全文获取类型
收费全文 | 2630篇 |
免费 | 229篇 |
专业分类
2859篇 |
出版年
2017年 | 27篇 |
2016年 | 33篇 |
2015年 | 64篇 |
2014年 | 60篇 |
2013年 | 87篇 |
2012年 | 117篇 |
2011年 | 105篇 |
2010年 | 96篇 |
2009年 | 68篇 |
2008年 | 101篇 |
2007年 | 113篇 |
2006年 | 75篇 |
2005年 | 77篇 |
2004年 | 69篇 |
2003年 | 73篇 |
2002年 | 69篇 |
2001年 | 58篇 |
2000年 | 57篇 |
1999年 | 55篇 |
1998年 | 34篇 |
1997年 | 38篇 |
1996年 | 40篇 |
1995年 | 28篇 |
1994年 | 30篇 |
1993年 | 35篇 |
1992年 | 52篇 |
1991年 | 61篇 |
1990年 | 59篇 |
1989年 | 51篇 |
1988年 | 48篇 |
1987年 | 57篇 |
1986年 | 46篇 |
1985年 | 44篇 |
1984年 | 39篇 |
1983年 | 29篇 |
1982年 | 27篇 |
1980年 | 30篇 |
1979年 | 34篇 |
1976年 | 24篇 |
1975年 | 32篇 |
1974年 | 25篇 |
1973年 | 34篇 |
1972年 | 25篇 |
1968年 | 28篇 |
1965年 | 39篇 |
1963年 | 33篇 |
1962年 | 25篇 |
1961年 | 34篇 |
1959年 | 28篇 |
1958年 | 28篇 |
排序方式: 共有2859条查询结果,搜索用时 15 毫秒
141.
Lectin-binding sites on the plasma membranes of rabbit spermatozoa: Changes in surface receptors during epididymal maturation and after ejaculation 总被引:2,自引:5,他引:2
下载免费PDF全文
![点击此处可从《The Journal of cell biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
GL Nicolson N Usui R Yanagimachi H Yanagimachi Smith JR 《The Journal of cell biology》1977,74(3):950-962
Modifications in rabbit sperm plasma membranes during epididymal passage and after ejaculation were investigated by used of three lectins: concanavalin A (Con A); Ricinus communis I (RCA(I)); and wheat germ agglutinin (WGA). During sperm passage from caput to cauda epididymis, agglutination by WGA drastically decreased, and agglutination by RCA(I) slightly decreased, although agglutination by Con A remained approximately unchanged. After ejaculation, spermatozoa were agglutinated to a similar degree or slightly less by Con A, WGA, and RCA(I), compared to cauda epididymal spermatozoa. Ultrastructural examination of sperm lectin-binding sites with ferritin- lectin conjugates revealed differences in the densities of lectin receptors in various sperm regions, and changes in the same regions during epididymal passage and after ejaculation. Ferritin-RCA(I) showed abrupt changes in lectin site densities between acrosomal and postacrosomal regions of sperm heads. The relative amounts of ferritin-RCA(I) bound to heads of caput epididymal or ejaculated spermatozoa. Tail regions were labeled by ferritin RCA(I) almost equally on caput and cauda epididymal spermatozoa, but the middle-piece region of ejaculated spermatozoa was slightly more densely labeled than the principal-piece region, and these two regions on ejaculated spermatozoa were labeled less than on caput and cuada epididymal spermatozoa. Ferritin-WGA densely labeled the acrosomal region of caput epididymal spermatozoa, although labeling of cauda epidiymal spermatozoa was relatively sparse except in the apical area of the acrosomal region. Ejaculated spermatozoa bound only a few molecules of ferritin-WGA, even at the highest conjugate concentrations used. Caput epididymal, but not cauda epididymal or ejaculated spermatozoa, bound ferritin-WGA in the tail regions. Dramatic differences in labeling densities during epididymal passage and after ejaculation were not found with ferritin-Con A. 相似文献
142.
143.
