全文获取类型
收费全文 | 2962篇 |
免费 | 144篇 |
国内免费 | 13篇 |
出版年
2017年 | 26篇 |
2016年 | 25篇 |
2015年 | 34篇 |
2014年 | 36篇 |
2013年 | 71篇 |
2012年 | 95篇 |
2011年 | 74篇 |
2010年 | 76篇 |
2009年 | 120篇 |
2008年 | 97篇 |
2007年 | 74篇 |
2006年 | 69篇 |
2005年 | 76篇 |
2004年 | 64篇 |
2003年 | 46篇 |
2002年 | 45篇 |
2001年 | 32篇 |
2000年 | 32篇 |
1999年 | 38篇 |
1998年 | 42篇 |
1997年 | 49篇 |
1996年 | 47篇 |
1995年 | 31篇 |
1994年 | 30篇 |
1993年 | 23篇 |
1992年 | 44篇 |
1991年 | 22篇 |
1990年 | 32篇 |
1989年 | 36篇 |
1988年 | 23篇 |
1987年 | 28篇 |
1986年 | 20篇 |
1985年 | 23篇 |
1984年 | 29篇 |
1983年 | 23篇 |
1981年 | 27篇 |
1980年 | 20篇 |
1979年 | 23篇 |
1977年 | 18篇 |
1959年 | 51篇 |
1958年 | 124篇 |
1957年 | 139篇 |
1956年 | 136篇 |
1955年 | 149篇 |
1954年 | 151篇 |
1953年 | 119篇 |
1952年 | 104篇 |
1951年 | 99篇 |
1950年 | 59篇 |
1948年 | 23篇 |
排序方式: 共有3119条查询结果,搜索用时 15 毫秒
151.
152.
P H Crowley 《Journal of theoretical biology》1979,80(1):129-144
I have constructed a spatially distributed analytical model of predators, superior prey competitors, and inferior prey competitors, based on the limiting deterministic version of a simulation model by Caswell (1978). Persistence regions for the three populations are mapped in parameter space. Conceptually shrinking the system from infinite size (i.e. infinitely many spatial “cells”) to some finite size introduces demographic stochasticity, increasing the chance of extinction of one or more populations within a given time interval. But some of the finite (stochastic) system's behavior, such as any tendency to damp perturbations, can be related to the behavior of the deterministic system at the same location in parameter space. 相似文献
153.
J. A. DE GREEF F. VEROUSTRAETE L. VAN WIEMEERSCH H. FREDERICQ 《Physiologia plantarum》1979,47(1):44-48
A mathematical analysis for fast changes of ethylene concentration in an open flow system (non-steady-state conditions) is presented and experimentally tested. In this way it becomes possible to determine true values of ethylene production in the minute range following physiological and environmental changes which influence ethylene evolution. By this procedure ethylene kinetics can also be compared in absolute values independent of flow rate and plant chamber volume. 相似文献
154.
155.
Treatment of Cucumis leaf discs with light and low temperature (1°C) resulted in degradation of the total lipids. In addition, a decrease of the linolenic acid content of the glycolipids of leaves and leaf discs took place while at the same time an increase in the content of unidentified degradation products from the glycolipids was observed. The decrease of the linolenic acid content was not due to galactolipase activity, since no monogalactosyl diglyceride acyl hydrolase activity was found. Dark and low temperature did not alter the fatty acid composition. Blue light and low temperature resulted in a decrease of the linolenic acid content, while yellow-, red- and far red light in combination with low temperature were ineffective. 相似文献
156.
157.
Behavior can be regarded as a result of various processes ofdecision based on the information provided by the sensory organs.In this review the role of the so-called additional heat sense,next to vision, smell and mechanoreception is discussed withrespect to the feeding behavior of snakes. The hierarchy ofthe sensory information in various phases of the feeding behaviordiffers between snakes possessing heat receptors (e.g., speciesof the Crotalinae and Pythoninae) and those without (e.g., speciesof the Viperinae and Colubrinae). Probably depending on theinfluence of ecological demands, visual or chemical cues arethe main information in the behavioral phases before the strikeHowever, in situations with little visual input, e.g., in darkness,rodents' burrows, etc., hunting behavior is guided in the firstplace by radiation of warm objects in Crotalus, Python and Trimeresurusflavoviridis, and by substrate vibrations in Vipera aspis, Pituophismelanoleucus and Boa constrictor. I suggest that in the sensoryhierarchy, heat information functionally replaces the mechanicalinformation which is utilized by snakes without pit organs.Poststrike behavior on the other hand is mainly guided by chemicalcues in all snakes. 相似文献
158.
