全文获取类型
收费全文 | 25843篇 |
免费 | 2524篇 |
国内免费 | 4389篇 |
出版年
2024年 | 87篇 |
2023年 | 468篇 |
2022年 | 538篇 |
2021年 | 736篇 |
2020年 | 935篇 |
2019年 | 1036篇 |
2018年 | 1000篇 |
2017年 | 959篇 |
2016年 | 1016篇 |
2015年 | 937篇 |
2014年 | 1054篇 |
2013年 | 1391篇 |
2012年 | 971篇 |
2011年 | 1059篇 |
2010年 | 850篇 |
2009年 | 1016篇 |
2008年 | 1116篇 |
2007年 | 1167篇 |
2006年 | 1125篇 |
2005年 | 1181篇 |
2004年 | 1228篇 |
2003年 | 1181篇 |
2002年 | 942篇 |
2001年 | 924篇 |
2000年 | 710篇 |
1999年 | 797篇 |
1998年 | 668篇 |
1997年 | 586篇 |
1996年 | 669篇 |
1995年 | 654篇 |
1994年 | 611篇 |
1993年 | 599篇 |
1992年 | 566篇 |
1991年 | 470篇 |
1990年 | 459篇 |
1989年 | 425篇 |
1988年 | 449篇 |
1987年 | 303篇 |
1986年 | 294篇 |
1985年 | 283篇 |
1984年 | 256篇 |
1983年 | 114篇 |
1982年 | 190篇 |
1981年 | 145篇 |
1980年 | 144篇 |
1979年 | 97篇 |
1978年 | 86篇 |
1977年 | 79篇 |
1976年 | 65篇 |
1974年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
221.
A. M. Cashmore M. S. Albury C. Hadfield P. A. Meacock 《Molecular & general genetics : MGG》1988,212(3):426-431
Summary The yeast 2 m circle encodes four major transcribed open reading frames, A, B, C and D. Products of ORF's A, B and C, together with the inverted repeats and the other cis-acting loci ORI and STB, have been shown to be involved in plasmid maintenance. However, the function of ORF D has remained unclear. We have therefore carried out studies on 2 m derivatives with both insertional and frameshift mutations in D. Our results indicate that there is a protein product encoded by ORF D, which is involved in plasmid maintenance. When the copy number of the C gene was reduced to one, by chromosomal integration, we observed striking differences in the efficiency of partitioning of D
+ and D
– plasmid derivatives. Absence of D function could be compensated by an increase in dosage of the C gene, indicating that the D product may act to regulate C expression. Since the C product has been implicated in copy number control as well as partitioning, our data suggest that the D product may also be involved in both of these processes. 相似文献
222.
Summary We present a method that allows positive selection and rapid analysis of mutations in Enterobacteriaceae. Mutations are detected in a 2630 bp selection cartridge inserted in two different bacterial mutlicopy plasmid vectors. Spontaneous mutations in Escherichia coli, Enterobacter cloacae and Citrobacter freundii include insertions, deletions and point mutations. The small size of the target sequence facilitates rapid analysis of DNA rearrangements by cleavage with restriction enzymes and of any type of mutation by DNA sequence analysis. While in E. coli insertions of the mobile elements IS1, IS2 and IS5 were readily found, insertions of putative new transposable elements were detected in Enterobacter cloacae. The selection cartridge can thus serve as a tool for studying the spectrum of insertion mutations in Enterobacteriaceae and probably other Gramnegative bacteria, and the dependency of this spectrum on physiological and environmental factors and the host's genetic background can be investigated. 相似文献
223.
224.
225.
Summary Fruit of the blackberry, Rubus pennsylvanicus Poir. (Rosaceae), were examined to determine variation in maturation characteristics. Maturation timing and rate varied greatly among individual fruits, resulting in a bi-colored fruiting display comprised largely of two maturation stages, pre-ripe (salmon and scarlet) and ripe (dark brown and black). While ripe fruit were generally larger and heavier than pre-ripe fruit, exhibiting greater fresh and dry fruit weight, diameter, water content, and total seed weight, no significant differences were found in energy content, i.e. numbers of calories per gram pulp, or in pulp:seed ratio. The differences between ripe and pre-ripe fruit appear to be due largely to an increase in water content and seed weight with maturity. The fact that little energetic benefit accrues to the preferential selection of ripe fruit suggests that bi-colored Rubus displays may be considered to be unicolored. 相似文献
226.
Volker Nicolai 《Oecologia》1988,75(4):575-579
Summary Factors influencing decomposition in European forests growing on different soils were studied in stands dominated by the European
beechFagus sylvatica L. Phenolic contents of freshly fallen leaves ofF. sylvatica growing on nutrient-poor soils (acid sandy soil) were higher than those of similar leaves on nutrient-rich soils (calcareous
mull soil). Analysis of fallen leaves of different ages showed rapid decay of phenolics during the first winter on the ground.
After 1 year the phenolic content of leaves ofF. sylvatica growing on nutrient-poor soils was still twice as high as in similar leaves on nutrient-rich soils. Field and laboratory
experiments showed that a major decomposer (Oniscus asellus, Isopoda) preferred leaves from trees on nutrient-rich soils. Mineral contents of leaves ofF. sylvatica growing on different soils differed: on rich soils leaves had higher contents of Ca, Mg, Na, and K. These elements are important
nutrients for decomposers. The distribution of major decomposers reflects the mineral content of their diet, which in turn
reflects soil type. Different rates of leaf turnover and nutrient turnover in different forest ecosystems (even when the same
tree species is dominant) are due to the decomposing system, which is influenced by the phenolic and mineral contents of the
leaves. 相似文献
227.
228.
Fructan biosynthesis in excised leaves of Lolium temulentum L. 总被引:10,自引:10,他引:0
229.
230.
The biology of mycorrhiza in the Ericaceae 总被引:6,自引:4,他引:2