1. In situ measurements of nitrogen fixation rates for Aphanizomenon in fertile Colorado lakes with low inorganic nitrogen concentrations demonstrated high efficiency of nitrogen fixation at low irradiance. 2. For study populations, rates of N2 fixation in darkness and with alternating exposure to light and darkness were a higher percentage of light‐saturated rates for Aphanizomenon than for Anabaena, suggesting storage of reduced metabolites at high irradiance that are used subsequently by Aphanizomenon when cells are forced by mixing into zones of low irradiance. Also, saturation of N2 fixation occurred over a lower range of irradiance for Aphanizomenon than for Anabaena. 3. High efficiency of N2 fixation in Aphanizomenon at low or fluctuating irradiance is complementary to its previously demonstrated high efficiency of photosynthesis at low irradiance. Nitrogen fixation rate was also strongly related to DIN concentration; fixation was highest at low DIN (maximum < 5 μg L?1) but was also most vulnerable to photoinhibition under such conditions. 4. The fixation capabilities of Aphanizomenon under weak or varying irradiance could explain its commonly observed domination over Anabaena when transparency is low and available nitrogen is scarce. 相似文献
144.
145.
Katja Fennel Damian Brady Dominic DiToro Robinson W. Fulweiler Wayne S. Gardner Anne Giblin Mark J. McCarthy Alexandra Rao Sybil Seitzinger Marie Thouvenot-Korppoo Craig Tobias 《Biogeochemistry》2009,93(1-2):159-178
Sediment denitrification is a major pathway of fixed nitrogen loss from aquatic systems. Due to technical difficulties in measuring this process and its spatial and temporal variability, estimates of local, regional and global denitrification have to rely on a combination of measurements and models. Here we review approaches to describing denitrification in aquatic sediments, ranging from mechanistic diagenetic models to empirical parameterizations of nitrogen fluxes across the sediment-water interface. We also present a compilation of denitrification measurements and ancillary data for different aquatic systems, ranging from freshwater to marine. Based on this data compilation we reevaluate published parameterizations of denitrification. We recommend that future models of denitrification use (1) a combination of mechanistic diagenetic models and measurements where bottom-waters are temporally hypoxic or anoxic, and (2) the much simpler correlations between denitrification and sediment oxygen consumption for oxic bottom waters. For our data set, inclusion of bottom water oxygen and nitrate concentrations in a multivariate regression did not improve the statistical fit. 相似文献
146.
Top‐down control of species distributions: feral cats driving the regional extinction of a threatened rodent in northern Australia
下载免费PDF全文
![点击此处可从《Diversity & distributions》网站下载免费的PDF全文](/ch/ext_images/free.gif)
147.
Wetlands are often highly effective nitrogen (N) sinks. In the Lake Waco Wetland (LWW), near Waco, Texas, USA, nitrate (NO3−) concentrations are reduced by more than 90% in the first 500 m downstream of the inflow, creating a distinct gradient in
NO3− concentration along the flow path of water. The relative importance of sediment denitrification (DNF), dissimilatory NO3− reduction to ammonium (DNRA), and N2 fixation were examined along the NO3− concentration gradient in the LWW. “Potential DNF” (hereafter potDNF) was observed in all months and ranged from 54 to 278 μmol N m−2 h−1. “Potential DNRA” (hereafter potDNRA) was observed only in summer months and ranged from 1.3 to 33 μmol N m−2 h−1. Net N2 flux ranged from 184 (net denitrification) to −270 (net N2 fixation) μmol N m−2 h−1. Nitrogen fixation was variable, ranging from 0 to 426 μmol N m−2 h−1, but high rates ranked among the highest reported for aquatic sediments. On average, summer potDNRA comprised only 5% (±2%
SE) of total NO3− loss through dissimilatory pathways, but was as high as 36% at one site where potDNF was consistently low. Potential DNRA
was higher in sediments with higher sediment oxygen demand (r
2 = 0.84), and was related to NO3− concentration in overlying water in one summer (r
2 = 0.81). Sediments were a NO3− sink and accounted for 50% of wetland NO3− removal (r
2 = 0.90). Sediments were an NH4+ source, but the wetland was often a net NH4+ sink. Although DNRA rates in freshwater wetlands may rival those observed in estuarine systems, the importance of DNRA in
freshwater sediments appears to be minor relative to DNF. Furthermore, sediment N2 fixation can be extremely high when NO3− in overlying water is consistently low. The data suggest that newly fixed N can support sustained N transformation processes
such as DNF and DNRA when surface water inorganic N supply rates are low. 相似文献
148.
149.
150.