全文获取类型
收费全文 | 39677篇 |
免费 | 135篇 |
国内免费 | 168篇 |
专业分类
系统科学 | 185篇 |
丛书文集 | 545篇 |
教育与普及 | 77篇 |
理论与方法论 | 126篇 |
现状及发展 | 17634篇 |
研究方法 | 1632篇 |
综合类 | 19156篇 |
自然研究 | 625篇 |
出版年
2013年 | 314篇 |
2012年 | 569篇 |
2011年 | 1174篇 |
2010年 | 258篇 |
2008年 | 696篇 |
2007年 | 824篇 |
2006年 | 766篇 |
2005年 | 817篇 |
2004年 | 822篇 |
2003年 | 720篇 |
2002年 | 732篇 |
2001年 | 1240篇 |
2000年 | 1188篇 |
1999年 | 805篇 |
1994年 | 388篇 |
1992年 | 774篇 |
1991年 | 553篇 |
1990年 | 618篇 |
1989年 | 601篇 |
1988年 | 566篇 |
1987年 | 614篇 |
1986年 | 655篇 |
1985年 | 819篇 |
1984年 | 596篇 |
1983年 | 519篇 |
1982年 | 478篇 |
1981年 | 487篇 |
1980年 | 516篇 |
1979年 | 1302篇 |
1978年 | 1008篇 |
1977年 | 932篇 |
1976年 | 756篇 |
1975年 | 780篇 |
1974年 | 1092篇 |
1973年 | 942篇 |
1972年 | 954篇 |
1971年 | 1074篇 |
1970年 | 1478篇 |
1969年 | 1149篇 |
1968年 | 1073篇 |
1967年 | 1067篇 |
1966年 | 1001篇 |
1965年 | 748篇 |
1959年 | 376篇 |
1958年 | 720篇 |
1957年 | 477篇 |
1956年 | 407篇 |
1955年 | 365篇 |
1954年 | 414篇 |
1948年 | 285篇 |
排序方式: 共有10000条查询结果,搜索用时 724 毫秒
971.
William S. Longland 《西北部美国博物学家》2011,67(3)
Heteromyid rodents in the deserts of North America have been shown to harvest large quantities of seeds of both native and introduced plants from soil seed banks, but rarely has the impact of this seed removal been demonstrated experimentally. I used a series of fenced plots (some of which excluded rodents) to demonstrate that heteromyids at a western Nevada study site can measurably reduce seed banks and subsequent seedling establishment of Salsola paulsenii , an introduced invasive weed that has become a significant problem over much of the desert Southwest. The frequency of S. paulsenii seedlings in both 2004 and 2005 was significantly greater around the interior perimeters of plots that permitted access by rodents than in plots that excluded rodents. Density of S. paulsenii seedlings was significantly greater inside than outside rodent exclusion plots, but there was no such difference in seedling density inside versus outside plots that permitted rodent access. Salsola paulsenii has such a conspicuous presence in many desert environments that the effect of rodents in reducing its abundance may not be visually apparent; however, rodents may still ameliorate competitive effects of this weed on coexisting plants. Heteromyid rodents disperse seeds through caching, and they also consume them. Caching may enhance establishment of native plant seedlings, but is unlikely to benefit exotics such as S. paulsenii . 相似文献
972.
Vegetation characteristics of 15 sagebrush community types identified on the Humboldt National Forest, northeastern Nevada, are described. A total of 218 plant species were found over the 372 relatively undisturbed rangeland communities sampled. The dominant plant families encountered were the Asteraceae (45 taxa), Poaceae (32 taxa), Scrophulariaceae (14 taxa), and the Fabaceae (12 taxa). Average annual dry weight production of the community types ranged from about 400 kg/ha/yr on types with Artemisia nova as the dominant sagebrush species to 1,200 kg/ha/yr on some A. tridentata ssp. vaseyana community types. A general increase in species richness and vegetation plus litter ground cover was observed within community types as the dominant sagebrush species changed from A. nova to A. arbuscula to A. longiloba to A. tridentata spp. tridentata to A. tridentata ssp. vaseyana . Major differences in plant species production and constancy exist between the sagebrush community types studied. 相似文献
973.
