecological mutualisms often feature highly specialised species' roles in which competitive exclusion may preclude multi‐species coexistence. Among the 800 fig (Ficus) species, it was originally considered that each was pollinated by their own wasp (Agaonidae). However, recent investigations show that this ‘one‐to‐one’ rule often breaks down, as fig species regularly host multiple agaonids but in ways suggesting that competitive processes still mediate biodiversity outcomes. 2. A phenological survey was conducted of the fig–fig wasp pair, Ficus microcarpa and its associated pollinating wasp, alongside its sister species, the cheating wasp, in Xishuangbanna, China. 3. Reproductive output underwent extreme seasonal variation. Seed and pollinator production fell markedly during cooler, drier months, although high levels of fig production continued. However, this resource was predominantly utilised by the cheater species, which offers no pollination services. Pollinators and cheaters rarely co‐occur, suggesting that temporal coexistence is constrained by competition for access to figs. 4. The overall findings indicate periodic rearrangements of mutualism dynamics, probably resulting from a strongly seasonal environment. Sympatric co‐occurrence may result from a window of opportunity for a functionally divergent agaonid, potentially due to constraints on the main pollinator in adapting to variable year‐round conditions that prevent competitive exclusion. 相似文献
2.
HAN-QI YANG HONG WANG DE-ZHU LI 《Botanical journal of the Linnean Society. Linnean Society of London》2008,156(3):411-423
The foliage leaf epidermis of 35 species representing 12 key genera of woody bamboos of the Asian tropics was investigated using light and scanning electron microscopy. The results indicated that papillae forms and distributional patterns around the stomatal apparatus of the abaxial foliage leaf epidermis were usually constant and were of great taxonomic significance at the specific and generic levels. However, papillae characters were not suitable for dividing subtribes within woody bamboos of the Asian tropics. On the basis of papillae characters, Schizostachyum s.s. and Cephalostachyum were confirmed, but their delimitations should be modified. The transfer of Leptocanna chinensis and Schizostachyum sanguineum into Cephalostachyum was supported, and Cephalostachyum virgatum and C . pergracile were confirmed to be members of Schizostachyum s.s. The subtribe Racemobambosinae did not obtain support and Racemobambos appeared to be better placed in subtribe Bambusinae. Neomicrocalamus was supported as a close relative and better treated as a synonym of Racemobambos . Gigantochloa was closely related to Dendrocalamus . © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 411–423. 相似文献
3.
WANG JC 《The Journal of general physiology》1948,31(3):259-268
1. Experiments were performed on six dogs to determine the rate of penetration of Cl(33) and Na(24) across the blood-aqueous humor and blood-cerebrospinal fluid barriers after intravenous injection of the radioactive ions. The radioactivity measurements were made with an immersion type of Geiger-Müller counter. 2. The concentrations of the labelled ions in the anterior chamber and the cisterna magna increase slowly to approach that of plasma. The rate of penetration k is calculated from a simple exponential equation with the half-value interval t(0.5) or the time required for the labelled-ion concentration in the fluid to reach 50 per cent of that of plasma. The average t(0.5) for Cl(38) and Na(24) in aqueous humor are 34.3 +/- 9 and 27.3 +/- 9 minutes, respectively, while those for cerebrospinal fluid are 90 +/- 6 and 95 +/- 6 minutes, respectively. 3. A study of the radioactivity in plasma was made to determine the per cent remaining after a steady state was reached. By means of this determination the sodium and chloride space was calculated to be 33 +/- 5 per cent. 相似文献
4.
5.
6.
多种天敌控制多种害虫的模糊数学模型 总被引:2,自引:0,他引:2
从生态经济学的原理出发,利用模糊集理论,探讨如何评价天敌控制多种害虫的为害作用,提出了多种天敌控制多种害虫的模糊数学模型。同时又得到了天敌对害虫的控制能力C,影响率G,影响强度H,及权数ω(x)的计算公式,以及在天敌作用下安全度的讨论与分析。 相似文献
7.
8.
9.
10.