全文获取类型
收费全文 | 1918篇 |
免费 | 153篇 |
专业分类
2071篇 |
出版年
2021年 | 27篇 |
2020年 | 17篇 |
2019年 | 16篇 |
2018年 | 20篇 |
2017年 | 15篇 |
2016年 | 47篇 |
2015年 | 74篇 |
2014年 | 54篇 |
2013年 | 88篇 |
2012年 | 94篇 |
2011年 | 92篇 |
2010年 | 63篇 |
2009年 | 65篇 |
2008年 | 72篇 |
2007年 | 96篇 |
2006年 | 84篇 |
2005年 | 66篇 |
2004年 | 82篇 |
2003年 | 66篇 |
2002年 | 55篇 |
2001年 | 34篇 |
2000年 | 53篇 |
1999年 | 45篇 |
1998年 | 17篇 |
1997年 | 21篇 |
1996年 | 18篇 |
1995年 | 19篇 |
1993年 | 13篇 |
1992年 | 30篇 |
1991年 | 31篇 |
1990年 | 37篇 |
1989年 | 31篇 |
1988年 | 27篇 |
1987年 | 32篇 |
1986年 | 28篇 |
1985年 | 26篇 |
1984年 | 34篇 |
1983年 | 29篇 |
1982年 | 17篇 |
1981年 | 16篇 |
1980年 | 15篇 |
1979年 | 32篇 |
1978年 | 17篇 |
1977年 | 19篇 |
1975年 | 16篇 |
1974年 | 26篇 |
1973年 | 28篇 |
1972年 | 13篇 |
1971年 | 12篇 |
1970年 | 17篇 |
排序方式: 共有2071条查询结果,搜索用时 15 毫秒
1.
2.
Hoshino Atsushi; Abe Yukihide; Saito Norio; Inagaki Yoshishige; Iida Shigeru 《Plant & cell physiology》1997,38(8):970-974
The Japanese morning glory carrying the recessive mutable speckledallele with the dominant speckled-activator bears colorlessflowers with fine and round colored spots distributed over thecorolla whereas the plant without the speckled-activator producespale yellow flowers. Previous chemical analysis has indicatedthat a mutation in the gene for flavanone 3-hydroxylase (F3H)is a likely candidate for the speckled allele. However, theF3HmRNA without sequence alteration accumulates normally inthe pale yellow flowers, indicating that the speckled alleleis neither the F3H gene nor a regulatory gene acting on theF3H gene expression. (Received April 4, 1997; Accepted June 2, 1997) 相似文献
3.
4.
5.
Kun-Nan Tsai Guang-Wu Chen Calvin Yu-Chian Chen 《Journal of biomolecular structure & dynamics》2013,31(5):1089-1099
Abstract The resonances of the protonated carbons of [d(TAGCGCTA)]2 have been assigned by the two-dimensional proton-detected double-quantum heteronuclear correlation experiment ([1H-l3C]-DQCOSY). 13C-coupled and l3C-decoupled versions of the experiment were used. The assignment method is discussed in detail. The deoxyribose cross peaks segregate into five well-resolved regions, and the base cross peaks have distinct features that are helpful for assignments. The cross peaks from the 1H-13C pairs at the Cyd5, Ado2 and ThdCH3 base positions fall in separate regions of the spectrum from each other; they also are resolved from the closely spaced Ado8, Guo8, Cyd6 and Thd6. Additional parameters for distinction of the base signals are their differing J-coupling values and long-range coupling patterns. 相似文献
6.
7.
8.