Characteristics of the Wave of Depolarization Induced by Wounding in Bidens pilosa L. 总被引:2,自引:0,他引:2
JULIEN J. L.; DESBIEZ M. O.; DE JAEGHER G.; FRACHISSE J. M. 《Journal of experimental botany》1991,42(1):131-137
Heating locally the hypocotyl of Bidens pilosa L. elicits awave of depolarization. The mechanism of the wave has been investigatedby means of microelectrophysiological techniques. The amplitudeof the transmembrane potential variation induced by an extracellularion concentration change (K+, Na+, Ca2+, Cl) was thesame in the resting conditions as during the slow wave. At pH4.0, the amplitude of the slow wave was reduced by 56% comparedwith the control performed at pH 7.0. In the presence of theuncoupler CCCP, the slow wave was not observed. The Ca2+ -chelatorEGTA and the Ca22+ -channel blocker La3+ reduced, respectively,the amplitude of the slow wave by 78% and 68%. These resultsindicate the involvement of Ca2+ in triggering the slow wave.A transient modification of the electrogenic H+ pump activity(inactivation-activation) and of the transmembrane H+ flux inthe slow wave are discussed. Key words: Slow wave (of depolarization), wounding, electrogenic pump, calcium, Bidens pilosa L 相似文献
159.
IZAGUIRRE-MAYORAL MARIA LUISA; CARBALLO O.; DE UZCTEGUI RAFAELA C.; DE MALLORCA MARGARITA S. 《Journal of experimental botany》1992,43(4):455-462
Three-day-old cowpea seedlings were inoculated with the severestrain of Cowpea Mosaic Virus (CpMV) and 24 h later with Bradyrhizobiumsp. cowpea, strain I-125, when virus translocation to rootsstill had not taken place. Plants were harvested at 22, 30,45 and 59 d after germination. Active virus replication wasassociated with increased protein content, detected in the leavesof 22-d-old plants. CpMV infection reduced the total leaf area,dry weight of shoots and the chlorophyll content of the firsttrifoliolate leaf at all experimental times. Low sugar contentwas recorded in leaves of 22- and 30-d-old CpMV-infected plantsand in nodules and roots of 30- and 45-d-old CpMV-infected plants.Up to 45 d, the nodule mass of CpMV-infected plants was lowerthan in controls, but reached control values at the 4th harvest.In CpMV-infected nodules, massive agglomeration of virus particles,crystalline virus inclusions and the proliferation of the endoplasmicreticulum were observed only in those cells containing bacteroids.In 30- and 45-d-old plants, CpMV infection decreased the contentof ureides in nodules, roots, and petioles. Virus infectiondid not alter the -amino-N content of roots and nodules butinduced a transient 74% reduction in the level of -amino-N inpetioles of 45-d-old plants. At the 1st and 2nd harvests theactivity of uricase (EC 1.7.3.3
[EC]
) in the nodules and of pyruvatekinase (EC 2.7.1.40
[EC]
) in the nodules and leaves were decreasedseverely by virus infection. CpMV did not hinder the allantoinase(EC 3.5.2.5
[EC]
) activity in the leaves but caused a 9% transitorydecrease in the activity of this enzyme in nodules of 45-d-oldplants. Measurements of NAD-malic enzyme (EC 1.1.1.38
[EC]
) in nodulesalso showed the non-effect of CpMV on this enzyme, except fora temporary 16% reduction at the 2nd harvest. As compared tocontrols, the relative abundance of ureides in 30-d-old CpMV-infectedplants indicated a 15%, 10%, and 51% reduction in the nodules,roots, and petioles, respectively. Results indicate that atthe time of the 4th harvest the symbiotic process, measuredin terms of ureide content and enzymatic activities, was functioningat a near normal level despite nodule infection by CpMV. Key words: Cowpea Mosaic Virus, nitrogen fixation, cowpea, enzymes, ultrastructure 相似文献
160.
A method is developed for obtaining tree-structured relative risk estimates for censored survival data. The first step of a full likelihood estimation procedure is used in a recursive partitioning algorithm that adopts most aspects of the widely used Classification and Regression Tree (CART) algorithm of Breiman et al. (1984, Classification and Regression Trees, Belmont, California: Wadsworth). The performance of the technique is investigated through stimulation and compared to the tree-structured survival methods proposed by Davis and Anderson (1989, Statistics in Medicine 8, 947-961) and Therneau, Grambsch, and Fleming (1990, Biometrika 77, 147-160). 相似文献