全文获取类型
收费全文 | 10984篇 |
免费 | 970篇 |
国内免费 | 4篇 |
专业分类
11958篇 |
出版年
2022年 | 51篇 |
2021年 | 114篇 |
2020年 | 72篇 |
2019年 | 116篇 |
2018年 | 149篇 |
2017年 | 126篇 |
2016年 | 178篇 |
2015年 | 320篇 |
2014年 | 377篇 |
2013年 | 491篇 |
2012年 | 678篇 |
2011年 | 735篇 |
2010年 | 466篇 |
2009年 | 420篇 |
2008年 | 577篇 |
2007年 | 597篇 |
2006年 | 600篇 |
2005年 | 602篇 |
2004年 | 589篇 |
2003年 | 545篇 |
2002年 | 558篇 |
2001年 | 143篇 |
2000年 | 108篇 |
1999年 | 162篇 |
1998年 | 165篇 |
1997年 | 124篇 |
1996年 | 112篇 |
1995年 | 111篇 |
1994年 | 122篇 |
1993年 | 138篇 |
1992年 | 115篇 |
1991年 | 107篇 |
1990年 | 93篇 |
1989年 | 113篇 |
1988年 | 90篇 |
1987年 | 94篇 |
1986年 | 81篇 |
1985年 | 113篇 |
1984年 | 114篇 |
1983年 | 125篇 |
1982年 | 131篇 |
1981年 | 113篇 |
1980年 | 124篇 |
1979年 | 87篇 |
1978年 | 62篇 |
1977年 | 67篇 |
1976年 | 71篇 |
1975年 | 68篇 |
1974年 | 73篇 |
1972年 | 49篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
22.
Rats were irradiated with one tibia shielded (95% marrow exposure), total body exposed (TBI, 100%), and only one tibia exposed (5%), or they were sham irradiated (SI, 0%). Plasma Fe-59 clearance time (T1/2) and Fe-59 content ratio in the right and left tibia (RT/LT) were assayed to determine the erythroid activity of the overall marrow of the animals and the relative marrow activity in the exposed and shielded tibias, respectively. When a major fraction of the overall marrow was shielded or irradiated, the overall erythroid activity levels were identical to those of the SI and TBI animals, respectively. Interestingly, enhanced normoblastosis was observed in the marrow of the exposed tibia of individual animals exhibiting normal erythroid activity in 95% of the marrow. Conversely, localized marrow with normal erythroid activity was found in a shielded tibia of individual rats, demonstrating an enhanced erythroid activity in a major fraction of the total body. It was concluded that 88 mrad can alter marrow functions in a small isolated skeletal region as effectively as in the whole body, and tandem assays of the Fe-59 T1/2 and Fe-59 RT/LT can facilitate ultra-low-dose X-ray studies involved with partial body exposures. 相似文献
23.
Bruce A. Menge 《Oecologia》1991,88(1):1-8
Summary Petraitis (1990) recently critized previous generalizations regarding the effects of predation in the New England rocky intertidal region (e.g., Menge 1976; Lubchenco and Menge 1978). Contrary to Lubchenco's and my conclusions, Petraitis concluded that (1) barnacles and not mussels are the favored prey of dogwhelks and (2) barnacles and not dogwhelks control mussel abundances in the mid and low rocky intertidal region. I provide evidence that these criticisms are unwarranted. First, Lubchenco and I never claimed that diet composition reflected prey preference. Moreover, predators can influence prey abundance without preferring the prey. Hence, claims regarding preferences have no bearing on our conclusions. Second, Petraitis' experiments do not invalidate Lubchenco's and my experimental results. Reanalyses of our experimental data support the earlier conclusion that at wave-sheltered sites, whelks reduce the abundance of mussels independently of barnacle abundances. Further, at all but one of Lubchenco's and my study sites, predator densities were higher than at Petraitis' site. Thus, the absence of a predator effect in Petraitis's study was most likely due to low predator density rather than a lack of generality of our earlier results. This reevaluation therefore suggests that within a broader conceptual framework, Petraitis' apparently divergent results are actually consistent with ours. 相似文献
24.
Binding of polonium-210 to liver metallothionein 总被引:1,自引:0,他引:1
The interactions of 210Po at the molecular level in biological systems have received little study even though this alpha-emitting radionuclide occurs widely in nature. Polonium-210 was given subcutaneously to rats and found to be incorporated into liver metallothionein as judged by a number of criteria including heat stability, acetone precipitation, and chromatography. In vitro studies confirmed this binding. The binding of 210Po to metallothionein has implications that may help explain some of the radiation damage 210Po causes intracellularly. 相似文献
25.
26.
Bruce H. Allen 《Brittonia》1988,40(2):180-187
Fontinalis welchiana, a new species in the concave-leaved group ofFontinalis restricted to Arkansas, Missouri, and Illinois, differs fromF. novaeangliae by its slender size, dimorphic leaves, erect and strongly concave branch leaves, and lightly papillose exostome with ventral lamellae that are strongly rounded at the corners.Fontinalis allenii is a synonym ofF. antipyretica var.oreganensis. 相似文献
27.
Bruce M. Campbell 《Plant Ecology》1986,66(1):3-16
The vegetation of the mountains of the Fynbos Biome is classified and described, mostly on the basis of vegetation structure and higher taxa. Various gradients can be recognized. A gradient in soil conditions runs from coarse textured, nutrient-poor soils in non-grassy fynbos (Mountain Fynbos) through grassy fynbos and grassy non-fynbos (Eastern Fynbos and Grasslands & Grassy Shrubland) to finer textured and less nutrient-poor soils in the non-fynbos Karroid & Renoster Shrubland. Another gradient of significance can be likened to the tropical gradient running from the dry conditions of hot semi-deserts to savannas or grasslands to woodlands. At the dry extreme an open shrubland occurs (Karroid & Renoster Shrubland, Asteraceous Fynbos), at intermediate positions a herbland occurs (Restioid Fynbos) and at the wet extreme another type of shrubland occurs (Ericaceous Fynbos). This kind of moisture gradient occurs on individual mountains with the xeric end being at the base of the north slopes, and the mesic end being on the upper south aspects. Soil characteristics are closely linked to this gradient; the better developed soils being found on the south aspects. Another moisture gradient is apparent in the vegetation. This gradient runs from the mesic southern coastal mountains to the north west where summer droughts are extremely severe although total rainfall may be similar.Nomenclature follows that used in the Government Herbarium, Stellenbosch.I thank William Bond, Richard Cowling, John Rushworth, Eddy van der Maarel and Marinus Werger for critical comments and discussion. 相似文献
28.
Gregory A. Osenga Bruce C. Coull 《Journal of experimental marine biology and ecology》1983,67(3):221-225
Meiofauna were collected from adjacent areas with and without Spartina alterniflora Loisel culms. Environmental complexity resulting from root structure cannot be determined by simply observing S. alterniflora culm distribution. There were no significant differences between the two areas with respect to root density or root sizes; however, the number of live roots was significantly higher in the area containing Spartina culms. Nematodes, the most abundant taxon were positively correlated with live root density. In contrast, the number of dead roots and harpacticoid copepods were higher in the area without culms. This study suggests that nematodes are positively associated with micro-oxygenated zones produced by live S. alterniflora roots. Other measures of biogenic structure, such as total root density or root size fractions are not important in affecting nematode abundance. 相似文献
29.
30.