全文获取类型
收费全文 | 1996篇 |
免费 | 148篇 |
国内免费 | 57篇 |
专业分类
2201篇 |
出版年
2024年 | 3篇 |
2023年 | 32篇 |
2022年 | 27篇 |
2021年 | 46篇 |
2020年 | 59篇 |
2019年 | 52篇 |
2018年 | 53篇 |
2017年 | 49篇 |
2016年 | 51篇 |
2015年 | 66篇 |
2014年 | 74篇 |
2013年 | 99篇 |
2012年 | 54篇 |
2011年 | 71篇 |
2010年 | 57篇 |
2009年 | 83篇 |
2008年 | 91篇 |
2007年 | 87篇 |
2006年 | 93篇 |
2005年 | 73篇 |
2004年 | 81篇 |
2003年 | 81篇 |
2002年 | 75篇 |
2001年 | 68篇 |
2000年 | 46篇 |
1999年 | 53篇 |
1998年 | 50篇 |
1997年 | 45篇 |
1996年 | 37篇 |
1995年 | 45篇 |
1994年 | 40篇 |
1993年 | 30篇 |
1992年 | 35篇 |
1991年 | 26篇 |
1990年 | 27篇 |
1989年 | 19篇 |
1988年 | 20篇 |
1987年 | 24篇 |
1986年 | 18篇 |
1985年 | 17篇 |
1984年 | 25篇 |
1983年 | 6篇 |
1982年 | 12篇 |
1981年 | 24篇 |
1980年 | 25篇 |
1979年 | 20篇 |
1978年 | 9篇 |
1977年 | 16篇 |
1976年 | 6篇 |
1975年 | 1篇 |
排序方式: 共有2201条查询结果,搜索用时 0 毫秒
1.
2.
D. D. PANT F.L.S. NUPUR BASU 《Botanical journal of the Linnean Society. Linnean Society of London》1977,75(3):271-282
Details of the cuticular, epidermal and anatomical features of the leaves of Cathaya argyrophylla Chun & Kuang, are described and compared with those of three species of Keteleeria (K. davidiana (Bertrand) Beissner, K. fortunei (Murray) Carriere, and K. chien-peii Flous). The study supports the creation of Cathaya Chun & Kuang as a new genus of the Pinaceae. 相似文献
3.
Gaël Clement 《Palaeontology》2002,45(3):577-593
Remains of two large sarcopterygians are described from Famennian deposits in Belgium. One of them is referred to Eusthenodon wängsjöi Jarvik; it is the first occurrence of this genus in Belgium. The other, much larger one, appears to be a tristichopterid. It has a postspiracular; size and shape of the mandible similar to those of Platycephalichthys skuenicus and P. bischoffi ; snout and cheek patterns close to those of Eusthenodon ; unusual shape of the supratemporal resembling that of Hyneria , Mandageria and Platycephalichthys skuenicus ; and tooth histology quite similar to that of Eusthenodon and Platycephalichthys . 相似文献
4.
OLIVIER RIEPPEL 《Biological journal of the Linnean Society. Linnean Society of London》2007,92(2):297-308
In view of their propositional content (i.e. they can be right or wrong), character statements (i.e. statements that predicate characters of organisms) are treated as low-level hypotheses. The thesis of the present study is that such character statements, as do more complex scientific theories, come with variable scope. The scope of a hypothesis, or theory, is the domain of discourse over which the hypothesis, or theory, ranges. A character statement is initially introduced within the context of a certain domain of discourse that is defined by the scale of the initial phylogenetic analysis. The doctrine of 'total evidence' requires the inclusion of previously introduced characters in subsequent studies. As a consequence, the initial scope of character statements is widened to the extent that the scale of subsequent analyses is broadened. Scope expansion for character statements may result in incomplete characters, in the subdivision of characters, or in ambiguity of reference (indeterminacy of the extension of anatomical terms). Character statements with a wide scope are desirable because they refer to characters with the potential to resolve deep nodes in phylogenetic analyses. Care must be taken to preserve referential unambiguity of anatomical terms if the originally restricted scope of a character statement is expanded to match a broad-scale phylogenetic analysis. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 297–308. 相似文献
5.
ECTOMYCORRHIZA FORMATION IN EUCALYPTUS 总被引:3,自引:3,他引:0
6.
