全文获取类型
收费全文 | 4860篇 |
免费 | 462篇 |
国内免费 | 393篇 |
出版年
2024年 | 14篇 |
2023年 | 99篇 |
2022年 | 82篇 |
2021年 | 169篇 |
2020年 | 207篇 |
2019年 | 240篇 |
2018年 | 233篇 |
2017年 | 157篇 |
2016年 | 199篇 |
2015年 | 205篇 |
2014年 | 270篇 |
2013年 | 293篇 |
2012年 | 234篇 |
2011年 | 310篇 |
2010年 | 274篇 |
2009年 | 287篇 |
2008年 | 309篇 |
2007年 | 276篇 |
2006年 | 257篇 |
2005年 | 213篇 |
2004年 | 184篇 |
2003年 | 141篇 |
2002年 | 135篇 |
2001年 | 114篇 |
2000年 | 89篇 |
1999年 | 92篇 |
1998年 | 89篇 |
1997年 | 60篇 |
1996年 | 57篇 |
1995年 | 51篇 |
1994年 | 50篇 |
1993年 | 46篇 |
1992年 | 37篇 |
1991年 | 30篇 |
1990年 | 37篇 |
1989年 | 19篇 |
1988年 | 24篇 |
1987年 | 25篇 |
1986年 | 11篇 |
1985年 | 23篇 |
1984年 | 20篇 |
1983年 | 7篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1973年 | 2篇 |
排序方式: 共有5715条查询结果,搜索用时 31 毫秒
71.
高原鼠兔与达乌尔鼠兔食物资源维生态位的研究 总被引:19,自引:5,他引:14
作者于1993年在青海省刚察县年诺索玛地区发现高原鼠兔(Ochotona curzoniae)和达乌尔鼠兔(0.daurica)在布哈河谷二阶同域且重叠分布之现象,并从这两种鼠兔的食物资源利用出发,研究了其生态位关系问题。在食物资源维上的生态位宽度(PS),高原鼠兔为0.4744,达乌尔鼠兔为0.4964,即它们具有相近似的生态位宽度。这两种动物的生态位在食物资源谱维上的重叠值(FT)为0.809 相似文献
72.
73.
Size-related selection of food plants by bumblebees 总被引:1,自引:0,他引:1
Abstract. 1. A positive correlation between the tongue length of conspecific workers collecting nectar from seven plant species and the corolla length of the flowers probed was found for B.lapidarius and B.pascuomm but not B.terrestris . No simple relationship was found between the volume, sugar weight or concentration of nectar in flowers and the tongue or wing length of probing bees.
2. B.terrestris workers collecting pollen from four plant species producing pollen only, were found to differ in size according to the type of pollen presentation mechanism and the pollen content per flower. Body size variation was also related to the foraging of pollen plus nectar from two other plant species. 相似文献
2. B.terrestris workers collecting pollen from four plant species producing pollen only, were found to differ in size according to the type of pollen presentation mechanism and the pollen content per flower. Body size variation was also related to the foraging of pollen plus nectar from two other plant species. 相似文献
74.
畜禽遗传资源冷冻保存中的取样方法探讨 总被引:1,自引:0,他引:1
本文基于遗传学和数理统计原理,提出了运用冷冻生殖细胞的方法长期保存畜禽遗传资源时,冷冻生殖细胞的4种取样方法,并推证了其误差公式和最低保存数量的估计公式。最后通过实例对公式的实际应用作了说明和讨论分析。 相似文献
75.
Protein variation in wild Atlantic salmon, with particular reference to southern Ireland 总被引:1,自引:0,他引:1
The extent of genetic variation in wild Atlantic salmon parr, Sulmo salur L., from river systems in Ireland, Iceland and eastern Canada, was investigated using starch gel electrophoresis. Within Ireland, seven polymorphic enzyme loci ( sAAT-4 *, GPI-1 *, IDDH-1 *, IDDH-2 *, IDHP-3 *, MDH-3 * and mMEP-2 *) were screened in nine different rivers and nine tributaries from the River Blackwater. Significant heterogeneity in gene frequencies occurred between riverine samples and between samples from tributaries of the River Blackwater. Variation between tributaries was as great as between rivers elsewhere in the country. Levels of population differentiation were comparable to those found in other regions throughout the range of the species, and temporal stability in gene frequencies was apparent when the results were compared with previously published data. Screening of riverine samples from Iceland and eastern Canada (Newfoundland and New Brunswick) allowed the Irish results to be considered in a broader context. Irish salmon cluster in the western European group, to which may be added Icelandic populations. Salmon from eastern Canada show a high level of genetic distinctiveness from the European group. 相似文献
76.
