全文获取类型
收费全文 | 4596篇 |
免费 | 483篇 |
国内免费 | 255篇 |
出版年
2024年 | 16篇 |
2023年 | 169篇 |
2022年 | 128篇 |
2021年 | 184篇 |
2020年 | 218篇 |
2019年 | 260篇 |
2018年 | 214篇 |
2017年 | 218篇 |
2016年 | 181篇 |
2015年 | 176篇 |
2014年 | 224篇 |
2013年 | 274篇 |
2012年 | 178篇 |
2011年 | 159篇 |
2010年 | 142篇 |
2009年 | 196篇 |
2008年 | 207篇 |
2007年 | 216篇 |
2006年 | 188篇 |
2005年 | 179篇 |
2004年 | 179篇 |
2003年 | 164篇 |
2002年 | 167篇 |
2001年 | 128篇 |
2000年 | 128篇 |
1999年 | 102篇 |
1998年 | 63篇 |
1997年 | 54篇 |
1996年 | 37篇 |
1995年 | 40篇 |
1994年 | 44篇 |
1993年 | 40篇 |
1992年 | 41篇 |
1991年 | 37篇 |
1990年 | 28篇 |
1989年 | 22篇 |
1988年 | 37篇 |
1987年 | 41篇 |
1986年 | 22篇 |
1985年 | 30篇 |
1984年 | 20篇 |
1983年 | 28篇 |
1982年 | 26篇 |
1981年 | 27篇 |
1980年 | 22篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 11篇 |
1976年 | 12篇 |
1971年 | 6篇 |
排序方式: 共有5334条查询结果,搜索用时 31 毫秒
71.
72.
An automatic system was installed for continuous analyses of ozone, sulphur dioxide, nitrogen monoxide and nitrogen dioxide in an experimental orchard with a canopy ofPrunus cerasifera plants in summer 1993. Air samples from three elevations (0.8 m, 1.6 m and 3 m above ground) were sequentially analyzed. Ozone concentrations above the canopy were usually higher than within the canopy; their relationships with stomatal resistance have been investigated. Sulphur dioxide levels were negligible. Nitrogen oxides showed a complex profile, with no particular trend, likely due to a reciprocal exchange between the atmosphere and the ground surface. 相似文献
73.
We have identified two types of structural elements in genomic DNA for annexin I that provide physical evidence of genetic events leading to conserved changes in gene structure. The sequence upstream of the transcribed region in human annexin I contained a rare, Alu-like repetitive element with flanking direct repeats, probably derived from the active BC200 gene via germline retroposition. Nucleotide substitutions in this BC200 insert relative to the 7SL gene and its absence in rodent annexins I identified it as a recent primate pseudogene. Phylogenetic analysis showed that the BC200 gene represents a new clade of primate Alu evolution that branched near the time of appearance of the progenitor to the free left Alu monomer, FLAM-C. Separate analysis identified a Z-DNA motif in pigeon annexin I intron 7 that may represent the vestigial recombination site involved in primordial assembly of the annexin tetrad. These distinct structural features in annexin I genes provide insight into the evolution of Alu repeats and the mechanism of annexin tetrad formation. 相似文献
74.
75.
Frequent measurement of below ground water levels at multiple locations is an important component of many wetland ecosystem studies. These measurements, however, are usually time consuming, labor intensive, and expensive. This paper describes a water-level sensor that is inexpensive and easy to construct. The sensor is placed below the expected low water level in a shallow well and, when connected to a datalogger, uses a pressure transducer to detect groundwater or surface water elevations. Details of pressure transducer theory, sensor construction, calibration, and examples of field installations are presented. Although the transducers must be individually calibrated, the sensors have a linear response to changing water levels (r
2 .999). Measurement errors resulting from temperature fluctuations are shown to be about 4 cm over a 35°C temperature range, but are minimal when the sensors are installed in groundwater wells where temperatures are less variable. Greater accuracy may be obtained by incorporating water temperature data into the initial calibration (0.14 cm error over a 35C temperature range). Examples of the utility of these sensors in studies of groundwater/surface water interactions and the effects of water level fluctuations on tree growth are provided. 相似文献
76.
Louise A. Lewis 《Journal of phycology》1997,33(2):279-285
Sequence data from the nuclear small-subunit ribosomal RNA gene was obtained for nine strains of Bracteacoccus Tereg, representing at least five morphological species and four distinct geographic locations. These, along with sequence data from two additional chlorophycean taxa, Spongiochloris spongiosa Starr and Ascochloris multinucleata Bold et MacEntee, and 48 published sequences from green algal taxa, were used to determine the phylogenetic placement of Bracteacoccus with respect to other chlorophycean green algae. Results support the monophyly of Bracteacoccus strains, contrasting with patterns observed so far for many other coccoid green algae. The range of variation among Bracteacoccus strains is similar to that of other congeners. Basal body orientation in Bracteacoccus has been interpreted as clockwise; however, the 18S data point to a relationship between Bracteacoccus and taxa with the directly opposed configuration of the flagellar apparatus. No close relationship was found to the multinucleated green coccoids with clockwise orientation of basal bodies, such as Spongiochloris, or to those with parallel basal bodies, such as Spermatozopsis. However, 18S data confirm that the motile and vegetative cells of Bracteacoccus are structurally distinct from the representatives of sphaeroplealean families currently studied. It is premature to reclassify Bracteacoccus until 18S comparisons can be made with additional sphaeroplealean taxa and with algae with similar flagellar structure such as Dictyochloris and Heterochlamydomonas. 相似文献
77.
Synopsis The ecological and evolutionary forces maintaining genetic polymorphism within populations is of continuing interest to evolutionary biologists. Male pygmy swordtails,Xiphophorus pygmaeus, are polymorphic at a Y-linked locus controlling body color. Fish with the + and cp alleles have blue bodies; those with the con allele have bright gold bodies. Male and female fish were tested in the laboratory to determine if there were any preferences for association with groups of male fish based on color. Single test fish were presented with two groups of males of different phenotypes, and times in which the test fish spent in proximity to each group were recorded. Males of different phenotypes showed no differences in preference, nor did phenotype of the males in the groups affect the behavior of the test fish. Females also showed no preference for males of a particular phenotype. Finally, female test fish showed no preference for groups containing a male with the rare phenotype compared to a group containing all males of the same phenotype. 相似文献
78.
Conserving biological diversity requires a major effort in conducting survey and inventories, establishing priorities, selecting protected areas, managing resources and monitoring the effects of management. Systematics has an important contribution to make to each of these five major activities. Further, the new Convention on Biological Diversity requires systematics information to support action under virtually all of its substantive conservation and sustainable use articles. It seems apparent that large reference collections contribute directly to development, and development assistance agencies should recognize that investing in maintaining these collections is a legitimate form of development assistance. 相似文献
79.
The asymptotic information in censored survival data 总被引:2,自引:0,他引:2
80.