全文获取类型
收费全文 | 7668篇 |
免费 | 682篇 |
国内免费 | 4篇 |
专业分类
8354篇 |
出版年
2023年 | 47篇 |
2022年 | 88篇 |
2021年 | 147篇 |
2020年 | 95篇 |
2019年 | 119篇 |
2018年 | 145篇 |
2017年 | 139篇 |
2016年 | 208篇 |
2015年 | 320篇 |
2014年 | 353篇 |
2013年 | 437篇 |
2012年 | 578篇 |
2011年 | 540篇 |
2010年 | 399篇 |
2009年 | 327篇 |
2008年 | 461篇 |
2007年 | 429篇 |
2006年 | 392篇 |
2005年 | 423篇 |
2004年 | 379篇 |
2003年 | 316篇 |
2002年 | 330篇 |
2001年 | 111篇 |
2000年 | 74篇 |
1999年 | 99篇 |
1998年 | 118篇 |
1997年 | 62篇 |
1996年 | 91篇 |
1995年 | 80篇 |
1994年 | 70篇 |
1993年 | 63篇 |
1992年 | 59篇 |
1991年 | 55篇 |
1990年 | 48篇 |
1989年 | 39篇 |
1988年 | 48篇 |
1987年 | 38篇 |
1986年 | 26篇 |
1985年 | 59篇 |
1984年 | 46篇 |
1983年 | 33篇 |
1982年 | 43篇 |
1981年 | 30篇 |
1980年 | 39篇 |
1979年 | 26篇 |
1977年 | 29篇 |
1976年 | 33篇 |
1975年 | 23篇 |
1973年 | 26篇 |
1971年 | 21篇 |
排序方式: 共有8354条查询结果,搜索用时 0 毫秒
1.
2.
The distribution of grooming and touching behaviours was recorded in a group of captive ring-tailed lemurs. Grooming was found
to be performed chiefly by older, higher ranking animals; touching (i.e., “reach out and touch” behaviour) was directed primarily
by younger, low ranking animals to older, high ranking individuals. It is suggested that such touching is a submissive gesture
in this species. 相似文献
3.
A E Rettie M Boberg A W Rettenmeier T A Baillie 《The Journal of biological chemistry》1988,263(27):13733-13738
The cytochrome P-450-mediated desaturation of valproic acid (VPA) to its hepatotoxic metabolite, 2-n-propyl-4-pentenoic acid (4-ene-VPA), was examined in liver microsomes from rats, mice, rabbits and humans. The highest substrate turnover was found with microsomes from rabbits (44.2 +/- 2.7 pmol of product/nmol P-450/15 min), while lower activities were observed in preparations from human, mouse, and rat liver, in that order. Pretreatment of animals with phenobarbital led to enhanced rates of formation of 4-ene-VPA in vitro and yielded induction ratios for desaturation ranging from 2.5 to 8.4, depending upon the species. Comparative studies in the rat showed that phenobarbital is a more potent inducer of olefin formation than either phenytoin or carbamazepine. The mechanism of the desaturation reaction was studied by inter- and intramolecular deuterium isotope effect experiments, which demonstrated that removal of a hydrogen atom from the subterminal C-4 position of VPA is rate limiting in the formation of both 4-ene- and 4-hydroxy-VPA. Hydroxylation at the neighboring C-5 position, on the other hand, was highly sensitive to deuterium substitution at that site, but not to deuteration at C-4. Based on these findings, it is proposed that 4-ene- and 4-hydroxy-VPA are products of a common P-450-dependent metabolic pathway, in which a carbon-centered free radical at C-4 serves as the key intermediate. 5-Hydroxy-VPA, in contrast, derives from an independent hydroxylation reaction. 相似文献
4.
5.
6.
7.
A controlled environment experiment investigated whether thered:far-red (R:FR) ratio of light at the apical bud of the mainstolon could alter plant morphogenesis in clonal cuttings ofwhite clover (Trifolium repens L.) The apical bud included theapical meristem, five to six developing leaf primordia withassociated axillary bud primordia and stipules and the firstemerged folded leaf until development was greater than 0·3on the Carlson scale. Three light regimes were imposed on theapical bud by collimating light from R or FR light-emittingdiodes so that the R:FR ratio of light incident at the apicalbud was set at 0·25, 1·6 or 2·1, withoutsignificantly altering photosynthetically active radiation.The effect of these light regimes on white clover seedling growthwas also tested. At a low R:FR ratio seedling hypocotyl and cotyledon lengthswere significantly longer. However, with the cuttings, the lighttreatments did not alter node appearance rate or internode lengthof the main stolon, petiole length, area of leaves or totalshoot dry matter. There was one significant photomorphogeneticresponse in the cuttings, a delay of 0·5 of a phyllochronin the appearance of branches from axillary buds in the lowR:FR ratio treatment relative to the other treatments. Wherebranch appearance was delayed plants had fewer branches. Thisdifference could be ascribed solely to a delay in branch appearanceas there were no significant treatment effects on either theinitiation of axillary bud primordia within the apical bud,the probability of branching or on the rate of growth of branchesafter appearance. Because treatment of the apical bud inducedonly one of the many previously observed responses of whiteclover to a decrease in the R:FR ratio of light, we concludethat other plant organs must also sense the quality of incidentlight.Copyright 1994, 1999 Academic Press White clover, Trifolium repens, apical bud, light quality, red:far-red ratio, light-emitting diode, branching, axillary buds, photomorphogenesis 相似文献
8.
9.
Using two-dimensional polyacrylamide gel electrophoresis, the GroEL homologue ofBacillus subtilis was shown to be induced upon infection with Ø105clz, a clear plaque mutant of the temperate bacteriophage Ø105. Western blotting of one dimensional polyacrylamide gels also showed the induction of the GroEL homologue when cells were infected with Ø105clz. 相似文献
10.