全文获取类型
收费全文 | 40561篇 |
免费 | 3071篇 |
国内免费 | 5359篇 |
专业分类
48991篇 |
出版年
2024年 | 201篇 |
2023年 | 849篇 |
2022年 | 1129篇 |
2021年 | 1332篇 |
2020年 | 1302篇 |
2019年 | 1836篇 |
2018年 | 1525篇 |
2017年 | 1354篇 |
2016年 | 1284篇 |
2015年 | 1282篇 |
2014年 | 1994篇 |
2013年 | 2765篇 |
2012年 | 1639篇 |
2011年 | 1839篇 |
2010年 | 1478篇 |
2009年 | 1925篇 |
2008年 | 1877篇 |
2007年 | 2067篇 |
2006年 | 1800篇 |
2005年 | 1676篇 |
2004年 | 1463篇 |
2003年 | 1355篇 |
2002年 | 1231篇 |
2001年 | 918篇 |
2000年 | 863篇 |
1999年 | 812篇 |
1998年 | 784篇 |
1997年 | 712篇 |
1996年 | 591篇 |
1995年 | 590篇 |
1994年 | 562篇 |
1993年 | 534篇 |
1992年 | 484篇 |
1991年 | 489篇 |
1990年 | 378篇 |
1989年 | 384篇 |
1988年 | 378篇 |
1987年 | 305篇 |
1986年 | 337篇 |
1985年 | 543篇 |
1984年 | 637篇 |
1983年 | 368篇 |
1982年 | 531篇 |
1981年 | 475篇 |
1980年 | 423篇 |
1979年 | 321篇 |
1978年 | 241篇 |
1977年 | 240篇 |
1976年 | 216篇 |
1973年 | 184篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
91.
Qualitative analyses were made from whole-body cuticular extracts of Scapteriscus abbreviatus, Scapteriscus acletus, Scapteriscus vicinus, and Neocurtilla hexadactyla by isothermal and temperature-programmed gas chromatography. Adults of both sexes and nymphs of each species were collected in Florida. The chromatographic profiles of peaks were distinct and easily recognizable for each species, regardless of sex or developmental stage. Distinct sexual differences were found in S. acletus and S. abbreviatus. Specimens of S. abbreviatus from Puerto Rico and S. vicinus from Bolivia produced gas chromatography (GC) traces very similar to those of conspecifics collected in Florida. Evidence is presented to illustrate the potential importance of volatile cuticular lipid analysis as a tool for mole cricket identification. Cuticular extracts of an undescribed short-winged species of Scapteriscus from Bolivia were also examined and produced GC traces unlike those of any other species analyzed to date. 相似文献
92.
The effect of phosphate feeding on the influence of low (2%) oxygen on photosynthetic carbon assimilation has been investigated in leaf discs of spinach (Spinacia oleracea L.) at 12°C. The following observations were made. First, after the transition from 20% O2 to 2% O2, the rate of CO2 uptake was inhibited at CO2 concentrations between about 250 and about 800 l CO2·l-1. Second, phosphate feeding stimulated the rate of CO2 uptake in 20% O2 at higher concentrations of CO2 (500–900 l·l-1). Third, phosphate feeding stimulated the rate of CO2 uptake in 2% O2 at all but the highest (900 l·l-1) and lowest 74 (l·l-1) concentrations of CO2 employed. Phosphate thereby restored the stimulation of photosynthesis by 2% O2 and it did so over a wide range of lower temperatures. Fourth, oscillatory behaviour, however generated, was dampened by phosphate feeding, even at very low concentrations of CO2. Contents of leaf metabolites were measured during the transition to 2% O2 in control and phosphate-fed leaf discs. During this period the ratio glycerate-3-phosphate/triose phosphate rose steeply, but fell again only in the phosphate-treated leaf discs. These data, taken together with measured ATP/ADP ratios, showed that assimilatory power, the ratio [ATP]·[NAD(P)H]/[ADP]·[Pi]·[NAD(P)], decreased when leaves were exposed to 2% O2, but that this decrease was minimised by previous feeding of phosphate. The mechanism of phosphate limitation is discussed in the light of the results.Abbreviations Ci
intercellular concentration of CO2
- RuBP
ribulose-1,5-bisphosphate 相似文献
93.
