全文获取类型
收费全文 | 22126篇 |
免费 | 1060篇 |
国内免费 | 124篇 |
专业分类
林业 | 1992篇 |
农学 | 825篇 |
基础科学 | 159篇 |
4166篇 | |
综合类 | 978篇 |
农作物 | 1146篇 |
水产渔业 | 1884篇 |
畜牧兽医 | 9776篇 |
园艺 | 454篇 |
植物保护 | 1930篇 |
出版年
2023年 | 108篇 |
2022年 | 179篇 |
2021年 | 347篇 |
2020年 | 383篇 |
2019年 | 365篇 |
2018年 | 732篇 |
2017年 | 827篇 |
2016年 | 784篇 |
2015年 | 697篇 |
2014年 | 784篇 |
2013年 | 1014篇 |
2012年 | 1495篇 |
2011年 | 1319篇 |
2010年 | 705篇 |
2009年 | 629篇 |
2008年 | 829篇 |
2007年 | 768篇 |
2006年 | 650篇 |
2005年 | 1368篇 |
2004年 | 1154篇 |
2003年 | 937篇 |
2002年 | 528篇 |
2001年 | 437篇 |
2000年 | 338篇 |
1999年 | 406篇 |
1998年 | 83篇 |
1992年 | 249篇 |
1991年 | 254篇 |
1990年 | 278篇 |
1989年 | 273篇 |
1988年 | 281篇 |
1987年 | 230篇 |
1986年 | 253篇 |
1985年 | 215篇 |
1984年 | 172篇 |
1983年 | 151篇 |
1979年 | 229篇 |
1978年 | 184篇 |
1977年 | 173篇 |
1976年 | 129篇 |
1975年 | 184篇 |
1974年 | 220篇 |
1973年 | 180篇 |
1972年 | 221篇 |
1971年 | 203篇 |
1970年 | 112篇 |
1969年 | 134篇 |
1968年 | 138篇 |
1967年 | 155篇 |
1966年 | 130篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Water soluble polysaccharides from Norway spruce, Scots pine, and Siberian larch were compared. For all species the total amount of polysaccharides isolated from the heartwood was higher than that from the sapwood. The heartwood polysaccharides had a high content of galactose and arabinose units, and some glucuronic acid units, suggesting the presence of acidic arabinogalactans. The total amounts of recovered water-soluble arabinogalactans were 1.9 mg/g for spruce heartwood, 5.3 mg/g for pine heartwood, and as much as 106 mg/g for larch heartwood. The other water-soluble polysaccharides were mainly glucomannans. The average ratio of Gal:Ara:GlcA in the water-soluble arabinogalactans of spruce heartwood was about 4.3:1:1, pine heartwood about 4.5:1:0.2, and larch heartwood about 6.7:1:0.1. The corresponding molar ratios then being about 3.6:1:0.8 for spruce, 3.8:1:0.2 for pine, and 5.6:1:0.08 for larch. Thus, the content of glucuronic acid units was especially high in the spruce heartwood arabinogalactans. The content of arabinose was slightly higher in spruce and pine than in larch heartwood arabinogalactans. 相似文献
992.
Masaki?HaradaEmail author Kohei?Komatsu Tomoyuki?Hayashi Masahiko?Karube 《Journal of Wood Science》2004,50(5):450-454
A wooden lattice floor with high stiffness and damping capacity has been developed to solve noise problems in wooden apartment houses. The lattice floor consisted of Douglas fir glulam beams with inserted steel plate joints and drift pins. To examine the structural performance of the floor, dynamic excitation and static loading tests were conducted on the full size floor. The first and second order resonance frequencies of the floor were 13.5Hz and 27.0Hz, respectively. These frequencies are similar to the peak frequency of a conventional wooden floor and the combined floor fabricated from glued laminated timber and iron. The maximum static load of the floor was 127kN. The apparent flexural rigidity was less than half the value of several floors studied in the past. However, it is considered that the stiffness is improved by constructing panels and this floor has almost equivalent performance. Relative deflection was not affected by the loading history.Part of this study was presented at the International Wood Engineering Conference, New Orleans, October 1996. 相似文献
993.
994.
Xiping?WangEmail author Robert?J.?Ross David?W.?Green Brian?Brashaw Karl?Englund Michael?Wolcott 《Wood Science and Technology》2004,37(6):531-537
Existing log grading procedures in the United States make only visual assessments of log quality. These procedures do not incorporate estimates of the modulus of elasticity (MOE) of logs. It is questionable whether the visual grading procedures currently used for logs adequately assess the potential quality of structural products manufactured from them, especially those for which MOE is of primary concern. The purpose of this study was to investigate the use of stress wave nondestructive evaluation techniques to sort red maple logs for the potential quality of lumber obtained from them. Ninety-five red maple logs were nondestructively evaluated using longitudinal stress wave techniques and sorted into four stress wave grades. The logs were then sawn into cants and lumber. The same procedure was used to obtain stress wave times in the cants and lumber. The lumber specimens were then dried and graded using a transverse vibration technique. The results of this study showed that good relationships existed between stress wave times measured in logs, cants, and the lumber produced from the logs. It was found that log stress wave grades have positive relationships with the lumber grades. Logs with high stress wave grades produced high-grade lumber. These findings indicate that the longitudinal stress wave technique has potential in sorting logs and cants for the production of high MOE products.The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. 相似文献
995.
