全文获取类型
收费全文 | 97230篇 |
免费 | 705篇 |
国内免费 | 893篇 |
专业分类
98828篇 |
出版年
2022年 | 42篇 |
2021年 | 67篇 |
2020年 | 52篇 |
2019年 | 60篇 |
2018年 | 11893篇 |
2017年 | 10713篇 |
2016年 | 7534篇 |
2015年 | 796篇 |
2014年 | 486篇 |
2013年 | 539篇 |
2012年 | 4494篇 |
2011年 | 13100篇 |
2010年 | 12203篇 |
2009年 | 8393篇 |
2008年 | 9996篇 |
2007年 | 11575篇 |
2006年 | 487篇 |
2005年 | 725篇 |
2004年 | 1211篇 |
2003年 | 1226篇 |
2002年 | 937篇 |
2001年 | 337篇 |
2000年 | 229篇 |
1999年 | 103篇 |
1998年 | 82篇 |
1997年 | 84篇 |
1996年 | 54篇 |
1995年 | 35篇 |
1994年 | 61篇 |
1993年 | 68篇 |
1992年 | 66篇 |
1991年 | 81篇 |
1990年 | 48篇 |
1989年 | 39篇 |
1988年 | 48篇 |
1987年 | 53篇 |
1986年 | 22篇 |
1985年 | 34篇 |
1984年 | 24篇 |
1983年 | 35篇 |
1982年 | 23篇 |
1980年 | 16篇 |
1979年 | 13篇 |
1978年 | 17篇 |
1975年 | 14篇 |
1974年 | 13篇 |
1972年 | 256篇 |
1971年 | 280篇 |
1965年 | 14篇 |
1962年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
It is widely believed that environmental variability is the main cause for fluctuations in commercially exploited small pelagic
fish populations around the world. Nevertheless, density-dependent factors also can drive population dynamics. In this paper,
we analyzed thirteen years of a relative abundance index of two clupeoids fish populations coexisting in the central-south
area off Chile, namely the common sardine, Strangomera bentincki, and anchovy, Engraulis ringens. We applied the classical diagnostic tools of time series analysis to the observed time-series. Also, the realized per capita
population growth rate was studied with the aim of detecting the feedback structure that is characterizing the population
dynamics of the two species. The analysis suggests that population fluctuations of the two species have an important density-dependent
component, displaying first-order (direct density-dependent) and second-order (delayed density-dependent) simultaneously.
The density-dependent component explained 70.5 and 55.6 % of the realized per capita population growth rate of common sardine
and anchovy, respectively. The deterministic skeleton model showed an asymptotic convergence to equilibrium density. In presence
of a stochastic environment, fluctuations were reproduced for the species showing a component of fluctuation with a period
of 4 year. The intrinsic dynamics of each species is typical of interacting species resulting from trophic interactions. It
is postulated that the second-order dynamics of S. bentincki and E. ringens in central-south Chile, may be the result from interactions with a specialist predator (the fishing fleet), interacting with
exogenous environmental factors. 相似文献
62.
Jacek Kęsy Beata Maciejewska Magdalena Sowa Magdalena Szumilak Krzysztof Kawałowski Maja Borzuchowska Jan Kopcewicz 《Plant Growth Regulation》2008,55(1):43-50
It has been shown that both IAA and ethylene application inhibit flower induction in the short-day plant Pharbitis nil. However application of IAA has elevated ethylene production in this plant, as well. Strong enhancement of ethylene production
is also correlated with the night-break effect, which completely inhibits flowering. In order to determine what the role of
IAA and ethylene is in the photoperiodic flower induction in Pharbitis nil, we measured changes in their levels during inductive and non-inductive photoperiods, and the effects of ethylene biosynthesis
and action inhibitors on inhibition of flowering by IAA. Our results have shown that the inhibitory effect of IAA on Pharbitis nil flowering is not physiological but is connected with its effect on ethylene biosynthesis. 相似文献
63.
64.
65.
66.
Apolipoprotein E has key functions in lipoprotein metabolism, and polymorphisms in the apolipoprotein E gene are associated with distinct lipoprotein patterns. The possibility of gene-nutrient interactions for apolipoprotein E has been addressed in many studies. Although results have generally been mixed, the indications for such an interaction have been more common in studies employing a metabolic challenge. Studies directly designed to examine apolipoprotein E gene-nutrient interactions are needed. 相似文献
67.
Cultured circular smooth muscle from the rabbit colon 总被引:1,自引:0,他引:1
H. W. Kao S. E. Finn A. M. Gown J. Lechago N. Lachant W. J. Snape Jr. 《In vitro cellular & developmental biology. Plant》1988,24(8):787-794
Summary Although cultured vascular smooth muscle cells have been extensively characterized and investigated, there are very few studies
of cultured intestinal smooth muscle cells. The aim of this study was to culture colonic smooth muscle (CSM) cells from the
rabbit colon. Freshly isolated CSM cells from the circular muscle layer of the distal colon were prepared by collagenase digestion.
In primary culture, CSM cells attached to the culture vessels by 48 to 72 h, proliferated by 3 to 7 d, and reached confluency
by 14 to 17 d with a “hill-and-valley” pattern. Spontaneous contractions were not observed at any time at 21° or 37° C. Confluent
primary cultures were greater than 95% CSM cells, as identified by intensely positive immunofluorescent staining to smooth
muscle actin-specific CGA7 and muscle-specific HHF-35 monoclonal antibodies. Transmission electron microscopy of freshly isolated
and proliferating CSM cells revealed ultrastructural features consistent with smooth muscle cells. We successfully cultured
CSM cells of the rabbit from freshly isolated cells and validated these CSM cells by electron microscopy and immunocytochemical
staining. These highly pure primary cultures may be used to investigate numerous aspects of CSM cell metabolism and physiology.
These studies were supported by the National Institutes of Health grant to the Inflammatory Bowel Disease Center (Bethesda,
MD) P30-AM-32200 and R01-DK-31147. Dr. Kao is the recipient of a Research Career Development Award from the National Foundation
for Ileitis and Colitis, Inc. A preliminary report of this work was presented at the American Motility Society Meeting, Houston,
TX, in October 1986, and appeared in abstract form inGastroenterology 91: 1057; 1986. 相似文献
68.
C-phycocyanin from two strains of the thermotolerant blue-green alga, Mastigocladus laminosus (NZ-DB2-m and I-30-m), that grow within different temperature ranges have been characterized with respect to aggregation, immunologic properties, subunit composition, and thermodenaturation. The critical thermal-denaturation temperature for phycocyanin from both strains of M. laminosus phycocyanin is 60 degrees C which is higher than that for mesophilic phyococyanin. Immunodiffusion studied have shown that these two strains of M. laminosus exhibit no antigenic differences and are closely related to the mesophilic Plectonema calothricoides and the thermophilic Synechococcus lividus (strains 3). Neither phenol nor alpha-naphthol has any effect on phycocyanin aggregation in these two strains of M. laminosus. There is also no enhancement of formation of large aggregates at their elevated temperature of cultivation. Furthermore, the phycocyanin of both strains of M. laminosus does not demonstrate any large amount of 19S or higher aggregates at any pH value. These observations suggest that the mode of adaptation of M. laminosus phycocyanin to high temperature is differnet from the previously encountered. It is also important to note that phycocyanin is essentially unchanged whether it is extracted from the same strain, M. laminosus (NZ-DBS-m), grown at either 50 degrees C or 37 degrees C. 相似文献
69.