全文获取类型
收费全文 | 1911篇 |
免费 | 123篇 |
国内免费 | 178篇 |
出版年
2024年 | 10篇 |
2023年 | 17篇 |
2022年 | 23篇 |
2021年 | 29篇 |
2020年 | 19篇 |
2019年 | 21篇 |
2018年 | 33篇 |
2017年 | 37篇 |
2016年 | 26篇 |
2015年 | 39篇 |
2014年 | 56篇 |
2013年 | 53篇 |
2012年 | 43篇 |
2011年 | 51篇 |
2010年 | 39篇 |
2009年 | 119篇 |
2008年 | 106篇 |
2007年 | 114篇 |
2006年 | 116篇 |
2005年 | 109篇 |
2004年 | 76篇 |
2003年 | 59篇 |
2002年 | 51篇 |
2001年 | 40篇 |
2000年 | 58篇 |
1999年 | 66篇 |
1998年 | 63篇 |
1997年 | 61篇 |
1996年 | 47篇 |
1995年 | 42篇 |
1994年 | 55篇 |
1993年 | 52篇 |
1992年 | 49篇 |
1991年 | 53篇 |
1990年 | 46篇 |
1989年 | 37篇 |
1988年 | 43篇 |
1987年 | 37篇 |
1986年 | 29篇 |
1985年 | 35篇 |
1984年 | 33篇 |
1983年 | 10篇 |
1982年 | 23篇 |
1981年 | 30篇 |
1980年 | 19篇 |
1979年 | 15篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1975年 | 3篇 |
1974年 | 2篇 |
排序方式: 共有2212条查询结果,搜索用时 15 毫秒
31.
The cellular parameters of leaf development in tobacco (Nicotiana tabacum L.) have been characterized using clonal analysis, an approach that provides unequivocal evidence of cell lineage. Our results indicate that the tobacco leaf arises from a group of around 100 cells in the shoot apical meristem. Each of these cells contributes to a unique longitudinal section of the axis and transverse section of the lamina. This pattern of cell lincage indicates that primordial cells contribute more or less equally to the growth of the axis, in contrast to the more traditional view of leaf development in which the leaf is pictured as arising from a group of apical initials. Clones induced prior to the initiation of the lamina demonstrate that the subepidermal layer of the lamina arises from at least six files of cells. Submarginal cells usually divide with their spindles parallel to the margin, and therefore contribute relatively little to the transverse expansion of the lamina. During the expansion of the lamina the orientation and frequency of cell division are highly regulated, as is the duration of meristematic growth. Initially, cell division is polarized so as to produce lineages that are at an oblique angle to the midrib; later cell division is in alternating perpendicular planes. The distribution of clones generated by irradiation at various stages of development indicates that cell division ceases at the tip of the leaf when the leaf is about one tenth its final size, and then ceases in progressively more basal regions of the lamina. Variation in the mutation frequency within the lamina reflects variation in the frequency of mitosis. Prior to the mergence of the leaf the frequency of mutation is maximal near the tip of the leaf and extremely low at its base; after emergence, the frequency of mutation increases at the base of the leaf. In any given region of the lamina the frequency of mutation is highest in interveinal regions, and is relatively low near the margin. Thus, both the orientation and frequency of cell division at the leaf margin indicate that this region plays a minor role in the growth of the lamina.Abbreviation MF
mutation frequency 相似文献
32.
Summary The objective of this in-vitro study was to examine whether the diencephalic floor or the mesenchyme is involved in differentiation of LH cells in the developing rat adenohypophysis. Overall growth of the adenohypophysial tissue was retarded when the adenohypophysial primordium was cultivated after enzymatic removal of the diencephalic floor on days 11.5 and 12.5 of gestation. This malgrowth was more marked when the brain was separated on day 11.5; most expiants retained a simple cystiform structure that consisted of a few layers of undifferentiated cells. Removal of the brain also caused a highly significant decrease (P < 0.001) in the number of immunoreactive LH cells, if it was performed on day 11.5 but not day 12.5. Mesenchyme had little effect on the adenohypophysial growth or the number of immunopositive cells. Cultivation of the adenohypophysial primordium with the diencephalic floor resulted in the appearance of many immunoreactive LH cells. The number of LH cells significantly decreased, however, when the co-cultivated brain completely surrounded the adenohypophysial tissue.These results indicate that in 11.5-day-old fetal rats the diencephalic floor is indispensable for the initial proliferation of adenohypophysial primordial cells and for the early determinating process of LH cells. Once determined, the development of LH cells may proceed without the surrounding tissues. The cytodifferentiation seems to be rather inhibited when in contact with the brain. The significance of the intimate spatial relationship between developing LH cells and the surrounding mesenchyme is also discussed. 相似文献
33.
