全文获取类型
收费全文 | 449232篇 |
免费 | 49829篇 |
国内免费 | 282篇 |
出版年
2018年 | 4323篇 |
2016年 | 5824篇 |
2015年 | 7750篇 |
2014年 | 9068篇 |
2013年 | 12537篇 |
2012年 | 14211篇 |
2011年 | 14564篇 |
2010年 | 10048篇 |
2009年 | 9190篇 |
2008年 | 13261篇 |
2007年 | 13670篇 |
2006年 | 12874篇 |
2005年 | 12224篇 |
2004年 | 12299篇 |
2003年 | 11531篇 |
2002年 | 11410篇 |
2001年 | 18130篇 |
2000年 | 18051篇 |
1999年 | 14521篇 |
1998年 | 5345篇 |
1997年 | 5593篇 |
1996年 | 5143篇 |
1995年 | 4861篇 |
1994年 | 4664篇 |
1993年 | 4764篇 |
1992年 | 12013篇 |
1991年 | 11916篇 |
1990年 | 11637篇 |
1989年 | 11237篇 |
1988年 | 10754篇 |
1987年 | 10331篇 |
1986年 | 9622篇 |
1985年 | 9537篇 |
1984年 | 8036篇 |
1983年 | 6956篇 |
1982年 | 5331篇 |
1981年 | 4810篇 |
1980年 | 4637篇 |
1979年 | 7642篇 |
1978年 | 6063篇 |
1977年 | 5615篇 |
1976年 | 5404篇 |
1975年 | 5802篇 |
1974年 | 6486篇 |
1973年 | 6347篇 |
1972年 | 5914篇 |
1971年 | 5374篇 |
1970年 | 4768篇 |
1969年 | 4700篇 |
1968年 | 4578篇 |
排序方式: 共有10000条查询结果,搜索用时 54 毫秒
81.
82.
E. G. Pringsheim 《American journal of botany》1964,51(8):898-913
From a variety of localities 14 strains of Beggiatoa, 1 ¼–3μ wide, were isolated in axenic heterotrophic culture. Most of these were freshwater forms, 2 were from brackish water, 1 was marine. The widths of the individual strains were constant, independent of conditions. The nutritional requirements of most of the strains are simple. Acetate at low concentrations, an ammonium salt as nitrogen source and the usual inorganic salts including trace elements supported growth. A few strains did not grow well without addition of an amino acid, and 2 (identical) strains required peptone or beef extract. Lactate, succinate, or pyruvate could often replace acetate. Multiplication was in most cases also possible with amino acids alone, without a further organic substrate. The appearance of the various strains on agar plates differs characteristically. Two types could be discerned: one forms spirals and one grows in tongues. These 2 types are not homogeneous for there are within them differences in width, growth rate, nutrition, and salt tolerance, so that a considerable number of independent forms exist even within the narrow limits in width of trichomes to which the investigations were restricted. 相似文献
83.
84.
85.
86.
87.
Temporal variability of top-down forces and their role in host choice evolution of phytophagous arthropods 总被引:1,自引:0,他引:1
The role of top-down forces in host choice evolution of phytophagous arthropods is the subject of a vividly animated debate. Empirical evidence for the evolutionary role of top-down forces comes from studies showing that phytophagous arthropods prefer hosts that entail enemy-free space. The aim of this paper is to draw the attention of plant–arthropod researchers to the potentially, temporally variable nature of third trophic level effects. We show that this aspect is largely neglected in studies on enemy-free space, despite the fact that relative enemy impact varies seasonally among plants in at least some studies. We conclude that rigorous testing of the enemy-free space hypothesis can only be performed when within and between season variation in higher trophic level effects is taken into account. 相似文献
88.
HIF-1 expression in healing wounds: HIF-1alpha induction in primary inflammatory cells by TNF-alpha 总被引:12,自引:0,他引:12
89.
90.