全文获取类型
收费全文 | 750篇 |
免费 | 57篇 |
出版年
2023年 | 4篇 |
2022年 | 7篇 |
2021年 | 9篇 |
2020年 | 10篇 |
2019年 | 11篇 |
2018年 | 14篇 |
2017年 | 12篇 |
2016年 | 17篇 |
2015年 | 30篇 |
2014年 | 33篇 |
2013年 | 55篇 |
2012年 | 77篇 |
2011年 | 43篇 |
2010年 | 31篇 |
2009年 | 31篇 |
2008年 | 44篇 |
2007年 | 39篇 |
2006年 | 44篇 |
2005年 | 48篇 |
2004年 | 44篇 |
2003年 | 37篇 |
2002年 | 28篇 |
2001年 | 9篇 |
2000年 | 7篇 |
1999年 | 10篇 |
1998年 | 8篇 |
1997年 | 6篇 |
1996年 | 3篇 |
1995年 | 7篇 |
1994年 | 9篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1973年 | 2篇 |
1970年 | 1篇 |
1969年 | 2篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1965年 | 1篇 |
排序方式: 共有807条查询结果,搜索用时 15 毫秒
801.
Donatella Magri 《Journal of Biogeography》2008,35(3):450-463
Aim Establishing possible relationships between the magnitudes of the glacial distribution of the European beech, Fagus sylvatica L., and its post-glacial spread.
Location Europe.
Methods A database of over 400 pollen records has been used to locate Fagus populations at the end of the last glacial and during the post-glacial in Europe and to assess the areal extent of their past distribution.
Results The rate of late-glacial and post-glacial increase in the number of pollen sites where Fagus was locally present conforms well to a logistic model of population growth. This suggests that the area occupied by beech populations expanded exponentially from the glacial refugia for a duration of over 10,000 years, until about 3500 yr bp . In the past three millennia beech populations increased at a slower rate, tending towards an equilibrium value.
Main conclusions The conformity of the increase in beech distribution to the classical logistic model of population growth indicates that: (1) a multiplicative biological process was the main factor shaping the pattern of the post-glacial expansion of F. sylvatica in Europe, (2) climate conditions, human activity and competition may have influenced its rate of spread, and (3) beech populations did not expand with a moving closed front, but with a diffuse spread from scattered nuclei. The distribution of Fagus in Europe at the end of the last glacial appears to have been of two orders of magnitude less extensive than at present. Pleistocene refugia were likely to have been a mosaic of sparse stands of small populations scattered in multiple regions. Fagus populations appear to have increased very slowly and to a moderate extent in southern Europe, where they are now declining slightly. The central European populations increased quickly and extensively, reaching northern Europe, and are now approaching their carrying capacity. 相似文献
Location Europe.
Methods A database of over 400 pollen records has been used to locate Fagus populations at the end of the last glacial and during the post-glacial in Europe and to assess the areal extent of their past distribution.
Results The rate of late-glacial and post-glacial increase in the number of pollen sites where Fagus was locally present conforms well to a logistic model of population growth. This suggests that the area occupied by beech populations expanded exponentially from the glacial refugia for a duration of over 10,000 years, until about 3500 yr bp . In the past three millennia beech populations increased at a slower rate, tending towards an equilibrium value.
Main conclusions The conformity of the increase in beech distribution to the classical logistic model of population growth indicates that: (1) a multiplicative biological process was the main factor shaping the pattern of the post-glacial expansion of F. sylvatica in Europe, (2) climate conditions, human activity and competition may have influenced its rate of spread, and (3) beech populations did not expand with a moving closed front, but with a diffuse spread from scattered nuclei. The distribution of Fagus in Europe at the end of the last glacial appears to have been of two orders of magnitude less extensive than at present. Pleistocene refugia were likely to have been a mosaic of sparse stands of small populations scattered in multiple regions. Fagus populations appear to have increased very slowly and to a moderate extent in southern Europe, where they are now declining slightly. The central European populations increased quickly and extensively, reaching northern Europe, and are now approaching their carrying capacity. 相似文献
802.
