全文获取类型
收费全文 | 1188517篇 |
免费 | 94480篇 |
国内免费 | 98787篇 |
专业分类
1381784篇 |
出版年
2023年 | 8468篇 |
2022年 | 9508篇 |
2021年 | 11000篇 |
2020年 | 9976篇 |
2019年 | 11198篇 |
2018年 | 11557篇 |
2017年 | 8027篇 |
2016年 | 9063篇 |
2015年 | 10332篇 |
2014年 | 13667篇 |
2013年 | 12626篇 |
2012年 | 107806篇 |
2011年 | 121695篇 |
2010年 | 29569篇 |
2009年 | 21956篇 |
2008年 | 102113篇 |
2007年 | 106037篇 |
2006年 | 99382篇 |
2005年 | 94545篇 |
2004年 | 91639篇 |
2003年 | 86929篇 |
2002年 | 77158篇 |
2001年 | 62018篇 |
2000年 | 77370篇 |
1999年 | 32670篇 |
1998年 | 7195篇 |
1997年 | 5459篇 |
1996年 | 4655篇 |
1995年 | 4403篇 |
1994年 | 4575篇 |
1993年 | 3779篇 |
1992年 | 4359篇 |
1991年 | 3846篇 |
1990年 | 3776篇 |
1989年 | 4294篇 |
1988年 | 4269篇 |
1987年 | 3919篇 |
1986年 | 3735篇 |
1985年 | 3572篇 |
1983年 | 3390篇 |
1959年 | 3879篇 |
1958年 | 6824篇 |
1957年 | 6798篇 |
1956年 | 6073篇 |
1955年 | 5733篇 |
1954年 | 5439篇 |
1953年 | 5173篇 |
1952年 | 4656篇 |
1951年 | 4278篇 |
1950年 | 3511篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
Estrous response, pregnancy rate, and the relationship between palpable corpora lutea (CL) and plasma progesterone concentrations were studied during the spring (n = 149) and fall (n = 146) breeding seasons using postpartum, crossbred beef cows consisting of 0-Brahman, 1 4 - Brahman , or 1 2 Brahman breeding. At the start of each breeding season, a jugular blood sample was collected for progesterone analysis; each cow was palpated for the presence of a CL and randomly alloted within breed to a non-treated control or Synchro-Mate-B (SMB) treatment. Fewer (P<0.01) fall than spring bred cows exhibited estrus within the first 21 d of breeding. Within 48 h of implant removal, 84 and 64% (spring and fall, respectively) of synchronized cows exhibited estrus. In both seasons, more SMB than control cows became pregnant by 7 d of breeding. Percentage of Brahman breeding did not influence estrous response or pregnancy rate. On the basis of 292 evaluations, palpation per rectum correctly assessed ovarian status 74% of the time. These data indicate that although season can influence response. SMB can be used effectively to synchronize Brahman crossbred cows. In addition, it was found that there is a close relationship between palpation per rectum and plasma concentrations of progesterone. 相似文献
912.
In past attempts, the experimental germination of the seeds of Euonymus americanus L. in vitro has had little success. However, treatment of seeds with ruminal fluid containing viable microflora has been successful in stimulating germination. In the presence of the cellulolytic ruminal bacterium, Clostridium cellobioparum ATCC 15832, seeds of E. americanus were stimulated to germinate. Subsequent studies were designed to determine whether the bacterium synthesized a cellulolytic enzyme responsible for initiating germination. The cell-free endocellulase from C. cellobioparum induced germination of the seeds. To support the hypothesis that the endocellulase from C. cellobioparum was responsible for triggering germination, a 1,4-beta-d-glucan glucanohydrolase (EC 3.2.1.4) from Penicillum funiculosum was used to treat the seeds. In addition, no germination was obtained from seeds treated with a commercial exocellulase enzyme. Also, Ruminococcus flavefaciens FD-1 was found to initiate germination of E. americanus seeds. Thus, cellulase activity is indicated in the degradation of the testa of the seed, allowing imbibition and germination. 相似文献
913.
Curry C Gilkes N O'neill G Miller RC Skipper N 《Applied and environmental microbiology》1988,54(2):476-484
We used the yeast MEL1 gene for secreted alpha-galactosidase to construct cartridges for the regulated expression of foreign proteins from Saccharomyces cerevisiae. The gene for a Cellulomonas fimi beta-1,4-exoglucanase was inserted into one cartridge to create a fusion of the alpha-galactosidase signal peptide to the exoglucanase. Yeast transformed with plasmids containing this construction produced active extracellular exoglucanase when grown under conditions appropriate to MEL1 promoter function. The cells also produced active intracellular enzyme. The secreted exoglucanase was N-glycosylated and was produced continuously during culture growth. It hydrolyzed xylan, carboxymethyl cellulose, 4-methylumbelliferyl-beta-d-cellobiose, and p-nitrophenyl-beta-d-cellobiose. A comparison of the recombinant S. cerevisiae enzyme with the native C. fimi enzyme showed the yeast version to have an identical K(m) and pH optimum but to be more thermostable. 相似文献
914.
Cutaneous leishmaniasis (CL) is one of the most important causes of chronic ulcerative skin lesions. The disease is endemic in many parts of the world, presenting a range of clinical forms - acute, chronic, recurrent and diffuse(1). Several species of Leishmania are involved, including L. major, L. tropica and L. aethiopica in the Old World, and several members of the L. braziliensis and L. mexicana complexes in the New World. Some forms of the disease produce only mild, self-limited lesions, while at the other extreme are the destructive mucocutaneous forms caused by L. braziliensis and L. panamensis(1-7). In all cases, chemotherapy tends to be difficult - often requiring prolonged parenteral administration of toxic drugs such as pentavalent antimonials or amphotericin B. Such drugs are also expensive and relatively inefficient in the sense that much of the active ingredient is excreted by the patient before reaching its target. Consequently, there is renewed interest in the development of active formulations suitable for topical application directly onto the lesions. 相似文献
915.
The eastern Caribbean island of Saint Lucia is now famous in parasitological history as the setting for a major programme of schistosomiasis control'. Perhaps less well-known are the island's effective control of many intestinal parasites, and elimination of malaria, such that the current patterns of mortality and other demographic indicators now resemble those of industrialized countries. More recently, the island has become the focus for another community-based health programme as the Caribbean region again comes to grips with Aedes aegypti and its recently imported relative, Aedes albopictus, important vectors of yellow fever and dengue viruses (see Box 1). 相似文献
916.
917.
918.
布氏田鼠G带染色体研究 总被引:1,自引:0,他引:1
布氏田鼠(Microtus brandti Radde)是田鼠亚科中的一种小型野鼠,分布在我国内蒙古东部地区,对牧草和农田危害甚大,同时又是鼠疫的贮存宿主。有关布氏田鼠的染色体研究国内外未见报道。作者得到内蒙古锡盟鼠防 相似文献
919.
920.