全文获取类型
收费全文 | 1224篇 |
免费 | 54篇 |
专业分类
林业 | 274篇 |
农学 | 31篇 |
126篇 | |
综合类 | 128篇 |
农作物 | 24篇 |
水产渔业 | 50篇 |
畜牧兽医 | 576篇 |
园艺 | 15篇 |
植物保护 | 54篇 |
出版年
2021年 | 18篇 |
2020年 | 12篇 |
2019年 | 16篇 |
2018年 | 19篇 |
2017年 | 21篇 |
2016年 | 26篇 |
2015年 | 21篇 |
2014年 | 25篇 |
2013年 | 59篇 |
2012年 | 43篇 |
2011年 | 53篇 |
2010年 | 19篇 |
2009年 | 33篇 |
2008年 | 33篇 |
2007年 | 45篇 |
2006年 | 51篇 |
2005年 | 32篇 |
2004年 | 32篇 |
2003年 | 25篇 |
2002年 | 30篇 |
2001年 | 29篇 |
2000年 | 28篇 |
1999年 | 28篇 |
1998年 | 10篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1993年 | 11篇 |
1992年 | 13篇 |
1991年 | 21篇 |
1990年 | 16篇 |
1989年 | 22篇 |
1988年 | 10篇 |
1987年 | 15篇 |
1986年 | 17篇 |
1985年 | 15篇 |
1984年 | 19篇 |
1983年 | 13篇 |
1980年 | 10篇 |
1979年 | 23篇 |
1978年 | 12篇 |
1975年 | 9篇 |
1973年 | 12篇 |
1972年 | 19篇 |
1971年 | 9篇 |
1970年 | 14篇 |
1969年 | 12篇 |
1968年 | 12篇 |
1967年 | 12篇 |
1965年 | 11篇 |
1951年 | 10篇 |
排序方式: 共有1278条查询结果,搜索用时 46 毫秒
71.
Adrian Patrut Karl F. von Reden Robert Van Pelt Diana H. Mayne Daniel A. Lowy Dragos Margineanu 《Annals of Forest Science》2011,68(5):993-1003
• Introduction
For large trees without a continuous sequence of growth rings in their trunk, such as the African baobab (Adansonia digitata L.), the only accurate method for age determination is radiocarbon dating. As of today, this method was limited to dating samples collected from the remains of dead specimens. 相似文献72.
Estimating balanced structure areas in multi-species forests on the Sierra Madre Occidental,Mexico 总被引:1,自引:0,他引:1
Christian Wehenkel José Javier Corral-Rivas José Ciro Hernández-Díaz Klaus von Gadow 《Annals of Forest Science》2011,68(2):385-394
• Introduction
This study presents a method for estimating the minimum area which exhibits a balanced diameter distribution, and the corresponding number of trees, for different tree species and forest types in the Santiago Papasquiaro region in the State of Durango, Mexico. The balanced structure area is defined as the minimum contiguous area that is required for sustainable management of a multi-sized selection forest. A multi-sized forest represents a balanced structure unit if the relationship between harvest and growth can be maintained, using a defined target diameter distribution and disregarding major natural disturbances. The study is based on 17,577 sample plots in uneven-aged forests, which are selectively harvested by local communities. 相似文献73.
Augmentation of small rare species populations is sometimes suggested on genetic grounds. However, outbreeding depression via dilution of local adaptation or break-up of genomic coadaptation may occur. These effects depend on the causes of population divergence. Here, we compare genetic measures of population divergence in Liatris scariosa var. novae-angliae, a rare New England perennial. We measured GST, neutral marker subdivision, and QST, quantitative subdivision of propagule and juvenile plant traits. GST was relatively high. QST for leaf shape exceeded GST, indicating local adaptation, while QST for other traits fell within or below the GST range. Local adaptation appears low for juvenile traits, although the high GST cautions against translocation because of potential coadaptation. If translocation is still required, however, donor populations should contain high quantitative genetic diversity. We assess population size and allozyme diversity as predictors of quantitative genetic variation, but find these poor proxies for direct measurement. 相似文献
74.
Lande B Andersen LF Veierød MB Baerug A Johansson L Trygg KU Bjørneboe GE 《Public health nutrition》2004,7(4):495-503
OBJECTIVE: To analyse factors associated with breast-feeding and use of sweetened drinks at 12 months, and to compare dietary habits among breast-fed and non-breast-fed infants. DESIGN: Data were collected by a semi-quantitative food-frequency questionnaire filled in by the parents. SETTING: National dietary survey in Norway. SUBJECTS: In total, 1932 12-month-old infants were included. RESULTS: At 12 months, 36% of the infants were breast-fed. The odds of breast-feeding at this age were more than doubled both for mothers > or =35 years compared with mothers <25 years and for mothers in the highest educational group compared with mothers in the lowest. A negative association was found for maternal smoking, and the odds of breast-feeding were 40% lower for mothers who smoked than for non-smokers. Some dietary differences were observed between breast-fed and non-breast-fed infants apart from intake of milk. In particular, breast-fed infants had a significantly lower daily intake of sweetened drinks than non-breast-fed infants and a 16% lower mean daily intake of added sugars (P<0.001). Furthermore, breast-fed infants had 30% higher odds of not receiving sweetened drinks daily, compared with non-breast-fed infants. CONCLUSIONS: Maternal age, education and smoking status were important factors for breast-feeding at 12 months. Breast-fed infants had lower intakes of sweetened drinks and added sugars than non-breast-fed infants. From a public health perspective, continued promotion of breast-feeding is needed to reduce inequalities in breast-feeding. Moreover, prevention of high intakes of sweetened drinks and added sugars should start in infancy. 相似文献
75.
