全文获取类型
收费全文 | 285篇 |
免费 | 19篇 |
国内免费 | 2篇 |
专业分类
306篇 |
出版年
2021年 | 3篇 |
2018年 | 3篇 |
2017年 | 6篇 |
2015年 | 3篇 |
2014年 | 5篇 |
2013年 | 10篇 |
2012年 | 6篇 |
2011年 | 7篇 |
2010年 | 17篇 |
2009年 | 18篇 |
2008年 | 12篇 |
2007年 | 5篇 |
2006年 | 10篇 |
2005年 | 9篇 |
2004年 | 3篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 5篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1988年 | 7篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1983年 | 7篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1971年 | 5篇 |
1969年 | 2篇 |
1965年 | 3篇 |
1963年 | 2篇 |
1959年 | 7篇 |
1958年 | 8篇 |
1957年 | 8篇 |
1956年 | 8篇 |
1955年 | 5篇 |
1954年 | 5篇 |
1953年 | 5篇 |
1952年 | 2篇 |
1951年 | 10篇 |
1950年 | 3篇 |
1948年 | 4篇 |
排序方式: 共有306条查询结果,搜索用时 15 毫秒
1.
2.
DAVID P. FOX BRUCE F. HILL DONALD M. SPOON GEORGE B. CHAPMAN 《The Journal of eukaryotic microbiology》1988,35(1):4-12
ABSTRACT. A sessile, tentacle-bearing protozoon, Heliophrya sp. (Suctoria, Ciliata), reproduces asexually by evaginative budding to form a ciliated swarmer, which begins metamorphosis to the adult form within 30 min of its release from the parent cell. Morphological features of embryogenesis were investigated using transmission and scanning electron microscopy and found to correspond, with certain exceptions, to the few previous reports concerning evaginative budding in suctorians. Following invagination of a portion of the pellicle to form an embryonic cavity within the parent cell, numerous kinetosomes, apparently formed de novo, organize into rows which surround the embryonic cavity and eventually develop cilia that project into the cavity. When the cavity is complete, its walls are extruded through an opening in the parent cell surface. Parent cell cytoplasm streams into the incipient swarmer, thus supplying it with at least the minimum requirement of all cytoplasmic organelles. The ciliated swarmer remains attached to its parent cell for several minutes before it detaches. A complete pellicle is formed in both parent and swarmer prior to detachment. The numerous mitochondria underlying the parent cell pellicle in the vicinity of the attachment area suggest that cross wall formation is an energy-dependent process. 相似文献
3.
The Growth and Carbon Allocation Patterns of White Clover (Trifolium repens L.) Plants of Contrasting Branching Structure 总被引:1,自引:0,他引:1
The growth, morphology and carbon allocation patterns of F1progeny white clover (Trifolium repens L.) plants selected foreither low (LBF) or high (HBF) frequencyof stolon branching were compared in two controlled-environmentexperiments. Selections from within both a small-leaved (GrasslandsTahora) and a large-leaved (Grasslands Kopu)clover cultivar were compared, and plants were grown under arelatively lenient defoliation treatment (expt 1) or under threelevels of defoliation seventy (expt 2). Carbon allocation patternswere measured by 14CO2 pulse-chase labelling using fully unfoldedleaves on the main (parent) stolon. LBF and HBF displayed consistent differences in the selectedcharacter though, within cultivars, the difference between selectionswas most pronounced for Kopu. The selections developed fundamentallydifferent branching structures resulting from differences inbranching frequency, with total branch weight per plant averaging122 mg for LBF and 399 mg for HBF (mean of both experiments).More C moved from parent stolon leaves to branches in HBF thanin LBF (mean 22.6% vs. 15.1% respectively of the 14C exportedfrom source leaves). More C also moved to stolon tissue in HBF,but, counterbalancing this and the difference in allocationto branches, less moved to developing leaves and roots on theparent stolon itself compared to LBF. However, the total weightof developing leaves and roots per parent stolon was generallygreater in HBF than in LBF, probably reflecting greater C importby these sinks from the higher number of branches present perplant in the former selection. HBF plants were consistentlylarger at harvest than LBF plants. There were no defoliationtreatment x selection interactions in C allocation patternsin expt 2. The implications of the results for plant performancein grazed pastures are discussed. Branching, carbon translocation, defoliation, growth, morphology, Trifolium repens, white clover 相似文献
4.
PRECIPITATION of collagen from solution in the presence of chondroitin sulphates (or certain other glycosaminoglycans), followed by dialysis against water, leads to the formation of the fibrous long spacing (FLS) polymorphic form of collagen. Electron microscopy shows this to consist of banded fibrils with a periodicity in the range 1200–2400 Å and a symmetric intraperiod structure, in contrast to the 600–700 Å periodicity and polarized fine structure characteristic of native collagen fibrils. FLS was originally reported by Highberger et al.1–3 and has been observed in three principal modifications, FLS I, FLS II and FLS III, differing in periodicity and arrangement of dense bands4–6. This letter reports a new form of FLS, denoted FLS IV. 相似文献
5.
KATHY GRIFFIS DAVID J. CHAPMAN 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1990,23(4):379-383
Griffis, K. & Chapman, D. J. 1990 10 15: Modeling Cretaceous-Tertiary boundary events with extant photosynthetic plankton: effects of impact-related acid rain. Lethaia , Vol. 23, pp. 379–383. OSIO. ISSN 0024–1164.
