全文获取类型
收费全文 | 701篇 |
免费 | 56篇 |
出版年
2021年 | 4篇 |
2019年 | 7篇 |
2018年 | 10篇 |
2017年 | 7篇 |
2016年 | 15篇 |
2015年 | 20篇 |
2014年 | 27篇 |
2013年 | 33篇 |
2012年 | 28篇 |
2011年 | 28篇 |
2010年 | 42篇 |
2009年 | 36篇 |
2008年 | 34篇 |
2007年 | 37篇 |
2006年 | 32篇 |
2005年 | 27篇 |
2004年 | 28篇 |
2003年 | 19篇 |
2002年 | 16篇 |
2001年 | 21篇 |
2000年 | 17篇 |
1999年 | 15篇 |
1998年 | 16篇 |
1997年 | 17篇 |
1996年 | 8篇 |
1995年 | 12篇 |
1994年 | 7篇 |
1993年 | 10篇 |
1992年 | 13篇 |
1991年 | 10篇 |
1989年 | 4篇 |
1988年 | 12篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 13篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 17篇 |
1981年 | 3篇 |
1979年 | 4篇 |
1977年 | 7篇 |
1976年 | 9篇 |
1975年 | 4篇 |
1973年 | 8篇 |
1972年 | 3篇 |
1971年 | 3篇 |
1951年 | 4篇 |
1946年 | 4篇 |
1923年 | 2篇 |
1921年 | 3篇 |
排序方式: 共有757条查询结果,搜索用时 968 毫秒
101.
J. E. Gilchrist J. E. Campbell C. B. Donnelly J. T. Peeler J. M. Delaney 《Applied microbiology》1973,25(2):244-252
A method is described for determining the number of bacteria in a solution by the use of a machine which deposits a known volume of sample on a rotating agar plate in an ever decreasing amount in the form of an Archimedes spiral. After the sample is incubated, different colony densities are apparent on the surface of the plate. A modified counting grid is described which relates area of the plate of volume of sample. By counting an appropriate area of the plate, the number of bacteria in the sample is estimated. This method was compared to the pour plate procedure with the use of pure and mixed cultures in water and milk. The results did not demonstrate a significant difference in variance between duplicates at the α = 0.01 level when concentrations of 600 to 12 × 105 bacteria per ml were used, but the spiral plate method gave counts that were higher than counts obtained by the pour plate method. The time and materials required for this method are substantially less than those required for the conventional aerobic pour plate procedure. 相似文献
102.
103.
104.
A S Gilchrist L Partridge 《Proceedings. Biological sciences / The Royal Society》1997,264(1386):1271-1275
Sperm displacement has been the subject of a large number of evolutionary studies because of its effects on relative male reproductive success. To understand better the evolutionary role of variation in sperm displacement ability (SDA), an obvious aim is to measure its heritability. In this paper, we show that a standard method used to measure the heritability of SDA can be misleading. First, we show that using conventional methods (based on counts of adult offspring of multiply mated females), SDA appears to be heritable. However, an examination of potentially confounding variables strongly suggests that this result is misleading, and that the heritable component is more likely to be pre-adult viability. Consequently, it is likely that there is little measurable heritable genetic variation for SDA in D. melanogaster. We conclude that, although conventional methods of measuring sperm displacement will usually be adequate for phenotypic measurements, greater care must be taken when measuring genetic variances. 相似文献
105.
ABSTRACTSleep deprivation impairs performance on cognitive tasks, but it is unclear which cognitive processes it degrades. We administered a semantic matching task with variable stimulus onset asynchrony (SOA) and both speeded and self-paced trial blocks. The task was administered at the baseline and 24 hours later after 30.8 hours of total sleep deprivation (TSD) or matching well-rested control. After sleep deprivation, the 20% slowest response times (RTs) were significantly increased. However, the semantic encoding time component of the RTs remained at baseline level. Thus, the performance impairment induced by sleep deprivation on this task occurred in cognitive processes downstream of semantic encoding. 相似文献
106.
Temperature is one of the main factors that determine sexual reproduction in terrestrial and emergent aquatic plant species. The effect of temperature on sexual reproduction and seed production of Glyceria maxima (Hartm.) Holmb. in the southern hemisphere is unknown. Glyceria maxima collections in February 2010 at three isolated infestations in KwaZulu-Natal failed to yield a single seed, only empty panicles. Laboratory experiments showed that vernalisation had no consistent effect on seed production. Field- and laboratory-grown plants produced seeds in the 2010/2011 season, because of having sufficient time at optimum temperatures required for seed production (1 491 and 1 585 hours, respectively), compared to a shorter period (1 352 hours) of suitable temperatures during the 2009/2010 growing season. An inadequate period of optimum temperatures (15–25°C) during seed production resulted in the lack of seeds in the field in the 2009/2010 growing season. This study showed that temperature and duration of exposure thereto during the seed-production period play vital roles in G. maxima sexual reproduction. 相似文献
107.
