全文获取类型
收费全文 | 23908篇 |
免费 | 2004篇 |
国内免费 | 1369篇 |
出版年
2024年 | 52篇 |
2023年 | 235篇 |
2022年 | 582篇 |
2021年 | 875篇 |
2020年 | 631篇 |
2019年 | 800篇 |
2018年 | 868篇 |
2017年 | 644篇 |
2016年 | 950篇 |
2015年 | 1455篇 |
2014年 | 1679篇 |
2013年 | 1810篇 |
2012年 | 2089篇 |
2011年 | 2020篇 |
2010年 | 1196篇 |
2009年 | 1155篇 |
2008年 | 1369篇 |
2007年 | 1250篇 |
2006年 | 1125篇 |
2005年 | 949篇 |
2004年 | 946篇 |
2003年 | 766篇 |
2002年 | 652篇 |
2001年 | 430篇 |
2000年 | 385篇 |
1999年 | 317篇 |
1998年 | 218篇 |
1997年 | 178篇 |
1996年 | 175篇 |
1995年 | 142篇 |
1994年 | 125篇 |
1993年 | 85篇 |
1992年 | 152篇 |
1991年 | 122篇 |
1990年 | 127篇 |
1989年 | 128篇 |
1988年 | 108篇 |
1987年 | 88篇 |
1986年 | 58篇 |
1985年 | 54篇 |
1984年 | 51篇 |
1983年 | 28篇 |
1982年 | 29篇 |
1981年 | 17篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 12篇 |
1976年 | 16篇 |
1973年 | 14篇 |
1969年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
在甜椒(Capsicum annuum L.)中,靠近花粉中部的绒毡层自药隔产生,由较大的细胞组成,而花药外部区域的其余的绒毡层细胞较小,来自于初生壁层,前者的细胞具有大液泡和较大的细胞核,甲基绿-派罗宁和汞-溴酚蓝染色反应较后者弱,在造孢组织时期,二者液泡内都含有较大的球形的酸性磷酸酶颗粒,在以后的发育中,这种颗粒消失,在减数分裂时期,两种绒毡层的DNA,RNA和蛋白质合成活动增强,来自药隔的绒毡层积累了更多的DNA,绒毡层在解体时酸性磷本酶活性很高,两种不同的绒毡层退化过程相似,在全部发育过程中绒毡层内无淀粉粒。 相似文献
63.
64.
Tomoji Endo 《Population Ecology》1988,30(1):107-121
The pattern of prey utilization of the orb-weaving spider Araneus pinguis was studies by comparing between arthropods restrained in the empty webs (spiders were removed) and those unattacked in the intact webs (spiders were not removed). The number of arthropods was larger in the empty webs than in the intact webs. In the empty webs, web area, mesh width, number of radii, and signal thread length were presumed to affect the number of arthropods left in a single web. As for the intact webs, web area, mesh width, and web-exposure time were important factors. In the empty webs, the density of arthropods decreased away from the hub. On the other hand, the density of unattacked arthropods in the intact webs was the same throughout the web. Arthropods in the empty webs were larger than those in the intact webs. The upper limit in size of unattacked arthropods increased along with the distance from the hub in the intact webs, but not in the empty webs. These results indicate increase in the minimum size of eaten arthropods increased in the former. This positive sizedistance relation may have resulted from the adaptive switching of spiders’ alternative foraging methods (i.e., the rapid attack at encounter and the later eating during web deconstruction) on the basis of the prey profitability. 相似文献
65.
Summary Substructured populations exhibit an overall deficiency of heterozygosity whose proportional magnitude depends on the nature of substructuring, i.e., the number of subpopulations (s), their time of divergence (t) from the ancestral population, and the rate of gene flow amongst them (m). Since apparent heterozygote deficiency could be caused by many factors other than population substructuring, one must examine the nature of substructuring that could produce the observed extent of heterozygote deficiency, in order to infer the substructuring from an observed heterozygote deficiency. Using the equivalence of proportional heterozygote deficiency and the coefficient of gene differentiation (G
ST), we can generate isolines of G
ST as functions of s, t (in units of 2N
e generations, N
e being the effective population size) and m. Analytical results suggest that large G
ST values cannot be reached by substructuring alone, unless the number of subpopulations are large and they remain isolated over a long period of time. Application of the theory to population data on six variable number of tandem repeats (VNTR) loci in US Caucasians and US Blacks demonstrates that the observed heterozygote deficiencies at these loci cannot be explained by substructuring within these populations alone. This is so because such large values of G
ST (3%–10%) would require an absence of gene exchange between the subpopulations and a divergence time from each other of at least 25000 years ago, neither of which is compatible with the demography and ethnohistory of US Caucasians and Blacks. In contrast, the inability to detect extreme-sized alleles and/or incomplete resolution of nearly similar-sized alleles following Southern gel electrophoresis could easily explain the observed heterozygote deficiencies. The implications of these results are discussed in the context of the forensic use of DNA-typing data, and justify the employment of population genetic principles in forensic genetics. 相似文献
66.