James A. Gessaman 《西北部美国博物学家》2011,44(1)
The weight of body water and fat-free dry weight of Uinta ground squirrels ( Spermophilus armatus ) is given as a function of body weight. Body fat is presented as a function of body weight and body length. An equation for calculating the total caloric content of Uinta ground squirrels is given. Fat index (weight of fat/fat-free dry weight) values of juveniles increased from 0.11 to 0.22 from 5 June to 28 July. In male yearlings and adults the fat index increased about 8 to 9 times in the latter half of June and early July. 相似文献
974.
D. Kendall Brown Anthony A. Echelle David L. Propst James E. Brooks William L. Fisher 《西北部美国博物学家》2011,61(2)
We used the computer program RAMAS to explore the sensitivity of an extinction-risk model for the Gila trout ( Oncorhynchus gilae ) to management of wildfires and number of populations of the species. The Gila trout is an endangered salmonid presently restricted to very few headwaters of the Gila and San Francisco river tributaries in southwestern New Mexico. Life history data for 10 extant populations were used to examine sensitivity of the species viability to changes in a variety of factors including population size, fecundity, life stage structure, number of populations, severity and probability of forest fires, and a regulated fishery. The probability and severity of forest fires and number of populations had the greatest effect on viability. Results indicate that successful conservation of Gila trout requires establishment of additional populations and reduction of the severity of forest fires through a program incorporating more frequent, but less severe, fires. 相似文献
975.
Fertilized ova of razorback sucker, Xyrauchen texanus, were adhesive for 3 to 4 hours after fertilization. Cleavage was completed at 24 hours, gastrulation occurred at 34 hours, and blood circulation was established at 117 hours. Hatching occurred from 5.2 to 5.5 days after fertilization. Larvae were from 6.8 to 7.3 mm TL at hatching. Yolk was assimilated at 13 days (10 mm TL). All fins were formed and had ossified rays at 64 days (27 mm TL). The unique nuchal keel appeared about 200 days after fertilization. 相似文献
976.
L.J.M. Coleby M.A. M.Sc. Ph.D. 《Annals of science》2013,70(2):101-123
The development of the circular dividing engine in England is traced from Henry Hindley and Jesse Ramsden through the improvements introduced by Ramsden's successors to the self-acting engine of William Simms. Particular emphasis is given to the invention, evolution and transmission of the methods used to achieve accuracy in: dividing the wheel; ratching the teeth and matching them to the endless screw; and mounting the cutter. The procedures adopted by Ramsden and Troughton for correcting initial dividing marks are also described. 相似文献
977.
Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Sixty-four cougar ( Felis concolor ) stomachs and 41 intestinal tracts were examined for food items in northeastern Oregon from 1976 through 1979. Food items, in order of decreasing frequency, were mule deer ( Odocoileus hemionus ), North American elk ( Cervus elaphus ), porcupine ( Erethizon dorsatum ), snowshoe hare ( Lepus americanus ), and deer mouse ( Peromyscus maniculatus ). 相似文献
978.
979.
P.A. Bretscher N. Ismail J.N. Menon C.A. Power J. Uzonna G. Wei 《Cellular and molecular life sciences : CMLS》2001,58(12-13):1879-1896
The occurrence of infectious disease represents a failure of the immune system, a failure that must be prevented by effective vaccination or remedied by treatment. Vaccination against acute diseases such as smallpox and polio are very effective, due to the rapid and increased immune response of vaccinated individuals upon natural infection. In contrast, effective vaccination against intracellular pathogens that cause chronic diseases, such as the leishmaniases, tuberculosis and AIDS, has not been achieved. Clinical observations suggest cell-mediated, Th1 responses, exclusive of antibody production and the generation of Th2 cells, are optimally protective against these intracellular pathogens. Effective vaccination must ensure the generation of such a protective response. We explore here whether understanding very broad features of the regulation of the immune response can accommodate modern findings on the immunological features of these diseases, and provide a perspective within which strategies for effective vaccination and treatment can be developed. 相似文献
980.