全文获取类型
收费全文 | 9334篇 |
免费 | 885篇 |
国内免费 | 3篇 |
出版年
2022年 | 65篇 |
2021年 | 131篇 |
2020年 | 65篇 |
2019年 | 73篇 |
2018年 | 114篇 |
2017年 | 107篇 |
2016年 | 195篇 |
2015年 | 334篇 |
2014年 | 340篇 |
2013年 | 519篇 |
2012年 | 548篇 |
2011年 | 509篇 |
2010年 | 368篇 |
2009年 | 295篇 |
2008年 | 473篇 |
2007年 | 506篇 |
2006年 | 441篇 |
2005年 | 437篇 |
2004年 | 426篇 |
2003年 | 436篇 |
2002年 | 424篇 |
2001年 | 148篇 |
2000年 | 116篇 |
1999年 | 128篇 |
1998年 | 157篇 |
1997年 | 97篇 |
1996年 | 91篇 |
1995年 | 107篇 |
1994年 | 66篇 |
1993年 | 79篇 |
1992年 | 93篇 |
1991年 | 103篇 |
1990年 | 110篇 |
1989年 | 93篇 |
1988年 | 80篇 |
1987年 | 84篇 |
1986年 | 81篇 |
1985年 | 80篇 |
1984年 | 98篇 |
1983年 | 65篇 |
1982年 | 87篇 |
1981年 | 95篇 |
1980年 | 69篇 |
1979年 | 61篇 |
1978年 | 80篇 |
1977年 | 57篇 |
1974年 | 73篇 |
1973年 | 72篇 |
1972年 | 57篇 |
1970年 | 58篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
121.
122.
Margaret Kain Balbach 《American journal of botany》1965,52(4):317-330
A reinvestigation of the 4 American species of Lepidocarpon described from petrifactions and of Illiniocarpon cadyi, also described from petrifactions, shows that they are all conspecific. When they, in turn, are compared with specimens of the British petrifaction L. lomaxi, no significant differences can be demonstrated. The criteria cited as diagnostic for the American petrifaction species of Lepidocarpon were found to fail as bases of discrimination when 400 specimens of Lepidocarpon found in coal balls of Pennsylvanian age were studied. Measurements and observations made of a sample size of 136 of these 400 specimens reveal a degree of variation compatible with that of a single species. All previously described American and British petrifaction taxa fall within (or not significantly beyond) this range of variability of a single species as determined from this sample. Additional evidence concerning the 3-dimensional sporophyll structure of this one species of Lepidocarpon and the configurations obtained by certain planes of section through it have served to demonstrate that Illiniocarpon cadyi is in reality a taxon based on sectioned structures of the sporophyll of this Lepidocarpon species. Under the rules of priority, this species of Lepidocarpon and the following are assigned to Lepidocarpon lomaxi: Illiniocarpon cadyi, Lepidocarpon ioense, L. magnificum (megasporangiate form), L. crenatum and L. palmerensis. On the basis of the morphological evidence, the British petrifaction described as Lepidocarpon wildianum is also equated with L. lomaxi. 相似文献
123.
Relative Stability of Pertussis Vaccine Preserved with Merthiolate, Benzethonium Chloride, or the Parabens 总被引:2,自引:2,他引:0 下载免费PDF全文
When stored at 4 C, or heated at 22 or 35 C followed by storage at 4 C, the potency of pertussis vaccines preserved with Merthiolate was more stable than the potency of vaccines preserved with benzethonium chloride or the parabens (methyl- and propyl-p-hydroxybenzoate). Without preservative, potency was more stable than in the presence of benzethonium chloride or the parabens, but less stable than when Merthiolate was present. The histamine-sensitizing factor of the vaccines likewise decreased with the loss of potency. The deleterious effect of benzethonium chloride and the absence of the stabilizing effect of Merthiolate were contributing factors, if not the sole cause, for the instability of pertussis vaccine in quadruple antigen vaccine (diphtheria and tetanus toxoids and pertussis and poliomyelitis vaccines). 相似文献
124.
125.
Susanne Richter 《Human genetics》1967,3(3):235-243
697 children with concomitant strabism, all patients who were seen in the Ophthalmalogical university hospital and outpatient service of the Charité, Berlin, during a certain period, have been examined together with their parents and siblings. In a second series, 3398 12-years old school children (born 1953) from three urban districts of Berlin were examined at the occasion of a vaccination term, and 179 children with strabism were ascertained. Probands as well as their parents and siblings were examined thoroughly according to the criteria of strabism diagnostics. Incidence of squinting among siblings was increased markedly when compared with the population frequency, but did not reach the expectations under rthe assumption of a single, monomeric mode of inheritance. Admixture of phenocopies or new mutants could be excluded. Twin investigations (12 monocygotic and 27 dicygotic pairs) showed a manifestation rate of 94,1% in monocygotic pairs, corresponding to a 3 1/2 times higher concordance in monocygotic as compared with dicygotic twins. From the results discussed, a multifactorial genetic system in combination with a threshold effect seems to be the most likely genetic interpretation. An analysis of pedigrees shows that slight sensoric as well as motoric anomalies might combine in different and continuously varying quantities, contributing to the syndrom of concomitant strabism. 相似文献
126.
Summary When grown with glucose, S. discophorus synthesized large amounts of poly--hydroxybutyrate which accumulated intracellularly as sudanophilic granules. The rate of endogenous oxygen consumption by such cells was markedly increased by Mn++ and even more by Mg++. It has been shown that these inorganic ions stimulate the oxidation of the intracellular poly--hydroxybutyrate.Dedicated by the senior author to Prof. C. B. van Niel on the occasion of his 70th birthday with gratitude for many unforgettable years of association, instruction and stimulation. 相似文献
127.
Margaret M. Clark 《BMJ (Clinical research ed.)》1967,2(5546):247-248
128.
Chromosome pairing was studied in the following hybrids: Hibiscus radiatus-meeusei (tetraploid F1), H. sabdariffa-meeusei (tetraploid F1 and spontaneous allooctoploid F2), and hexaploid H. acetosella-(sabdariffa-meeusei). Genome constitutions of the species adduced from these data are symbolized as follows: H. radiatus and H. acetosella, AABB; H. meeusei, AAXX; H.sabdariffa, XXYY or AAYY. 相似文献
129.
One hundred and forty-eight basal cell carcinomas were treated by curettage and electrosurgery. Twenty-six recurrent carcinomas were treated and 24 did not recur during a minimum two-year follow-up. Seventy-two newly diagnosed carcinomas were treated by the same method, and a two-year recurrence-free rate of 97.4% was obtained. About 50 new and recurrent lesions were treated in three patients in whom extensive cutaneous changes from actinic atrophy and previous therapy made the distinction between new and recurrent lesions difficult or impossible. This technique has a particular place in the management of multiple lesions. Patient acceptance is good. Lesions at some sites, e.g. on the nose, where closure of a wound is difficult, are better managed by this method than by surgical excision. Secondary infection is rare and the cosmetic results are excellent. 相似文献
130.