The partitioning of limited resources commonly explains how different species can coexist within the same ecological community. In this 2010 study, the diets of three coexisting freshwater fishes (Cape galaxias Galaxias zebratus, n = 27; Cape kurper Sandelia capensis, n = 60; Breede River redfin Pseudobarbus burchelli, n = 77) were characterised and compared in three headwater streams in South Africa's Cape Fold Ecoregion using gut contents and stable isotope analyses. These data were analysed to ascertain whether the three species exploit distinct trophic niches. Both approaches provided evidence that these species occupy different trophic niches, though with some overlap. However, dietary differences among sites were not consistent and were probably influenced by site-specific factors like resource availability. Pseudobarbus burchelli had a broader niche breadth at Tierkloof Stream than the other two species, but not at Waaihoek or Tierstel Streams. Our results also suggest that P. burchelli consumed a more omnivorous diet than do the other two species, whereas S. capensis occupied a higher trophic position than the other two species and consumed vertebrates. Our findings suggest that these species occupy non-equivalent feeding niches in Cape Fold Ecoregion headwater streams, and that diet partitioning might facilitate their coexistence in these systems. 相似文献
108.
Gilchrist James S.C. Palahniuk Chris Bose Ratna 《Molecular and cellular biochemistry》1997,172(1-2):159-170
In this report we describe the application of spectroscopic methods to the study of Ca2+ release by isolated native sarcoplasmic reticulum (SR) membranes from rabbit skeletal muscle. To date, dual-wavelength spectroscopy of arsenazo III and antipyrylazo III difference absorbance have been the most common spectroscopic methods for the assay of SR Ca2+ transport. The utility of these methods is the ability to manipulate intraluminal Ca2+ loading of SR vesicles. These methods have also been useful for studying the effect of both agonists and antagonists upon SR Ca2+ release and Ca2+ uptake. In this study, we have developed the application of Calcium Green-2, a long-wavelength excitable fluorescent indicator, for the study of SR Ca2+ uptake and release. With this method we demonstrate how ryanodine receptor Ca2+ channel opening and closing is regulated in a complex manner by the relative distribution of Ca2+ between extraluminal and intraluminal Ca2+ compartments. Intraluminal Ca2+ is shown to be a key regulator of Ca2+ channel opening. However, these methods also reveal that the intraluminal Ca2+ threshold for Ca2+-induced Ca2+ release varies as a function of extraluminal Ca2+ concentration. The ability to study how the relative distribution of a finite pool of Ca2+ across the SR membrane influences Ca2+ uptake and Ca2+ release may be useful for understanding how the ryanodine receptor is regulated, in vivo. 相似文献
109.
L-Lactate dehydrogenase (L-LDH, E.C. 1.1.1.27) is encoded by two or three
loci in all vertebrates examined, with the exception of lampreys, which
have a single LDH locus. Biochemical characterizations of LDH proteins have
suggested that a gene duplication early in vertebrate evolution gave rise
to Ldh-A and Ldh-B and that an additional locus, Ldh-C arose in a number of
lineages more recently. Although some phylogenetic studies of LDH protein
sequences have supported this pattern of gene duplication, others have
contradicted it. In particular, a number of studies have suggested that
Ldh-C represents the earliest divergence among vertebrate LDHs and that it
may have diverged from the other loci well before the origin of
vertebrates. Such hypotheses make explicit statements about the
relationship of vertebrate and invertebrate LDHs, but to date, no closely
related invertebrate LDH sequences have been available for comparison. We
have attempted to provide further data on the timing of gene duplications
leading to multiple vertebrate LDHs by determining the cDNA sequence of the
LDH of the tunicate Styela plicata. Phylogenetic analyses of this and other
LDH sequences provide strong support for the duplications giving rise to
multiple vertebrate LDHs having occurred after vertebrates diverged from
tunicates. The timing of these LDH duplications is consistent with data
from a number of other gene families suggesting widespread gene duplication
near the origin of vertebrates. With respect to the relationships among
vertebrate LDHs, our data are not consistent with previous claims that
Ldh-C represented the earliest divergence. However, the precise
relationships among some of the main lineages of vertebrate LDHs were not
resolved in our analyses.
相似文献
110.
Paul M. Lehrer Stuart M. Hochron Tracy Mayne Susan Isenberg Ann Marie Lasoski Victor Carlson James Gilchrist Stephen Porges 《Applied psychophysiology and biofeedback》1997,22(3):183-191
This paper reports the relationships among changes in cardiovagal activity, surface EMG, and measures of pulmonary function in a study of relaxation therapy for asthma. Changes in FEV
1
/FVC were negatively correlated with those in cardiac interbeat interval, consistent with the hypothesis that relaxation-induced changes in airway function are mediated autonomically, with increased vagal tone and/or decreased sympathetic arousal producing bronchoconstriction. Contrary to Kotses's theory of a vagal-trigeminal reflex as mediator for relaxation-induced improvement in asthma, decreases in pulmonary function occurred during relaxation sessions, accompanied by increases in cardiovagal activity, and within-session changes in frontal EMG in the first session of training were positively associated with changes in a measure of pulmonary function (FEV1/FVC). However, consistent with this hypothesis, first-session frontalis EMG changes were positively associated with changes in respiratory sinus arrhythmia, and last-session changes in cardiac interbeat interval were positively associated with changes in FEV1/FVC. The results suggest that the immediate effects of generalized relaxation instruction can be associated with a parasympathetic rebound, which, in turn, may induce countertherapeutic changes in asthma. However, the effects of specific facial muscle relaxation remain uncelar. 相似文献