77.
78.
S. Kresovich J. G. K. Williams J. R. McFerson E. J. Routman B. A. Schaal 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1992,85(2-3):190-196
Summary Effective conservation and the use of plant genetic resources are essential for future agricultural progress. Critical to this conservation effort is the development of genetic markers which not only distinguish individuals and accessions but also reflect the inherent variation and genetic relationships among collection holdings. We have examined the applicability of the random amplified polymorphic DNA (RAPD) assay for quick, cost-effective, and reliable use in addressing these needs in relation to collection organization and management. Twenty-five decamer oligonucleotide primers were screened individually with a test array composed of individuals representing a range of genetic relationships in Brassica oleracea L. (vegetable and forage cole crops). Over 140 reproducible, polymorphic fragments were generated for study. Each individual of the test array exhibited a unique molecular genotype and composites specific for accessions and botanical varieties could be established. An analysis of similarity based on amplified DNA fragments reflected the known genetic relationships among the selected entries. These results demonstrated that RAPD markers can be of great value in gene bank management for purposes of identification, measurement of variation, and establishment of genetic similarity at the intraspecific level. 相似文献
79.
Takao Itino 《Population Ecology》1992,34(1):203-211
There are, at least, three possible ways in which similar species coexist; resource partitioning, interference competition, and exploitation competition. Here, I investigated which way contributed to the coexistence of leafroller-hunting eumenid wasp species. Resource partitioning and, in addition, differential diet breadths proved to promote species coexistence in this case. First, I analyze the prey records and diet overlap of four eumenid species in a local area. The larger two eumenids hunted similar-sized prey items and had similar potential taxonomic prey uses. But the diet breadth of the subsocial eumenid was much wider than that of the solitary one. As a result, the diet overlap between the two large eumenids decreased. This was because the solitary eumenid attend repeatedly to the same hunting site inhabited by one abundant prey species, while the subsocial one made random hunting. On the other hand, the two medium-sized eumenids partitioned resources according to prey size. Secondly, I related these results to prey choice by several other species of eumenid obtained from literature sources. Ten Japanese common eumenids were divided into four groups according to their prey size. In each of the four groups, 2 to 3 wasp species differentiated the habitat (1 group) or coexisted by means of differential diet breadths (parallel with differential sociality, 2 groups). 相似文献
80.
Toxoplasma gondii, growing exponentially in heavily infected mutant Chinese hamster ovary cells that had a defined defect in purine biosynthesis, did not incorporate [U-14C]glucose or [14C]formate into the guanine or adenine of nucleic acids. Intracellular parasites therefore must be incapable of synthesizing purines and depend on their host cells for them. Extracellular parasites, which are capable of limited DNA and RNA synthesis, efficiently incorporated adenosine nucleotides, adenosine, inosine, and hypoxanthine into their nucleic acids; adenosine 5′-monophosphate was the best utilized precursor. Extracellular parasites incubated with ATP labeled with 3H in the purine base and 32P in the α-phosphate incorporated the purine ring 50-fold more efficiently than they did the α-phosphate. Thus, ATP is largely degraded to adenosine before it can be used by T. gondii for nucleic acid synthesis. Two pathways for the conversion of adenosine to nucleotides appear to exist, one involving adenosine kinase, the other hypoxanthine—guanine phosphoribosyl transferase. In adenosine kinase-less mutant parasites, the efficiency of incorporation of ATP or adenosine was reduced by 75%, which indicates the adenosine kinase pathway was predominant. Extracellular parasites incorporated ATP into both the adenine and the guanine of their nucleic acids, so ATP from the host cell could supply the entire purine requirement of T. gondii. However, ATP generated by oxidative phosphorylation in the host cell is not essential for parasites because they grew normally in a cell mutant that was deficient in aerobic respiration and almost completely dependent upon glycolysis. 相似文献