András Cseri Mátyás Cserháti Maria von Korff Bettina Nagy Gábor V. Horváth András Palágyi János Pauk Dénes Dudits Ottó Törjék 《Euphytica》2011,181(3):341-356
In the present study, allele mining was conducted on a panel of drought related candidate genes in a set of 96 barley genotypes
using EcoTILLING, which is a variant of the targeting induced local lesions in genomes (TILLING) technology. Analyzing approximately
1.5 million basepairs in barley a total number of 94 verified unique haplotypes were identified in 18 amplicons designed for
9 genes. Overall, 185 single nucleotide polymorphisms (SNPs) and 46 insertions/deletions (INDELs) were detected with a mean
of 1SNP/92 bp and 1INDEL/372 bp genomic sequence. Based on overlapping haplotype sequences, markers were developed for four
candidate genes (HvARH1, HvSRG6, HvDRF1, HVA1), which allows distinguishing between the main haplotypes showing either differences in amino acid sequence or which have
larger INDELs in the promoter region. As “proof of concept”, the HvARH1 and HvSRG6 haplotypes were tested for the level of abscisic acid-induced gene expression in subsets of genotypes belonging to different
haplotype categories. An integrated database was developed to contain information about the genes, genotypes, and haplotypes
analyzed in this study. The database supplies profound information about the natural variation in the tested drought related
candidate genes providing a significant asset for further mapping studies dealing with this highly polygenic trait. 相似文献
76.
Hypoxic dilation of coronary arteries is mediated by ATP-sensitive potassium channels 总被引:44,自引:0,他引:44
J Daut W Maier-Rudolph N von Beckerath G Mehrke K Günther L Goedel-Meinen 《Science (New York, N.Y.)》1990,247(4948):1341-1344
The function of the heart depends critically on an adequate oxygen supply through the coronary arteries. Coronary arteries dilate when the intravascular oxygen tension decreases. Hypoxic vasodilation in isolated, perfused guinea pig hearts can be prevented by glibenclamide, a blocker of adenosine triphosphate (ATP)-sensitive potassium channels, and can be mimicked by cromakalim, which opens ATP-sensitive potassium channels. Opening of potassium channels in coronary smooth muscle cells and the subsequent drop in intracellular calcium is probably the major cause of hypoxic and ischemic vasodilation in the mammalian heart. 相似文献
77.
Tsivion D Schvartzman M Popovitz-Biro R von Huth P Joselevich E 《Science (New York, N.Y.)》2011,333(6045):1003-1007
The large-scale assembly of nanowires with controlled orientation on surfaces remains one challenge preventing their integration into practical devices. We report the vapor-liquid-solid growth of aligned, millimeter-long, horizontal GaN nanowires with controlled crystallographic orientations on different planes of sapphire. The growth directions, crystallographic orientation, and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. This paves the way to highly controlled nanowire structures with potential applications not available by other means. 相似文献
78.
Madakamutil LT Christen U Lena CJ Wang-Zhu Y Attinger A Sundarrajan M Ellmeier W von Herrath MG Jensen P Littman DR Cheroutre H 《Science (New York, N.Y.)》2004,304(5670):590-593
Memory T cells are long-lived antigen-experienced T cells that are generally accepted to be direct descendants of proliferating primary effector cells. However, the factors that permit selective survival of these T cells are not well established. We show that homodimeric alpha chains of the CD8 molecule (CD8alphaalpha) are transiently induced on a selected subset of CD8alphabeta+ T cells upon antigenic stimulation. These CD8alphaalpha molecules promote the survival and differentiation of activated lymphocytes into memory CD8 T cells. Thus, memory precursors can be identified among primary effector cells and are selected for survival and differentiation by CD8alphaalpha. 相似文献
79.
80.
Grassmé H Kirschnek S Riethmueller J Riehle A von Kürthy G Lang F Weller M Gulbins E 《Science (New York, N.Y.)》2000,290(5491):527-530
Pseudomonas aeruginosa causes severe infections, particularly of the lung, that are life threatening. Here, we show that P. aeruginosa infection induces apoptosis of lung epithelial cells by activation of the endogenous CD95/CD95 ligand system. Deficiency of CD95 or CD95 ligand on epithelial cells prevented apoptosis of lung epithelial cells in vivo as well as in vitro. The importance of CD95/CD95 ligand-mediated lung epithelial cell apoptosis was demonstrated by the rapid development of sepsis in CD95- or CD95 ligand-deficient mice, but not in normal mice, after P. aeruginosa infection. 相似文献