H I Calvin K Grosshans S R Musicant-Shikora S I Turner 《Journal of reproduction and fertility》1987,81(1):1-11
Essentially all of the selenium in the rat spermatozoon is bound to a polypeptide of Mr 15,000-17,000 confined to the capsule that surrounds the sperm mitochondria. Isoelectric focussing of isolated 75Se-labelled, carboxymethylated mitochondrial capsule protein (MCP) reveals the presence of at least four radioactive components, with a predominant charge isomer at pI4.6. The sperm selenoprotein appears to be identical with MCP, as judged by the exact coincidence of radioactivity and protein stain during two-dimensional electrophoresis. The temporal pattern of 75Se-labelling of rat caput epididymal spermatozoa after intratesticular 75Se injection suggests that maximum incorporation of 75Se into MCP occurs in step 7-step 12 spermatids and that 75Se uptake ceases during step 15 of spermiogenesis. The developmental appearance of sperm selenoprotein in rat testis therefore appears to lag several days behind that reported for MCP in mouse testis, suggesting the presence of selenium-free MCP in immature germ cells. SDS gel electrophoretic analysis of testis subcellular fractions 24 h after 75Se injection into rat testis at 21, 28 and 90 days of age indicates that sperm selenoprotein first appears in very low concentration during late meiosis and that its concentration increases sharply during early spermiogenesis. Additional 75Se-labelled polypeptides were detected on the gels, most of them of higher molecular weight than MCP. At least two of these (Mr 47,000 and 54,000) displayed a marked decrease in labelling between 5 and 24 h after injection into adult testis, coincident with a comparable increase in 75Se-labelled MCP, indicating that they may be precursors of MCP. 相似文献
9.
Reproductive success,spontaneous embryo abortion,and genetic load in flowering plants 总被引:15,自引:0,他引:15
Summary Reproductive success is divided into two phases: preemergent (the number of viable seeds that enter the ambient environment) and postemergent (the percentage of progeny that survive to reproduce). We studied preemergent reproductive success (PERS) in flowering plants by measuring the fruit/flower (Fr/Fl) ratio and the seed/ovule (S/O) ratio in a number of species of outcrossing and inbreeding plants, where PERS=the product of (Fr/Fl) and (S/O). In order to determine the influence of the ambient environment (including resource availability) we studied pairs of outcrossing and inbreeding species occurring in the same habitat. Among outcrossing species PERS averaged about 22%, whereas in inbreeding species the average was approximately 90%. The progeny/zygote (P/Z) ratio was studied in hand-pollinated populations in Epilobium angustifolium (a strongly outcrossing species) from populations in Oregon and Utah, by direct observation of embryogenesis at twoday intervals throughout the course of seed development. The P/Z ratio in both populations averaged near 30%, and the developing embryos showed a surprising array of abnormalities that resulted in embryo death. During early development >95% of the ovules had normally developing globular embryos, but beginning with differentiation (cotyledon formation) about 70% of the original globular embryos aborted during the course of embryogenesis and seed development. The clustering of developmental lethals during peroids of major differentiation events parallels the animal model of development. We found little evidence that PERS was limited by the ambient environment (including resource availability), pollination, or factors associated with the inbreeding habit. Instead, PERS was found to be inextricably linked to outcrossing plants, whose breeding systems promote genetic variability. The high incidence of developmental lethals in E. angustifolium and the resulting low P/Z ratio (ca. 30%) is attributed to genetic load (any lethal mutation or allelic combination) possibly working in combination with developmental selection (interovarian competition among genetically diverse embryos). Examples of maternally controlled, fixed patterns of ovule abortion with respect to position or number are discussed. However, we found no need to employ female choice as a hypothesis to explain our results for the extensive, seemingly random patterns of embryo abortion in E. angustifolium and other outcrossing species. A more parsimonious, mechanistic explanation based on genetic load-developmental selection is sufficient to account for the differential survivorship of embryos. Likewise, the traditional concept of a positive growth regulator feedback system based on the number of surviving ovules in an ovary can account for subsequent fruit survivorship. 相似文献
10.
Calvin E. Olson 《Chronobiology international》1987,4(1):19-29
Several models of erosive peptic disease have used drug-induced lesions to examine protective mechanisms of the gastric mucosa. Physiological processes such as acid secretion, motility, or epithelial cell turnover have circadian rhythms which may modulate the susceptibility of the gastric mucosa to injury. In this review are described recent studies which demonstrated that susceptibility to gastric mucosal injury by acidified aspirin and absolute ethanol varied with the phases of the light-dark cycle. Acidified aspirin caused significantly more gastric mucosal lesions when administered early in the light phase compared to administration early in the dark phase. The differences in susceptibility were not altered by pretreatment conditions such as immobilization or length of the fasting period. Absolute ethanol also caused significantly greater gastric mucosal injury when administered in the light than in the dark phase, but this difference was only evident in rats immobilized during the pretreatment fasting period. Further studies are needed to correlate circadian susceptibility to drug-induced gastric mucosal injury with physiological defense mechanisms. Careful attention to circadian timekeeping may allow us to refine therapy to optimize physiological defense mechanisms in the stomach. 相似文献