Long bone growth is analyzed for 180 children from a Medieval population at Kulubnarti in Sudanese Nubia (550–1450 A.D.). A regional interpopulation comparison is made with growth data from Wadi Halfa in Lower Nubia, and an intrapopulation analysis is undertaken to assess diachronic changes in growth at Kulubnarti. Changes in growth patterns are interpreted in the context of mortality and morbidity profiles and relationships between the three variables are discussed. It is suggested that changes in the sociopolitical environment may have been responsible in part for altering levels of biological stress and impacting growth. 相似文献
94.
95.
Chromosome numbers for 26 different species of the generaPiper, Peperomia andPothomorphe (Piperaceae) are reported. The basic chromosome numbers are 2n = 26, x = 13 (Piper, Pothomorphe) and 2n = 22, x = 11 (Peperomia), polyploid series are characteristic forPiper andPeperomia. Piper has the smallest chromosomes and prochromosomal interphase nuclei,Peperomia the largest ones and mostly reticulate to euchromatic nuclei.Pothomorphe is intermediate in both characters. The karyomorphological differences betweenPothomorphe andPiper underline their generic separation. Interspecific size variation of chromosomes occurs inPiper andPeperomia. Infraspecific polyploidy was observed inPiper betle. C-banding reveals different patterns of heterochromatin (hc) distribution between the genera investigated. The genome evolution is discussed. 相似文献
96.
97.
Multivariate statistical analysis has been applied to time series measurements of aerosol elemental composition from PIXE
analysis of filter samples, and principal components have been resolved that represent distinct particle types in an external
mixture in the atmosphere. In this study, it is argued that a combination of chemical and statistical analyses of the data
may be more powerful in determining chemical species in atmospheric aerosols than studies that employ mainly direct chemical
analysis of chemical species in unresolved mixtures of aerosol particle samples. Sulfur is generally associated with mineral
dust elements. It is reasoned that the association may represent sulfuric acid coatings on particles that can lead to mineral
dissolution and solubilization of significant amounts of aluminum, iron, and other metals.
Upon wet or dry deposition to the surface, the fluxes of these metals in biologically-available form may be sufficient to
affect primary productivity in the world ocean and cause ecological damage in lakes. As a consequence, the fluxes of biogenic
trace gases to the atmosphere may be changed, possibly leading to changes in the tropospheric concentration of ozone. The
inputs to lakes of soluble aluminum, which is toxic to fish, may be partly by deposition directly from the atmosphere, thus
not limited to leaching of soils by acid deposition. Human inhalation of soluble aluminum and other solulilized mineral metals
may account, in part, for the observed geographic pattern of deaths attributed to chronic obstructive pulmonary disease (COPD)
that show high rates in cities of the Western US and the southeast region, but low in most of the midwest and northeast. 相似文献
98.
99.
J. D. Paterson 《International journal of primatology》1996,17(4):585-611
I analyzed somatometric measurements from subsets of the Texas and Oregon transplanted troops of Japanese macaques(Macaca fuscata) to reveal secular changes in body size and shape. Body weights of the Texas population (N = 59) are lower than those of the
Oregon population(N = 49) and the founding population from Arashiyama. The adult weights of the Oregon population are significantly higher than the
founding population from Mihara. There are significant differences in adult circumferential measures and in skinfolds, which
are correlated with the increased weight of the Oregon macaques. The adult Texas macaques have longer limb segments in comparison
with the adult Oregon troop members, while the latter have significantly longer heads and trunks. Examination of the developing
morphological trends through regression analyses on the complete sample suggests distinctive growth patterns for each population.
Members of the Texas population start with smaller initial measurements but hold a steeper growth pattern for limb segments,
while the Oregon macaques start larger in most measures and show lower growth rates. I argue that these differences in both
somatometry and growth patterns are related to the differing climatic conditions under which the translocated macaques have
lived. This set of analyses supports the basic arguments for Bergmann’s rule and Allen’s rule. 相似文献
100.