From the isopropyl acetate fraction obtained by partition of the leaves ethanolic extract of Alchornea glandulosa, six phenolic compounds were isolated and identified. General lethality of the extract, fractions and compounds were assayed with Brine Shrimp Test (BST) and with Antifeedant Activity Bioassay (AAB) against neonate larvae of Spodoptera frugiperda. Aqueous fraction and ethyl gallate, one of the major constituents of the polar fraction, have shown the highest toxicity in BST. Ethanolic extract, isopropyl acetate fraction and gallic acid reduced significantly larval growth of S. frugiperda neonates. 相似文献
996.
D. Matthies P. Rydval R. Federl W. Wahl H. L?ffler 《Forstwissenschaftliches Centralblatt》1994,113(1):113-124
Vorliegende Arbeit beschreibt ein neuartiges Verfahren zur “In-situ-Bestimmung” der Gasleitf?higkeit von B?den. Es bedient
sich des natürlich vorkommenden, radioaktiven Edelgases Radon. Hierbei wird in einer Me?kammer, die auf dem Boden aufsitzt,
die H?he der Radonexhalation registriert. Sie ist u. a. direkt von der Gasleitf?higkeit des Bodens abh?ngig. Da das Verfahren
zerst?rungsfrei arbeitet. er?ffnen sich eine Vielzahl von Applikationsm?glichkeiten, wie z. B. Beobachtungen befahrungsbedingter
Strukturver?nderungen w?hrend und nach Holzerntema?nahmen sowie Untersuchungen zur Regeneration strukturell ver?nderter B?den.
Dies kann beliebig oft und zu beliebigen Zeitpunkten am identischen Bodenausschnitt geschehen.
The paper describes a new analytical tool for the “in situ” measurement of the gas permeability of soils. It uses on the exhalation
of the redioactive rare gas radon. A chamber covering a certain surface area on the ground registers the amount of radon exhald.
One of the factors, which directly controls the exhalation rate is the gas permeability. As this is a non-destructive method
several possibilities for applications arise, as for example observation and analysis of changes in soil structure during
and after regular timber harvesting, i.e. such caused by forest machines, as well as investigations about the regeneration
of damaged sites. These investigations can be repeated as often as desired, at any time and, moreover, on the identical spot. 相似文献
997.
V. P. Sharma T. K. Wan G. H. S. Hooper B. Würzer Klara B. Balász J. Bodor 《Journal of pest science》1969,42(1):14-15
Ohne Zusammenfassung 相似文献
998.
Pastur Guillermo Martínez Peri Pablo Luis Fernández María Cecilia Staffieri Gabriela Lencinas Mará Vanessa 《Journal of Forest Research》2002,7(3):165-174
Human commercial activities must be compatible with sustainable management to achieve species conservation in the natural
environment. After harvesting, diversity and abundance of understory species differ substantially from the pre-harvest situation.
Have knowledge of the changes that logging produces in all components of the system allows us to develop management alternatives
with reduced impacts. The objective of this work was to evaluate changes in density, biomass and diversity of understory species
along a forest shelterwood cut system management cycle in aNothofagus pumilio forest. Six forest management situations (postharvesting stands and different phases of forest development) were sampled
in a pureN. pumilio forest of Tierra del Fuego (Argentina) of middle-high site class (fromclass I toIII). Species number, density and biomass were modified substantially after a shelterwood cut. These were being greatest after
cutting and falling substantially when the forest canopy closes during theinitial growth phase, returning to values similar to this virgin forest when themature phase was reached. The impact of the forest management on understory species diversity is analyzed and discussed. The silvicultural
practices applied toN. pumilio forest in Tierra del Fuego permit the introduction of alien flora to the system, and produce great changes during the forest
management cycle.
To Celina Matteri, Itatí Ferraro, and Rosa Kofalt for their aid in taxonomical identification. To Juan Manuel Cellini, Ricardo
Vukasovic and Gregorio Martínez Pastur for their participation in the field work; and to the sawmillLos Castores, Universidad Nacional de la Patagonia Austral, Centro Austral de Investigaciones Científicas and Instituto Nacional de Tecnología
Agropecuaria for the institutional and financial support. 相似文献
999.
V. Rypáček 《Wood Science and Technology》1977,11(1):59-67
Mycelium of twenty species of wood-destroying fungi was cultivated in nutrient solutions with one of the following sugars as the only carbon source: glucose, arabinose, galactose, mannose and xylose. The growth of the species occurring in nature only on the wood of Angiosperms was equal or lower in the solutions with galactose and mannose than in the solution with glucose. The utilization of mannose was practically the same as that of xylose. The species which are specialized in nature in the wood of conifers grow better in the solutions with galactose and mannose, than in the solution with glucose; likewise mannose is beter utilized than xylose. Fungi attacking both kinds of wood do not exhibit any dependence on the utilization of the sugars used in these investigations. 相似文献
1000.