Summary Leaf water potentials, osmotic properties and structural characteristics were examined in the Australian tropical rainforest tree species, Castanospermum australe. These features were compared for individuals growing in the understorey and canopy of the undisturbed forest and in an open pasture from which the forest had been cleared. Leaf water potentials during the day declined to significantly lower values in the open-grown and canopy trees than in the understorey trees. During most of the day the opengrown tree experienced the lowest water potentials. These differences were paralleled by significant differences in tissue osmotic properties. The tissue osmotic potential at full hydration was lowest in the open-grown tree (-1.80 MPa), intermediate in the canopy trees (-1.38 MPa), and highest in the understorey trees (-0.80 MPa). As a result, the degree to which high and positive turgor pressures were maintained as water potentials declined was highest in the open-grown tree, intermediate in the canopy trees, and lowest in the understorey trees. The differences in tissue osmotic properties between individuals in the three crown positions were paralleled, in turn, by differences in leaf structual characteristics. Relative to leaves of the canopy and open-grown trees, leaves of the understorey trees had significantly larger epidermal cells with thinner cell walls, larger specific leaf areas and turgid weight: dry weight ratios, and a higher proportion of intercellular air space.Abbreviations 1
Leaf tissue water potential
- min
Lowest value of 1 during the day ( noon)
-
P=0 1
zero turgor
-
R
Relative water content
-
P
Tissue turgor pressure
-
Tissue osmotic potential
- 0
at full hydration 相似文献
7.
C. A. Atkins 《Plant and Soil》1987,100(1-3):157-169
Summary Nitrogen (N2) fixed by Rhizobium bacteroids in the legume nodule is excreted as ammonia to the surrounding host cell where it is efficiently
assimilated into the amide group of glutamine. Generally glutamine is a minor exported solute of nitrogen, being further metabolised
to asparagine in temperate species and to the ureides, allantoin and allantoic acid in tropical species. These solutes serve
as the principal translocated forms of nitrogen in xylem. Compartmentalisation of the pathways of nitrogen metabolism and
the role of ammonia in regulation of their activity is examined in nodules of both asparagine-forming (Lupinus albus L.) and ureide-forming (Vigna unguiculata L. Walp) symbioses. 相似文献
8.
Akira Takahashi 《Journal of plant research》1988,101(4):387-395
Very recentlySarcandra, which had long been known as the only vesselless genus in Chloranthaceae, was found by Carlquist to have vessels in root
secondary xylem. The present study further shows on the basis of observations of the xylem ontogeny that vessels occur in
stem metaxylem ofSarcandra glabra as well, thus offering additional evidence for the occurrence of vessels in the genus, virtually in all Chloranthaceae. Metaxylem
elements of the stem are thicker than the other tracheary elements in general and have scalariform pittings at the end wall,
and their ontogeny indicates that, as the surrounding cytoplasm disintegrates, pit membranes at the end wall disappear at
least in some elements, resulting in a perforated end wall, i.e., vessel perforation. The present study further shows thatChloranthus spicatus, which is closely related toSarcandra, may have an incomplete perforation plate because of retaining membranes at places on the plate. An evolutionary state of
the “vesselless” condition in Chloranthaceae is discussed. 相似文献
9.
10.
Résumé En vue d'une étude, au laboratoire, de la nodulation et de la symbiose plante-bactérie des préparations de protoplastes de mésophylle et de nodule racinaire ont été effectuées chez le haricot (Phaseolus vulgaris variété Constant). Pour chaque préparation, les conditions opératoires et les concentrations optimales de cellulase et de pectinase en solution, dans du mannitol 0,6M ont été définies.Áprès 15 jours, 10 à 15% des protoplastes de mésophylle mis en culture dans le milieu B5 de Gamborg se divisent 2 à 3 fois. Des essais de fusion réalisés avec des faibles densités cellulaires entre protoplastes de mésophylle et protoplastes de nodule ont, donné des taux de fusion de l'ordre de 2,43%.
Study of the symbiosis Rhizobium-legume: Preparation of isolated cells forin vitro study
Summary With a view to studyin vitro the processes of nodulation and plant-bacteria symbiosis, preparation of mesophyll protoplasts and root nodule protoplasts have been realized fromPhaseolus vulgaris (Constant var.).For every preparation the process of experimentation and optimum concentrations of cellulase and pectinase diluted in mannitol 0,6M have been defined.After a 15 days' period, 10 to 15% of mesophyll protoplasts divided twice or even three times in B5 Gamborg's medium. Although we used low rate cell concentrations, root nodule protoplasts and mesophyll protoplasts have been able to fuse in the proportion of 2,43%.相似文献