67.
Augmentation rhinoplasty: observations on 1200 cases 总被引:1,自引:0,他引:1
Over the past 14 years, from January of 1975 to December of 1988, we have done 1263 aesthetic rhinoplasties using ear cartilage. In the field of augmentation rhinoplasty, many kinds of materials, such as bone, septal cartilage, ear cartilage, and prostheses, were used. In this paper, we limit discussion to our experience with the technique for the augmentation of the nasal dorsum using the ear cartilage and compare this with other materials. Patient ages ranged from 15 to 72 years, with an average of 24 years. Some 95 percent of patients (1199) were female, and only 5 percent (64) were male. Patients were followed for a minimum of 6 months and a maximum of 20 months, with average follow-up only 8 months. Of course, we know that this is a very short follow-up period, but we could not follow patients longer because if they had no complaint about the results at the 6-month visit, they never returned, despite our efforts. Five-hundred and ten of the 1263 patients (40 percent) had been augmented elsewhere, and the silicone prosthesis was already in place. However, 753 patients (60 percent) had no previous operation. For the 510 patients (secondary rhinoplasty patients), too-high or too-large a prosthesis was the largest complaint in number, totaling 378 cases (74 percent), and psychological dissatisfaction, such as pain or an uncomfortable sensation, was the second largest in number, totaling 104 cases (20 percent). For the 753 patients (primary rhinoplasty patients), the main complaint was too-short or too-flat a nose (100 percent).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
68.
To determine the relative survival of porcine embryos after co-culture with cells producing an avian retrovirus, four-cell stage embryos were obtained from sows following synchronization with altrenogest and superovulation with gonadotropins. These embryos were randomly assigned to the following treatments: no manipulation (zona-intact); zona removed with acidified Tyrode's solution (zona-free); and zona removed followed by co-culture with D-17 canine cells producing an avian retrovirus vector derived from spleen necrosis virus (zona-free + co-culture). The survival rates of four-cell stage embryos to morulae or early blastocysts during a 48-h culture period were 93.3, 80.0 and 57.7% in zona-intact, zona-free and zona-free + co-culture groups, respectively. Following embryo transfer, the development of embryos to fetuses at six weeks of gestation was 37.5, 30.0 and 11.7% in zona-intact, zona-free and zona-free + co-culture groups. These results indicate that early preimplantation porcine embryos can develop to apparently normal fetuses following co-culture with cells producing a retrovirus, and the feasibility of this method for retrovirus-mediated gene transfer in pigs was demonstrated. 相似文献
69.
Terhi Siimes Mikio Nakajima Hideo Yada Hajime Asama Teruyuki Nagamune Pekka Linko Isao Endo 《Biotechnology Techniques》1992,6(5):385-390
Summary A knowledge based system has been shown to be a powerful tool for diagnosing microbial activities during a fermentation process.
Knowledge about lactic acid fermentation was collected by an experimental study ofLactobacillus casei. The effects of the inoculum properties and sterilization time on the cultivation were expressed in a form of a fuzzy rule-based
knowledge network. The system was able to detect abnormal inoculum or sterilization conditions which caused malfunctions in
the cultivations. 相似文献
70.
Endo S Inada K Inoue Y Otsu T Kasai T Kuwata Y Hoshi S Yoshida M 《Mediators of inflammation》1992,1(1):45-48
Plasma levels of endotoxin and various cytokines were assessed in 70 patients with gastrointestinal tract perforation. Sepsis developed in 29 of them, and eight of these (27.6%) had on admission endotoxin levels higher than 9.8 pg ml(-1). The clinical outcome correlated with the level of tumour necrosis factor alpha (TNFalpha), rather than with the endotoxin level. The high interleukin 6 (IL-6) level was shown in septic patients and no correlation was observed between the IL-6 level and the clinical outcome. Plasma TNFalpha levels tended to change independently from endotoxin levels, suggesting that TNFalpha may have been locally produced in inflammatory lesions. 相似文献