首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   535574篇
  免费   57651篇
  国内免费   762篇
  593987篇
  2018年   6327篇
  2017年   5966篇
  2016年   8470篇
  2015年   12385篇
  2014年   13829篇
  2013年   18616篇
  2012年   22228篇
  2011年   22219篇
  2010年   14555篇
  2009年   12577篇
  2008年   19229篇
  2007年   19613篇
  2006年   18244篇
  2005年   17342篇
  2004年   17122篇
  2003年   15930篇
  2002年   15251篇
  2001年   20449篇
  2000年   20410篇
  1999年   16431篇
  1998年   6237篇
  1997年   6118篇
  1996年   5800篇
  1995年   5588篇
  1994年   5315篇
  1993年   5198篇
  1992年   13422篇
  1991年   13314篇
  1990年   12969篇
  1989年   12406篇
  1988年   11516篇
  1987年   10777篇
  1986年   10285篇
  1985年   10122篇
  1984年   8417篇
  1983年   7291篇
  1982年   5492篇
  1981年   4989篇
  1980年   4612篇
  1979年   7802篇
  1978年   6329篇
  1977年   5623篇
  1976年   5284篇
  1975年   6107篇
  1974年   6656篇
  1973年   6489篇
  1972年   5714篇
  1971年   5341篇
  1970年   4508篇
  1969年   4425篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
Tree size, survival, and coppicing of micropropagated plantlets, macropropagated cuttings, and seedlings of Eucalyptus grandis Hill ex Maiden were monitored through 57 months in a study in southern Florida to assess propagation options. Two plantlet lines developed by direct micropropagation and orchard open-pollinated seedlings from three ortets were compared in the main study. Rooted cuttings from up to four ramets of each of the three ortets and another ortet were examined in an adjacent supplemental study. Freezes at six and 16 months killed most initial and first-coppice stems to the ground. Most developmental differences in the main study were consistent from ages 2 to 57 months. Propagation by ortet interactions were observed beginning at 21 months, due to the poor performance of seedlings of one ortet after the second freeze. At 57 months, no differences in tree height, DBH, volume, or survival were detected between plantlet lines and between rooted cuttings and plantlets, but seedlings were inferior to plantlets and cuttings. Vegetative propagules had more uniform tree size at every age, with typically less than one-half the variability observed among seedlings. Even though plantlets and cuttings may be more expensive to produce, they have numerous advantages over seedlings for E. grandis plantation establishment in Florida.  相似文献   
942.
Auromomycin and macromomycin from the organism Streptomyces macromomyceticus have been crystallized. The X-ray diffraction pattern of crystals of each molecule is consistent with space group P21212 with cell parameters a = 46.45 A?, b = 54.34 A? and c = 42.03 A? for auromomycin, and a = 46.45 A?, b = 54.52 A? and c = 41.54 A? for macromomycin. Diffraction analysis of auromomycin is in progress.  相似文献   
943.
Histological and electron-microscopic radioautographic investigations of regenerating medullar tissue were performed in rabbits following curettage. Low differentiated connective tissue cells were shown to possess the highest proliferative activity. DNA synthesis mainly took place in low differentiated, endothelial and osteogenic cells. It is suggested that low differentiated cells take part in histogenesis of regenerating medullar tissue, including osteo- and angiogenesis.  相似文献   
944.
Experiments on E. coli used as a model have revealed that fatty-acid composition is one of the characteristics which determine the viability of bacteria in the air. The viability of microbial cells in the air has been shown to increase with the increase of the pool of cyclopropane acids and the palmitic acid/palmitoleic acid ratio in the cells, irrespective of their genotype and the phase of their growth.  相似文献   
945.
946.
947.
948.
949.
Fowler's central slip tenotomy for old mallet deformity is a well-known, but apparently seldom used procedure. Although the vast majority of patients with mallet finger can be successfully treated by well-known conservative techniques, occasional patients, such as the 11 reported here, will have enough difficulty with a chronic mallet finger to warrant rebalancing of the extensor apparatus by means of central slip tenotomy.  相似文献   
950.
The authors study by means of immunoperoxidase method the pattern of thyroglobulin, triiodothyronine and thyroxine distribution in 58 cases of thyroid disorders: 15 euthyroid goiters, 10 Graves' disease, 7 Hashimoto's thyroiditis, 11 folliculo-papillary carcinomas (6 primary tumors and 5 lymph node metastases), 8 follicular carcinomas, 4 anaplastic carcinomas and 3 medullary carcinomas. Thyroglobulin, triiodothyronine and thyroxine were present in most of the thyroid disorders, excepting anaplastic and medullary carcinomas. Thyroglobulin and thyroxine were localized both in the follicular epithelium and in the colloid, whereas triiodothyronine was present especially in the follicular cells. The thyroid hormones distribution in benign lesions is rather similar. In carcinomas, the pattern of thyroglobulin, triiodothyronine and thyroxine is more heterogeneous, but generally the triiodothyronine distribution is similar to that of thyroglobulin. In some carcinomas, triiodothyronine and thyroxine showed a weak or negative immunostaining. The immunoperoxidase method is a valuable tool in the study of functional disturbances in the thyroid pathology and in the diagnosis of thyroid carcinoma metastases as well. Positive thyroid hormones staining clearly indicates the thyroid origin of metastases.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号