全文获取类型
收费全文 | 9336篇 |
免费 | 1077篇 |
国内免费 | 3篇 |
出版年
2021年 | 136篇 |
2020年 | 56篇 |
2019年 | 91篇 |
2018年 | 105篇 |
2017年 | 92篇 |
2016年 | 157篇 |
2015年 | 288篇 |
2014年 | 290篇 |
2013年 | 368篇 |
2012年 | 504篇 |
2011年 | 530篇 |
2010年 | 288篇 |
2009年 | 271篇 |
2008年 | 469篇 |
2007年 | 475篇 |
2006年 | 475篇 |
2005年 | 439篇 |
2004年 | 437篇 |
2003年 | 391篇 |
2002年 | 382篇 |
2001年 | 248篇 |
2000年 | 237篇 |
1999年 | 193篇 |
1998年 | 124篇 |
1997年 | 115篇 |
1996年 | 113篇 |
1995年 | 91篇 |
1994年 | 118篇 |
1993年 | 106篇 |
1992年 | 175篇 |
1991年 | 158篇 |
1990年 | 147篇 |
1989年 | 131篇 |
1988年 | 125篇 |
1987年 | 139篇 |
1986年 | 127篇 |
1985年 | 129篇 |
1984年 | 137篇 |
1983年 | 102篇 |
1982年 | 114篇 |
1981年 | 106篇 |
1980年 | 84篇 |
1979年 | 98篇 |
1978年 | 76篇 |
1977年 | 69篇 |
1976年 | 76篇 |
1975年 | 68篇 |
1974年 | 68篇 |
1973年 | 87篇 |
1970年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Monoterpenoids from wood of Pinus quadrifolia and Pinus monophylla from south of the San Jacinto Mountains in southern California were analysed by gas-liquid chromatography (GLC). The number of needles per fascicle, the number of resin canals, and the number of abaxial and adaxial stomatal rows in the needles were determined. Percentages of mycrene, α-pinene, and to a minor extent of camphene, β-pinene, limonene, and β-phellandrene, the above mentioned morphological characteristics and previously obtained data were all used for identification and characterisation of species intermediacy in the San Jacinto area. It was concluded that many P. quadrifolia trees in the mixed Buck Ridge stand and some trees in two pure stands of the same area were hybrids and that sympatric and to a lesser extent allopatric introgression of P. monophylla into P. quadrifolia takes place in the San Jacinto region. Introgression of P. quadrifolia into P. monophylla could not be demonstrated. 相似文献
12.
13.
14.
Alexander V. Kirdyanov Eugene A. Vaganov Malcolm K. Hughes 《Trees - Structure and Function》2007,21(1):37-44
We propose a technique for separating the climatic signal which is contained in two tree-ring parameters widely used in dendroclimatology.
The method is based on the removal of the relationship between tree-ring width (TRW) and maximum latewood density (MXD) observed
for narrow tree rings from high latitudes. The new technique is tested on data from three larch stands located along the northern
timberline in Eurasia. Correlations were calculated between the temperatures of pentads (five consecutive days), TRW chronologies
and MXD chronologies calculated according to the standard and proposed methods. The analysis confirms the great importance
of summer temperature for tree radial growth and tree-ring formation. TRW is positively correlated with the temperature of
four to eight pentads (depending on the region) at the beginning of the growth season, but MXD as obtained by the standard
technique is correlated with temperature over a much longer period. For maximum density series from which the relationship
between MXD and TRW has been removed (MXD′), there is a clear correlation with temperatures in the second part of the growing
season. These results are consistent with the known dynamics of tree-ring growth in high latitudes and mechanisms of tree-ring
formation. 相似文献
15.
The computer program HYLAS generates from a standard DNA lettersequence a three-dimensional space curve (H curve) which embodiesthe entire information content of the original nucleotide sequence.The program can display H curves either as two-dimensional (frontand side view) projections or as stereo-pair images. The curvescan be marked at specific nucleotide locations, annotated, rotatedfor observation from any viewing angle, and manipulated forconvenient side-by-side comparisons. Unlike the cumbersome lettersequences, H curves can be drastically condensed in size withoutlosing their ability to reflect the global nucleotide-distributionpattern of the entire DNA sequence. Often, biologically importantloci can be visually identified on the H curves. HYLAS is writtenin FORTRAN with separate mainframe (IBM- VM/CMS) and microcomputer(MS-DOS) versions. It uses the Tektronix-TCS library of graphicsubroutines.
Received on October 24, 1988; accepted on July 15, 1989 相似文献
16.
Cycling virgin female rats were treated with clomiphene citrate (CC) during dioestrus of the reproductive cycle. Animals were sacrificed 2 days after the initial injection and their vaginal tissue was examined by scanning electron microscopy. Animals treated with one dose of CC had an epithelium consistent with a prolonged dioestrus. Treatment with CC for 2 days induced changes in the epithelium that had no resemblance to any normal hormonally controlled event in the vagina. It was found that CC had effects consistent with progesterone alone as well as effects unique to this superovulatory drug. 相似文献
17.
18.
Abstract Azide, an inhibitor of ATPase, and a specific inhibitor of protein export was used in order to select for protein secretion mutants in Acinetobacter calcoaceticus A2. Two such mutants were isolated that were azide-resistant and defective in the general protein transport system. The mutation also conferred additional phenotypic changes, including an inability to grow on minimal media or at 40°C. The existence of protein secretion mutants with a selectable phenotype may be useful for the genetic study of protein export. 相似文献
19.
Ultrastructure and elemental composition of dormant and germinating Diplodia maydis spores. 总被引:1,自引:0,他引:1
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The ultrastructure of Diplodia maydis spores was studied in thin sections with a transmission electron microscope. Storage vacuoles were evenly distributed in the two cells. Some of the vacuoles that contained a dense osmiophilic sphere(s) were surrounded by a membrane, and had membranous aggregates around their periphery. The sport wall was composed of an electron-dense layer and an electron-translucent layer. An inner cytoplasmic membrane was present. Dormant and germinating spores were studied with scanning electron microscopy and also with a Si (Li) energy-dispersive X-ray analyzer. The dormant spore was ovate and usually two-celled with a central septum. Germination proceeded via a germ tube from the side of one end of the cell. Of several methods for preparation of specimens for X-ray analysis studied, freeze-dried spores mounted on carbon stubs and then further carbon coated gave the best results. X-ray analyses revealed that spore populations contained large amounts of Si, P, Cl, and K, smaller amounts of S and Ca, and trace amounts of Mg and Al. Analyses of single spores revealed high K and Cl and low P and Mg at one end of the cell with concomitant low K and Cl and high P and Mg in the central portion and other end of the cell. In two-celled germinating spores, high K and Cl occurred in the end of the nongerminating spore cell, whereas the germinating cell contained high P and Mg and low K and Cl. X-ray image maps revealed that K and Cl were located together at one end of the spore. 相似文献
20.