全文获取类型
收费全文 | 19637篇 |
免费 | 1884篇 |
国内免费 | 644篇 |
专业分类
22165篇 |
出版年
2023年 | 132篇 |
2022年 | 315篇 |
2021年 | 473篇 |
2020年 | 321篇 |
2019年 | 413篇 |
2018年 | 473篇 |
2017年 | 350篇 |
2016年 | 592篇 |
2015年 | 967篇 |
2014年 | 1057篇 |
2013年 | 1253篇 |
2012年 | 1452篇 |
2011年 | 1431篇 |
2010年 | 937篇 |
2009年 | 740篇 |
2008年 | 1009篇 |
2007年 | 944篇 |
2006年 | 891篇 |
2005年 | 819篇 |
2004年 | 750篇 |
2003年 | 716篇 |
2002年 | 642篇 |
2001年 | 551篇 |
2000年 | 484篇 |
1999年 | 450篇 |
1998年 | 217篇 |
1997年 | 203篇 |
1996年 | 186篇 |
1995年 | 167篇 |
1994年 | 151篇 |
1993年 | 121篇 |
1992年 | 245篇 |
1991年 | 244篇 |
1990年 | 203篇 |
1989年 | 216篇 |
1988年 | 189篇 |
1987年 | 152篇 |
1986年 | 144篇 |
1985年 | 168篇 |
1984年 | 123篇 |
1983年 | 98篇 |
1982年 | 90篇 |
1981年 | 97篇 |
1979年 | 109篇 |
1978年 | 91篇 |
1977年 | 71篇 |
1976年 | 68篇 |
1975年 | 88篇 |
1974年 | 89篇 |
1973年 | 81篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
Chang CW 《Plant physiology》1979,63(5):973-977
During cotton leaf development, starch accumulation was characterized by an initial rise to a maximum at the second to the fourth leaf from the apex. Then, starch content progressively decreased with leaf age. Starch accumulation was inversely related to the ratio of amylopectin to amylose. Differences between leaves in this ratio resulted from variations in both amylose and amylopectin levels. Fluctuations in amylose levels were more extreme than those of amylopectin. 相似文献
63.
64.
65.
Construction and characterization of amplifiable multicopy DNA cloning vehicles derived from the P15A cryptic miniplasmid. 总被引:819,自引:431,他引:388 下载免费PDF全文
Construction and characterization of a class of multicopy plasmid cloning vehicles containing the replication system of miniplasmid P15A are described. The constructed plasmids have cleavage sites within antibiotic resistance genes for a variety of commonly employed site-specific endonucleases, permitting convenient use of the insertional inactivation procedure for the selection of clones that contain hybrid DNA molecules. Although the constructed plasmids showed DNA sequence homology with the ColE1 plasmid within the replication region, were amplifiable by chloramphenicol or spectinomycin, required DNA polymerase I for replication, and shared other replication properties with ColE1, they were nevertheless compatible with ColE1. P15A-derived plasmids were not self-transmissible and were mobilized poorly by Hfr strains; however, mobilization was complemented by the presence of a ColE1 plasmid within the same cell. 相似文献
66.
Zona-free eggs were introduced to fresh or preincubated sperm suspensions and the penetration of eggs by foreign spermatozoa was examined, as evidenced by enlargement of the sperm head and formation of the male pronucleus. It was found that zona-free hamster eggs can be penetrated by guinea-pig, deer mouse and rabbit spermatozoa but zona-free rat, mouse and rabbit eggs cannot be penetrated by guinea-pig spermatozoa. Furthermore, zona-free rat and mouse eggs cannot be penetrated by spermatozoa from two species of deer mice and the Mongolian gerbil. The zona pellucida of a few intact rat eggs can be penetrated by mouse (6%) and by P. leucopus spermatozoa (14%) but enlargement of the sperm head and formation of pronuclei were observed in the former but not in the latter. It seems that (1) sperm capacitation is required for the penetration of zona-free eggs, (2) the attachment of foreign spermatozoa to eggs may indicate their potential ability of penetration in some cases, (3) there is a certain affinity between the vitellus of one species and spermatozoa from another species, (4) the block to the entry of foreign spermatozoa is not only in the zona pellucida but also in the vitelline membrane, (5) zona-free hamster eggs can be penetrated by spermatozoa of six species, (6) mouse spermatozoa can penetrate zona-free eggs of three species, and (7) fertilization of intact P. maniculatus eggs can be achieved in vitro. 相似文献
67.
Five cases of primary amoebic meningoencephalitis associated with swimming in freshwater lakes have been recorded in Florida over the past 14 years. The present study demonstrated that pathogenic Naegleria, the causative agent, is relatively widespread. Twelve of 26 lakes sampled only once yielded the amoeba. Populations in three of five lakes sampled routinely reached levels of one amoeba per 25 ml of water tested during the hot summer months. Overwintering in freshwater lake bottom sediments was demonstrated, showing that thermal-discharge pollution of waters plays a miniscule, if any, role in the maintenance of pathogenic Naegleria in nature in this semitropical area. 相似文献
68.
Sei-itsu Murota Midori Abe Katsuhiro Otsuka Wen-Chang Chang 《Prostaglandins & other lipid mediators》1977,13(4):711-717
Early effects of various prostaglandins on the production of hexosamine-containing substances by cultured fibroblasts, which were derived from a rat carrageenin granuloma, were studied. At the stationary phase, the cells were exposed for 6 h to one of the prostaglandin A1 (PGA1), A2, B1, B2, D2, F1α, F2α, E1, E2 or arachidonic acid in various concentrations ranging from 0.01 to 10 μg/ml for all the stimuli and from 10 pg to 10 μg/ml for PGF2α. The activity of the cells in incorporating 3H-glucosamine into hexosamine-containing substances (acidic) glycosaminoglycans and glycoproteins) during this period was compared with that of control cells. All the stimuli tested showed more or less stimulative effect on the synthesis of hexosamine-containing substances at their specific concentrations. PGF2α was found to be the most potent stimulant and its stimulative effect was found significant even at the low concentration of 100 pg/ml. PGD2, F1α and E2 were the next potent stimuli. Their optimum dose were around 1 μg/ml but they still had significant stimulation at the concentration of 0.01 μg/ml. Effect of PGE2 was rather mild. Stimulation by PGA1, A2, B1 and B2 or arachidonic acid was seen at high dose, and its seemed to be non-specific. The results suggested that these prostaglandins such as PGF2α, D2, F1α and E2 play some important role on regulating the production of intercellular ground substances. 相似文献
69.
The effect of PGF2α on the synthesis of hexosamine-containing substances (acidic glycosaminoglycans and glycoproteins) was studied in cultured fibroblasts derived from a rat carrageenin granuloma. Treatment with PGF2α ranging from 0.01 μg/ml to 20 μg/ml resulted in a significant increase of the production of these macromolecules by the cells. The stimulatory effect was found significant even at the low concentration of 10 ng/ml, and could be seen as early as 3h after exposure to PGF2α. The hexosamine-containing substances increased by PGF2α revealed that 80% of the increase was due to acidic glycosaminoglycans and the rest was due to glycoproteins. 相似文献
70.