全文获取类型
收费全文 | 1274823篇 |
免费 | 109503篇 |
国内免费 | 1481篇 |
专业分类
1385807篇 |
出版年
2021年 | 17727篇 |
2020年 | 12572篇 |
2019年 | 16126篇 |
2018年 | 17236篇 |
2017年 | 16005篇 |
2016年 | 27717篇 |
2015年 | 42143篇 |
2014年 | 50162篇 |
2013年 | 76362篇 |
2012年 | 34000篇 |
2011年 | 21828篇 |
2010年 | 42391篇 |
2009年 | 44165篇 |
2008年 | 21376篇 |
2007年 | 18870篇 |
2006年 | 24808篇 |
2005年 | 25835篇 |
2004年 | 25159篇 |
2003年 | 22837篇 |
2002年 | 20993篇 |
2001年 | 29021篇 |
2000年 | 26096篇 |
1999年 | 27361篇 |
1998年 | 24841篇 |
1997年 | 24686篇 |
1996年 | 24322篇 |
1995年 | 22339篇 |
1994年 | 22285篇 |
1993年 | 21283篇 |
1992年 | 24226篇 |
1991年 | 22818篇 |
1990年 | 21397篇 |
1989年 | 22546篇 |
1988年 | 20541篇 |
1987年 | 19548篇 |
1986年 | 18576篇 |
1985年 | 20617篇 |
1984年 | 20893篇 |
1983年 | 18510篇 |
1982年 | 19639篇 |
1981年 | 19062篇 |
1980年 | 17749篇 |
1979年 | 17037篇 |
1978年 | 16403篇 |
1977年 | 16061篇 |
1976年 | 15275篇 |
1975年 | 14581篇 |
1974年 | 15024篇 |
1973年 | 15382篇 |
1972年 | 12887篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
The forty-two days long experiment was conducted on a total of 1000 Arbor Acres broilers, divided into two groups. Both groups
of broilers were fed with a commercial feed mixture which consisted of standard feedstuffs and contained enough nutrients
regarding the requirements. During the first three weeks of the trial, corn naturally contaminated with AFB1 0.0445 ppm per kg dry matter was used in the amount of 20% in the experimental group, while AFB1 free corn was given to birds in the control group. After the period of toxin administration until the end of the trial, broilers
from both groups were normally fed another 5 weeks with uncontaminated feed.
In the first phase of the trial, broilers in the control groups had an average daily gain (ADG) of 31 g, average daily feed
intake (ADFI) of 45 g and a feed: gain ratio (FCR) of 1.42. In the same time, experimental broilers achieved an ADG of 25
g, ADFI of 35 g and a FCR of 1.39. During the whole period of the trial, control and experimental broilers achieved 49.12
g, 95.24 g and 1.94 and 39.71 g, 86.90 g and 2.19, respectively. 相似文献
962.
Elena C. Guzman Alfonso Jimenez-Sanchez Elisha Orr Robert H. Pritchard 《Molecular & general genetics : MGG》1988,212(2):203-206
Summary A temperature shift-up accompanied by a reduction in RNA polymerase activity in Escherichia coli causes an increased rate of initiation leading to a 1.7- to 2.2-fold increase in chromosome copy number. A temperature shift-up without a reduction in polymerase activity induces only a transient non-scheduled initiation of chromosome replication caused by heat shock with no detectable effect on chromosome copy number. 相似文献
963.
STEVEN C. BEADLE 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1988,21(1):1-12
The cyclocrinitids are an extinct tribe of dasycladacean green algae. They were anatomically very similar to certain Recent dasyclads, even at early growth stages. The morphology and preservation of cyclocrinitids strongly suggest that they had a siphonous cellular organization with extracellular, aragonitic calcification; these features are characteristic of living dasyclads. The light surficial calcification of cyclocrinitids and other dasyclads had important paleoecological effects. It restricted them to low-energy waters, as it provided relatively little structural support. It also confined them to warm, tropical waters; they are good paleoequatorial indicators. The decline of these algae during the late Ordovician and early Silurian may therefore reflect the simultaneous cooling and glaciation. Receptaculitids are entirely unrelated organisms. Their meroms have several distinctive features; they are not homologous to the lateral branches of cyclocrinitids or dasyclads. Receptaculitid calcification was extensive and their thalli were apparently quite sturdy; they often occurred in reefs. Receptaculitids also lived in high-latitude, cold-water environments. Thus, they were ecologically unlike any calcareous green algae, and cannot be used as paleoequatorial indicators. Receptaculitids remain problematical, although the arrangement of meroms suggests plant affinities. □ Calcareous algae, Problematica, Dasycladales, Cyclocriniteae, Receptaculitales, morphology, classification, paleoecology, paleogeography . 相似文献
964.
965.
Microsporidia 2003: IWOP-8 总被引:1,自引:0,他引:1
LOUIS M. WEISS 《The Journal of eukaryotic microbiology》2003,50(S1):566-568
966.
On the measurement of growth with applications to the modelling and analysis of plant growth 总被引:3,自引:0,他引:3
M. L. Roderick 《Functional ecology》2000,14(2):244-251
967.
Human long-latency auditory evoked potentials were studied during simulation with variable-amplitude pulse sequences from a sound source moving to and from the subject. The N1 peak parameters were shown to depend on an accurate estimate of the direction of the change in the distance to the sound source. Differences in the processing of signals that simulated the approaching and/or distancing of the sound source were found in the N1 and P2 component parameters of on- and off-responses as was a more pronounced long negative potential shift in the evoked response to the approaching source as compared to the distancing source. 相似文献
968.
Yeast cell mortality related to a high-pressure shift: occurrence of cell membrane permeabilization 总被引:3,自引:0,他引:3
The shrinkage of yeast cells caused by high-pressure treatment (250 MPa, 15 min) was investigated using direct microscopic observation. A viable staining method after treatment allowed the volume variation of two populations to be distinguished: an irreversible volume decrease (about 35% of the initial volume) of pressure-inactivated cells during pressure holding time, and viable cells, which were less affected. A mass transfer was then induced during high-pressure treatment. Causes of this transfer seem to be related to a pressure-induced membrane permeabilization, allowing a subsequent leakage of internal solutes, where three ions (Na+, K+ and Ca2+), plus endogenous glycerol, were verified. This glycerol leakage was found to occur after yeast pressurization in a medium having low water activity, although the yeast was not inactivated. All these observations lead to the hypothesis that pressure-induced cell permeabilization could be the cause of yeast inactivation under pressure. 相似文献
969.
970.
J. Dierschke T. Krüger F. Hüttmann F. Bairlein Kerstin Müller G. Scheiffarth H. -W. Helb W. Irsch W. Lantermann B. Haubitz W. Winkel und J. Haffer 《Journal of Ornithology》2003,144(4):484-498
Ohne Zusammenfassung 相似文献