Donatella Serio Mario Cormaci Giovanni Furnari Fernando Boisset 《Phycological Research》2010,58(1):9-16
The first occurrence of Palisada maris-rubri (K.W. Nam et Saito) K.W. Nam (Ceramiales, Rhodomelaceae) from the Mediterranean Sea, is reported. To date the species was known only from tetrasporic specimens from the type locality (Ras Muhammed, Sinai, Egypt, Red Sea). Mediterranean thalli share nearly all vegetative and reproductive features with Red Sea specimens showing more robust thalli with axes to 3 mm broad and ultimate branchlets to 1000 µm broad, absence of intercellular spaces between medullary cells and epidermal cells in transverse section with a palisade arrangement. Male and cystocarpic thalli are recorded for the first time. Moreover, the analysis of characters of three species of Chondrophycus previously known from the Mediterranean Sea ( C. patentirameus (Montagne) K.W. Nam, C. tenerrimus (Cremades) G. Furnari et al. and C. thuyoides (Kützing) G. Furnari) led us to conclude that they belong to the genus Palisada . The following new combinations are formally proposed: P. patentiramea (Montagne) Serio et al., P. thuyoides (Kützing) Serio et al., P. tenerrima (Cremades) Serio et al. 相似文献
803.
Camillo Branca Vincenzo Plazzi Marina Vitto Fabrizio Bordi Donatella Serafini Fracassini Nello Bagni 《Phytochemistry》1975,14(12):2545-2550
The 1,2-benzisothiazol-3-yl-acetic and -3-yl-butyric acids and their ethyl esters, amides and nitriles are generally active in the split pea stem test, induce an increase in both length and fresh weight of pea internodes, inhibit the development of pea roots, and, with some exceptions (1,2-benzisothiazol-3-yl-butyric amide and nitrile), induce the production of ethylene by pea segments. Moreover they stimulate cell multiplication and raise the degree of hydration of Helianthus tuberosus explants grown in vitro. These activities are often similar or sometimes higher than those of IAA. By contrast, the 1,2-benzisothiazole derivatives having a side chain with an odd number of carbon atoms (-3-yl-carboxylic and propionic acids, amides, ethyl esters and nitriles) are inactive or show a far lower activity. 相似文献
804.
805.
Guido Greco Jr. Donatella Albanesi Maria Cantarella Liliana Gianfreda Rosaria Palescandolo Vincenzo Scardi 《Applied microbiology and biotechnology》1979,8(4):249-261
Summary An ultrafiltration membrane enzymatic reactor is used in connection with different reacting systems.The experimental conditions are such that the enzyme, which operates at fairly high concentration levels because of the concentration polarization phenomena taking place in the reactor, is still in soluble form.The analysis of the system unsteady-state response enables the identification of the mechanism of enzyme deactivation and the extraction of the kinetic parameters of both the deactivation and the main reaction.The stabilizing effect observed in connection with enzyme entrapment within an inert gel deposited onto the U.F. membrane active surface is also discussed.List of Symbols A
U.F. membrane cross sectional area cm2
- CE
Enzyme concentration mg/ml
- CEI
Enzyme concentration at the active membrane surface mg/ml
- CE
Mean enzyme concentration mg/ml
- c
s
o
Substrate concentration in the feed m moles/ml
- c
s
u
Substrate concentration in the outlet m moles/ml
- DE
Enzyme diffusivity cm2/s
- Km
michaelis constant mM
- k2
Kinetic constant of the enzymatic reaction m moles/mg s
- kd
Kinetic constant of the enzyme deactivation reaction s–1
- No
Initial amount of active enzyme mg
- N(t)
Active enzyme amount at reaction time t mg
- Q
Flow rate ml/s
- r
Rate of the main reaction m moles/ml s
- t
Reaction time s
- t*
Reaction time at which product concentration in the outlet is within ± 2% of the steady-state value s
- v
Fluid velocity cm/s
- V
Cell volume ml
- VB
Volume within which 99% of the enzyme fed is contained at the steady-state ml
- VS
Volume within which 99% of the total substrate concentration drop occurs at the steady-state ml
- x
Distance upstream the membrane measured from the membrane surface cm 相似文献
806.
We report on the effects that the presence of ethylene glycol or glycerol has on the oxygen affinity of hemoglobin. We attribute these effects to an altered equilibrium between T and R quaternary conformations of hemoglobin and separate them into bulk-electrostatic and non-bulk-electrostatic contributions to the standard free-energy difference between the R and T states. 相似文献
807.