首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   195篇
  免费   19篇
  2017年   2篇
  2015年   4篇
  2014年   4篇
  2012年   4篇
  2010年   7篇
  2009年   5篇
  2008年   3篇
  2007年   6篇
  2006年   6篇
  2005年   3篇
  2002年   2篇
  1999年   3篇
  1998年   6篇
  1997年   2篇
  1996年   2篇
  1995年   4篇
  1993年   3篇
  1992年   3篇
  1991年   3篇
  1990年   5篇
  1989年   4篇
  1988年   3篇
  1987年   5篇
  1986年   2篇
  1985年   7篇
  1983年   3篇
  1982年   3篇
  1981年   2篇
  1980年   4篇
  1979年   4篇
  1978年   5篇
  1976年   3篇
  1975年   4篇
  1974年   4篇
  1972年   4篇
  1971年   2篇
  1966年   2篇
  1958年   5篇
  1957年   6篇
  1956年   9篇
  1955年   7篇
  1954年   6篇
  1953年   9篇
  1952年   5篇
  1951年   3篇
  1950年   2篇
  1949年   4篇
  1948年   5篇
  1942年   1篇
  1938年   1篇
排序方式: 共有214条查询结果,搜索用时 15 毫秒
1.
2.
Adventitious bud formation on Sitka spruce [ Picca sitchensis (Bong.) Carr.] needle explants was strongly dependent upon the rigidity of the culture medium. In general, of organogenesis was greatest on weak gels and poorest on more rigid gels resulting from increased medium pH or agar strength. There was a significant interaction between agar strength and pH, with the optimum pH for organogenesis declining with increasing agar strength. Poor organogenesis at high agar concentrations was not due to toxic impurities since increased adventitious bud production could be stimulated by decreasing the medium pH whilst maintaining a high agar strength and an agar washing treatment had no significant effect. Although high levels of organogenesis could be sustained on weak gels the resultant adventitious shoots often showed severe vitrification. The frequency of shoots showing vitrification could be reduced by growing the tissues on harder media but this resulted in reduced shoot elongation. Vitrification of needle tissues did not stimulate the formation of adventitious buds in the absence of cytokinins.  相似文献   
3.
Abstract. The wood-degrading white-rot fungus Phanerochaete chrysosporium , has been the subject of intensive research in recent years and, based upon isolation of the extracellular enzyme ligninase, major advances have now been made toward elucidating the mechanism by which this fungus degrades lignin. From these developments, a model emerges which could explain the process by which wood-degrading fungi in general, attack lignin.  相似文献   
4.
The distribution and stability of the cellular tumor antigen p53 were studied in baby rat kidney cells transformed by region E1 sequences of nononcogenic adenovirus (Ad) type 5 (Ad5) or oncogenic type 12 (Ad12). In transformed cells expressing the large E1B T antigen of Ad5, p53 was associated with this T antigen. The complexed proteins were concentrated in a cytoplasmic body, which has been shown to consist of a cluster of 8-nm filaments (A. Zantema et al., Virology 142:44-58, 1985). In transformed cells expressing the E1B region of Ad12, however, no association between the viral large T antigen and p53 was detectable. In the latter case, both proteins were found almost exclusively in the nucleus. The stability of p53 in both Ad5- and Ad12-transformed cells was increased relative to that in primary cells or cells immortalized by the E1A region only. Thus, the increased stability of p53 in Ad-transformed cells is not caused by association with a viral T antigen, but it correlates with expression of E1B and with morphological transformation.  相似文献   
5.
In a field experiment on infested sandland from 1955 to 1967, numbers of Heterodera avenae increased only in 1957 and then decreased under both continuous and rotational spring oats (1955-62) and subsequently under spring barley (1963-67). Fluctuations in numbers of Pratylenchus neglectus and Trichodorus primitivus were recorded from 1963 to 1967. Cereal grain yields were generally unsatisfactory, but were consistently higher for cereals grown in rotation rather than continuously. Extra nitrogen fertilizer gave slightly improved yields with higher post-cropping numbers of H. avenae.  相似文献   
6.
Crocodylus porosus is a mound-nesting crocodilian in which incubationtemperature influences the rate of embryonic development, theprobability that embryos will survive to hatching, post-hatchinggrowth rates and the probability of hatchlings surviving to2 yr of age. Similar responses have been described in Alligatormississippiensis (Joanen et al., 1987) and C. niloticus (Hutton,1987), and they reflect a suite of "non-sexual" effects of incubationtemperature. Temperature-dependent sex determination allocatessex on the basis of these "non-sexual" effects. In C. porosus,it results in maleness being assigned to embryos with high probabilitiesof surviving and good potential for post-hatching growth. Withinthe limits of survival, effects of the moisture environmenton embryological development rate and hatchling fitness seemminor relative to those of the temperature environment. Reptilian orders have either obligate oviparity (chelonians,crocodilians and rhynchocephalians) or facultative oviparity(squamates), depending on the extent of embryonic developmentwithin the oviducts. The distinction is equally one betweenembryos which are buffered from thermal effects within a female'sbody (facultative oviparity) and those that are not (obligateoviparity). Facultative oviparity and internal thermal bufferingmay be the primitive condition within the Class Reptilia, andthe "shell-less" eggs of extant squamates may reflect the originalamniote egg. Obligate oviparity, which also exists in birds,appears to have been a specialized development, and is a blindend in the evolution of viviparity among vertebrates. The significanceof thermal buffering being lost in obligate oviparous reptilesremains unclear.  相似文献   
7.
A differential equation model of vegetative growth of the soyabean plant (Glycine max (L.) Merrill cv. ‘Ransom’)was developed to account for plant growth in a phytotron systemunder variation of root temperature and nitrogen concentrationin nutrient solution. The model was tested by comparing modeloutputs with data from four different experiments. Model predictionsagreed fairly well with measured plant performance over a widerange of root temperatures and over a range of nitrogen concentrationsin nutrient solution between 0.5 and 10.0 mmol in the phytotron environment. Sensitivity analyses revealedthat the model was most sensitive to changes in parameters relatingto carbohydrate concentration in the plant and nitrogen uptakerate. Key words: Glycine max (L.) Merrill, dry matter, nitrogen uptake, partitioning, photosynthesis, respiration, sensitivity analysis  相似文献   
8.
Abstract.
  • 1 The influence of fluctuating asymmetry on lifetime mating success of males of the damselfly Coenagrion puella (Odonata: Coenagrionidae) was investigated. Fluctuating asymmetry was measured as the difference in length of left and right fore and hind wings.
  • 2 Males with more symmetrical wings enjoyed higher lifetime mating success.
  • 3 Larger males, in contrast to a previous study of this species, also had higher mating success. This may be attributed to differences in the weather conditions prevailing at the time of the studies.
  相似文献   
9.
10.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号