全文获取类型
收费全文 | 833921篇 |
免费 | 100459篇 |
国内免费 | 500篇 |
专业分类
934880篇 |
出版年
2018年 | 7319篇 |
2017年 | 6990篇 |
2016年 | 10202篇 |
2015年 | 14966篇 |
2014年 | 17055篇 |
2013年 | 24399篇 |
2012年 | 27637篇 |
2011年 | 28180篇 |
2010年 | 18840篇 |
2009年 | 17396篇 |
2008年 | 24873篇 |
2007年 | 25731篇 |
2006年 | 23758篇 |
2005年 | 23246篇 |
2004年 | 22697篇 |
2003年 | 21872篇 |
2002年 | 21094篇 |
2001年 | 36155篇 |
2000年 | 36261篇 |
1999年 | 29200篇 |
1998年 | 11280篇 |
1997年 | 11658篇 |
1996年 | 11203篇 |
1995年 | 10288篇 |
1994年 | 10253篇 |
1993年 | 10172篇 |
1992年 | 24140篇 |
1991年 | 23311篇 |
1990年 | 22536篇 |
1989年 | 22303篇 |
1988年 | 20239篇 |
1987年 | 19561篇 |
1986年 | 18128篇 |
1985年 | 18116篇 |
1984年 | 15485篇 |
1983年 | 13407篇 |
1982年 | 10678篇 |
1981年 | 9688篇 |
1980年 | 9131篇 |
1979年 | 14540篇 |
1978年 | 11583篇 |
1977年 | 10642篇 |
1976年 | 10051篇 |
1975年 | 10679篇 |
1974年 | 11362篇 |
1973年 | 11196篇 |
1972年 | 9991篇 |
1971年 | 9261篇 |
1970年 | 7752篇 |
1969年 | 7525篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
In vitro directly micropropagated plantlets from three selected five-year-old Eucalyptus grandis Hill ex. Maiden hybrids were compared to their related half-sib seedlings for growth and growth pattern parameters under greenhouse conditions used for operational seedling production. The oven dry weights were determined from stem, leaf, and root samples collected every 40 days for four times. Relative growth rate, net assimilation rates and shoot:root ratio were calculated. Survival was 98% and 95% for plantlets and seedlings, respectively. Significant differences were observed between parents in terms of shoot and root dry weights and their ratios with similar ranking among plantlets and seedlings, suggesting genetic control over these traits. Plantlets started with significantly higher root: shoot ratios and stem, leaf, root, and total dry weight. Although seedlings had higher relative growth and net assimilation rates, all the initial differences decreased sharply over time. 相似文献
942.
One hundred and forty-four isolates of dairy origin were identified using a number of commercially available test kits. Agreement between these kits was acceptable for assignment of isolates to genera, but there was some variation between the kits for the identification of the isolates at specific level. The reasons for these variations are discussed. In terms of cost, ease of performance and accuracy, the identification systems manufactured by Roche Products Ltd (viz. the Oxi/Ferm tube and the Enterotube) are considered to be the best suited to the routine identification of dairy micro-organisms. 相似文献
943.
The regenerating scale and tissues comprising the scale pocket of Fundulus olivaceus were examined microscopically at specific intervals. Scale removal resulted in a thickening of the epidermis which persisted through the early stages of regeneration. This thickening was due in part to the appearance of columnar basal cells which divided producing cells that became mucous cells and squamous cells. The scale regenerated as a relatively large plate of bone which first appeared between layers of scleroblasts on the floor of the scale pocket and then grew producing circuli and radii. By the fourth day of regeneration, calcium was observed in the cytoplasm of the scleroblasts and at randomly distributed foci in the osseous portion of the scale. The osseous layer was completely calcified by 15 days of regeneration. 相似文献
944.
945.
D B Snead A Weltman J Y Weltman W S Evans J D Veldhuis M M Varma C D Teates E A Dowling A D Rogol 《Journal of applied physiology》1992,72(6):2149-2156
We examined the relationships among reproductive hormone concentrations and bone mineral density (BMD) in 43 women runners classified as eumenorrheic (n = 24), oligomenorrheic (n = 8), or amenorrheic (n = 11). Results were compared with a eumenorrheic nonrunner control group (n = 11). Serum 17 beta-estradiol, progesterone, and dehydroepiandrosterone sulfate concentrations were determined in daily blood samples for 21 days, and integrated concentrations (areas under the curve) were calculated. BMD was assessed at the lumbar spine and proximal femur by dual-photon absorptiometry. As expected, 17 beta-estradiol, progesterone, and lumbar spine BMD were higher in the control and eumenorrheic runner groups than in the oligomenorrheic and amenorrheic runner groups (P less than 0.05). Progesterone concentration was significantly correlated with lumbar spine BMD in the eumenorrheic runners (r = 0.61). None of the steroid hormones was significantly related to BMD in the oligomenorrheic/amenorrheic group. The present data suggest that circulating levels of gonadal steroid hormones affect axial BMD in eumenorrheic runners. 相似文献
946.
D Crews 《Hormones and behavior》1984,18(1):22-28
A common observation for seasonally breeding vertebrates is that the reproductive processes of gamete production, sex steroid hormone secretion, and mating behavior coincide, and further, that sex steroid hormones activate mating behavior. The postulate of hormone-dependence of mating behavior is based primarily on detailed studies of laboratory and domesticated species. However, comparison of a wide array of vertebrates reveals numerous exceptions to this supposed rule. Consideration of these species indicates that there is no fixed or intrinsic causal association among gamete production, sex hormone secretion, and mating behavior within any of the classes of vertebrates. 相似文献
947.
J C McDonald 《BMJ (Clinical research ed.)》1981,282(6257):83-4,88
948.
949.
Linda L. Laack Michael E. Tewes Aaron M. Haines John H. Rappole 《Acta theriologica》2005,50(4):505-514
The ocelotLeopardus pardalis Linnaeus, 1758 is an endangered felid in the United States currently restricted to southern Texas. The objectives of our
study were to obtain data on ocelot parturition dates, fecundity, sex ratios, den characteristics, and first year survival,
all of which are critical in development of population viability models. Sixteen parturition events were recorded ranging
from mid-April to late December for 12 wild ocelots. Cumulatively, litters consisted of 1 or 2 kittens (ˉ = 1.2 ± 0.44 SD).
Cumulative sex ratio was 1∶2.5 (male:female); however, there was no significant difference between the observed sex ratio
and a 1∶1 sex ratio. Ten den sites were in close proximity (≤ 10 m) to dense thornshrub. Adult female ocelots used 2 to 4
den sites for each litter with distance between consecutively occupied dens ranging from 110 to 280 m (ˉ = 158 m ± 93 SD).
An estimated annual survival for ocelots 0 to 1 year of age was 0.68. Evidence suggests that ocelots in the wild may breed
more frequently than had been previously hypothesized. 相似文献
950.