全文获取类型
收费全文 | 785040篇 |
免费 | 85950篇 |
国内免费 | 900篇 |
专业分类
871890篇 |
出版年
2018年 | 6939篇 |
2016年 | 9280篇 |
2015年 | 12992篇 |
2014年 | 15193篇 |
2013年 | 21685篇 |
2012年 | 24521篇 |
2011年 | 25342篇 |
2010年 | 17021篇 |
2009年 | 15543篇 |
2008年 | 22370篇 |
2007年 | 23416篇 |
2006年 | 21969篇 |
2005年 | 21030篇 |
2004年 | 21090篇 |
2003年 | 20122篇 |
2002年 | 19537篇 |
2001年 | 31854篇 |
2000年 | 31824篇 |
1999年 | 25660篇 |
1998年 | 9609篇 |
1997年 | 9896篇 |
1996年 | 9366篇 |
1995年 | 8931篇 |
1994年 | 8590篇 |
1993年 | 8602篇 |
1992年 | 22035篇 |
1991年 | 21712篇 |
1990年 | 21581篇 |
1989年 | 20992篇 |
1988年 | 19623篇 |
1987年 | 18523篇 |
1986年 | 17449篇 |
1985年 | 17761篇 |
1984年 | 14790篇 |
1983年 | 12715篇 |
1982年 | 9798篇 |
1981年 | 8857篇 |
1980年 | 8291篇 |
1979年 | 13978篇 |
1978年 | 11161篇 |
1977年 | 10262篇 |
1976年 | 9705篇 |
1975年 | 10875篇 |
1974年 | 11838篇 |
1973年 | 11600篇 |
1972年 | 10610篇 |
1971年 | 9700篇 |
1970年 | 8313篇 |
1969年 | 8232篇 |
1968年 | 7613篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
M. L. Rabinovich 《Applied Biochemistry and Microbiology》2006,42(1):1-26
Development of national research of cellulose-degrading microorganisms and enzymes is reviewed, with emphasis on the prospects of producing ethanol from cellulose materials using cellulolytic enzymes. Leading Russian research groups in this field are introduced. A section of the review analyzes problems and prospects of setting up environmentally friendly production of motor biofuels from renewable raw materials of plant origin (an approach developed in Russia). 相似文献
993.
Harbor seal ( Phoca vitulina ) haul-out site use may be affected by natural or anthropogenic factors. Here, we use an 11-yr (1997–2007) study of a seal colony located near a mariculture operation in Drakes Estero, California, to test for natural (El Niño-Southern Oscillation (ENSO), density-dependence, long-term trends) and anthropogenic (disturbance or displacement related to oyster production activities) factors that may influence the use of haul-out subsites. Annual mariculture related seal disturbance rates increased significantly with increases in oyster harvest ( r s = 0.55). Using generalized linear models (GLMs) ranked by best fit and Akaike's Information Criteria, ENSO and oyster production (as a proxy for disturbance/displacement) best explained the patterns of seal use at all three subsites near the mariculture operations, with effects being stronger at the two subsites closest to operations. Conversely, density-dependence and linear trend effects poorly explained the counts at these subsites. We conclude that a combination of ENSO and mariculture activities best explain the patterns of seal haul-out use during the breeding/pupping season at the seal haul-out sites closest to oyster activities. 相似文献
994.
Recent reports suggest that prostaglandins, rather than cAMP, play a major role in mediating cholera toxin-induced water and electrolyte secretion from rabbit intestinal loops. We examined the role of prostaglandins in mediating toxin-induced pancreatic and gastric exocrine secretion. In these tissues, indomethacin, a potent inhibitor of prostaglandin synthesis, did not alter the stimulatory effects of cholera toxin on increases in cellular cAMP or enzyme secretion. Moreover, the addition of cholera toxin did not alter prostaglandin E2 release from either tissue. In contrast to their effects in rabbit intestinal loops, prostaglandins do not regulate cholera toxin-induced enzyme secretion from the guinea pig pancreas or stomach. 相似文献
995.
996.
997.
The seasonal chronology of the events of the reproductive cycle, and changes in the structure and function of the primary and accessory organs of the male bent-winged bat, Miniopterus schreibersii, were studied at latitude 37 degrees S in temperate southeastern Australia. The testicular cycle commenced in late spring (November), and sperm appeared in the seminiferous tubules and epididymides in early fall (March). The cycle of the accessory sex gland complex generally paralleled the testicular cycle, reaching maximum hypertrophy at the time of insemination in late fall (April/May). Thereafter, the primary and secondary sex glands (except the ampullary gland) involuted as the animals entered winter torpor. However, a cauda epididymal store of sperm persisted until late spring, and sperm were often observed, as well, in the ampullary gland duct and alveoli throughout winter. This study has confirmed that male Miniopterus differs from other vespertilionids in that accessory gland activity declines following the fall breeding in keeping with the fact that, unlike in other vespertilionids, insemination, ovulation and conception are concurrent events in the fall in this species. The reduced secretory status of the Leydig cells and exceptionally low levels of circulating androgens throughout the year, in combination with the presence of viable epididymidal sperm for most of gestation, are all interesting features of this reproductive cycle. 相似文献
998.
999.
1000.
Early events of mycorrhizal and nonmycorrhizal fungal colonization in newly-emerging roots of mature apple (Malus domestica Borkh) trees were characterized to determine the relationship of these events to fine root growth rate and development. New roots were traced on root windows to measure growth and then collected and stained to quantify microscopically the presence of mycorrhizal and nonmycorrhizal fungal structures. Most new roots were colonized by either mycorrhizal or nonmycorrhizal fungi but none less 25 days old were ever internally colonized by both. Compared to nonmycorrhizal colonization, mycorrhizal colonization was associated with faster growing roots and roots that grew for a longer duration, leading to longer roots. While either type of fungi was observed in roots as soon as 3 days after root emergence, intraradical colonization by mycorrhizal fungi was generally faster (peaking at 7 to 15 days) than that by nonmycorrhizal fungi and often occurred more frequently in younger roots. Only 15 to 35% of the roots had no fungal colonization by 30 days after emergence. This study provides the first detailed examination of the early daily events of mycorrhizal and nonmycorrhizal fungal colonization in newly emerging roots under field conditions. We observed marked discrimination of roots between mycorrhizal and nonmycorrhizal fungi and provide evidence that mycorrhizal fungi may select for faster growing roots and possibly influence the duration of root growth by non-nutritional means. 相似文献