An acid rain phenomenon has previously been proposed as one of the consequences of a bolide impact contributing to the extinctions at the Cretaceous-Tertiary boundary. This hypothesis has been tested by observing the growth responses of four organisms under simulated acid rain conditions. Two of these phytoplankton, Ditylum (a diatom) and Thoracosphaera (a dinoflagellate). are genera that persisted through the boundary, while the other two, Coccolithus (a coccolithophorid) and Gonyaulax (a dinoflagellate), are post-boundary genera. Ditylum and Coccolithus survive the acid rain simulation. but with the loss of scales in Coccolithus . The two dinoflagellate are sensitive to acid rain simulations. with Gonyaulax unable to survive beyond seven days. The results indicate that acid rain may have contributed to the Cretaceous-Tertiary boundary extinctions. but that the changes resulting from the acid rain were not as severe as postulated or were short-lived and quickly dissipated. ▭ Acid rain, coccolithophorids. Cretaceous/Tertiary extinctions, diatons, dinoflagellates, phytoplankton . 相似文献
An acid rain phenomenon has previously been proposed as one of the consequences of a bolide impact contributing to the extinctions at the Cretaceous-Tertiary boundary. This hypothesis has been tested by observing the growth responses of four organisms under simulated acid rain conditions. Two of these phytoplankton, Ditylum (a diatom) and Thoracosphaera (a dinoflagellate). are genera that persisted through the boundary, while the other two, Coccolithus (a coccolithophorid) and Gonyaulax (a dinoflagellate), are post-boundary genera. Ditylum and Coccolithus survive the acid rain simulation. but with the loss of scales in Coccolithus . The two dinoflagellate are sensitive to acid rain simulations. with Gonyaulax unable to survive beyond seven days. The results indicate that acid rain may have contributed to the Cretaceous-Tertiary boundary extinctions. but that the changes resulting from the acid rain were not as severe as postulated or were short-lived and quickly dissipated. ▭ Acid rain, coccolithophorids. Cretaceous/Tertiary extinctions, diatons, dinoflagellates, phytoplankton . 相似文献
6.
The ATP-binding-cassette transmembrane transporters (ABC transporters)
known from vertebrates belong to four major subfamilies: (1) the P-
glycoproteins (Pgp); (2) the cystic fibrosis transmembrane conductance
regulators (CFTR); (3) the Tap proteins encoded with the major
histocompatibility complex of mammals; and (4) the peroxisomal membrane
proteins. Both Pgp and CFTR have a structure suggesting a past internal
gene duplication; a phylogenetic analysis indicated that these duplications
occurred independently, while an independent tandem gene duplication
occurred in the case of the Tap family. Both the Pgp and Tap proteins show
evidence of relationship to bacterial ABC transporters lacking internal
duplication, and both are significantly more closely related to the HlyB
and MsbA families of transporters from purple bacteria than they are to ABC
transporters from nonpurple bacteria. The simplest hypothesis to explain
this observation is that eukaryotic Pgp and Tap genes are descended from a
mitochondrial gene or genes that were subsequently translocated to the
nuclear genome. The Pgp genes of eukaryotes are characterized by a
remarkable degree of convergent evolution between the ATP-binding cassettes
of their N- terminal and C-terminal halves, whereas no such convergence is
seen between the two halves of CFTR genes or between the duplicated Tap
genes. Exon 13 of the CFTR gene, which encodes a putative regulatory domain
not found in other ABC transporters apart from CFTR, showed high levels of
both synonymous and nonsynonymous difference in comparisons among different
mammalian species, suggesting that this region is a mutational hot spot.
相似文献
7.
8.
A sample consisting of 2,252 persons among 20,199 Los Angeles civil service employees was observed for the occurrence of heart disease. The first examination measured the prevalence. Based upon the diagnosis of 165 cases of heart disease, the prevalence was 73 per 1,000 persons examined. Two reexaminations, at intervals of 12 to 18 months, of persons with normal heart on the first examination were carried out and 52 additional cases were diagnosed. There were also 13 deaths of heart disease in persons first diagnosed as having normal heart, making a total of 65 "new" cases (36.6 per 1,000) during the 30-month period of observation. An annual estimated heart disease incidence of 15 per 1,000 appears reasonable. Based on 89 deaths, the cardiovascular disease death rate was 11 per 1,000 among persons entering the study with normal heart, and 133 per 1,000 persons diagnosed as having heart disease at entry. The ratio of newly diagnosed cases to deaths of heart disease was 4 to 1.Among men diagnosed as having normal heart there was little difference in death rates whether their jobs were physically strenuous or sedentary. Among the men with heart disease, however, the highest death rates are observed among those employed at sedentary jobs and at light exertion. This may, of course, be an indication of the employee's selection of the job rather than the effect of inactivity. The relative usefulness of minifilm x-ray, electrocardiograms and questioning as to history were considered. 相似文献
9.
10.
X Xiao G Hintermann AL Demanin J Piret 《Journal of industrial microbiology & biotechnology》1996,16(4):261-262
Streptomyces glaucescens is shown to possess -lactamase activity which is inhibitable by clavulanate. This is important in regard to its use as a cloning host for enzymes of \-lactam biosynthesis. 相似文献