Peter B. Reich 《Physiologia plantarum》1984,61(4):541-548
Cycling of stomatal conductance in three hybrid poplar ( Populus sp.) cultivars was observed under a variety of conditions. Illumination of plants kept previously in the dark induced very large oscillations with a period of about 40 min and large oscillations with a shorter period (< 10 min) were superimposed on the longer cycles. During these oscillations, large changes in conductance could occur very rapidly (1.0 cm s−1 in 3 min). Plants in constant light also displayed both long and short term cycles in conductance, but these were smaller in amplitude than those induced by sudden illumination. Stomatal oscillations were also observed in darkness and after darkening of previously illuminated plants. These oscillations had shorter (< 30 min) and less regular periods than those observed in the light. Such cycling in the dark is rare. Cycling of the two leaf surfaces was sometimes in synchrony in the light, and more so after a perturbation. Little synchrony between the two surfaces was observed in the dark. Stomatal movements of different leaves on a plant were usually relatively independent. Transient stomatal opening occurred following leaf excision in the light or dark, and often after sudden darkening of intact leaves. Also, stomata of intact leaves sometimes transiently closed following illumination. 相似文献
34.
O. I. Joarder N. H. Joarder P. J. Dale 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1986,73(2):286-291
Summary Immature gramineous leaves provide a convenient system for comparing the response of cells in culture with their state of differentiation. Callusing frequency is compared with leaf segment position, leaf age and in vivo mitotic activity in Lolium multiflorum. (1) In a succession of one millimeter sections from the immature leaf base, callus was formed from the first and second sections but not the third or subsequent sections. The frequency of those explants callusing decreased with distance from the base of the leaf and with leaf age (or leaf extension growth). (2) In vivo, the proportion of cells in mitosis declined from around 10–14% at the base of young leaves to zero at 5 mm from the base and beyond. Mitotic activity also declined in leaves as they aged, and dividing cells were not observed in leaves 30 days from initiation or older. (3) A high frequency of callus formation was associated with a high mitotic index in the explant. But for corresponding mitotic indices, cells further away from the leaf base were less responsive in culture. (4) It is proposed that cells are becoming differentiated even in highly meristematically active regions of the leaf and concomitantly losing their ability to respond in culture. 相似文献
35.
Ken-ichi Miyaji 《Ecological Research》1986,1(3):303-322
In order to demonstrate in detail the relationship between the longevity and productivity of leaves within a canopy, a new
life table approach, the ‘bioeconomic life table’, was applied to the leaves of kidney bean plants (Phaseolus vulgaris L.) in relation to planting density and their position within the canopy.
The net photosynthetic rate for upper leaves under full daylight tended to decline gradually due to leaf senescence from about
20 days after leaf emergence, and for the lower leaves the decrease was very rapid due to both shading and senescence about
10 days after emergence.
Analysis of the survivorship curves and daily surplus production of leaves suggested that the lower and middle leaves, especially
the latter, survived without surplus production of dry matter after they had reached mean longevity, and while the upper leaves
at high density had a much shorter mean longevity, they had very large values of daily surplus production throughout the survival
period.
For the total foliage, the summed value of accumulated surplus production during the survival period was about five times
as large as the summed value of the dry weight of the dead leaves, regardless of planting density.
The daily rate of canopy leaf respiration was almost proportional to that of canopy gross photosynthesis for the various leaf
area indices of the canopy, so that there was no optimum leaf area index that maximized canopy daily surplus production. 相似文献
36.
The photosynthetic capacity of detached leaves of a non-yellowing mutant of Festuca pratensis Huds. declined during senescence at a similar rate to that in a normal cultivar. Respiratory oxygen uptake in the dark continued at similar rates in both genotypes during several days of senescence. In chloroplasts isolated from leaves at intervals after excision, the rate of photosystem I (PS I)-mediated methyl viologen reduction using reduced N,N,N,N-tetramethyl-p-phenylene diamine as electron donor also declined in both genotypes, possibly due to loss of integrity of the photosynthetic apparatus in the cytochrome f-plastocyanin region. There was a similar fall in PS II electron transport using water as electron donor and measured at the rate of reduction of 2,6-dichlorophenolindophenol. Partial restoration of this activity by the addition of diphenyl carbazide was evidence for lability of the oxygen-evolving complex during senescence. An accentuated difference between mutant and normal material in this case indicated that the mutant retains a greater number of functional PS II centres. Changes in the light-saturation characteristics of the two photosystems have been discussed in relation to the organization of the photosynthetic membranes during senescence.Abbreviations and symbols DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- DCPIP
2,6-dichlorophenolindophenol
- DMSO
dimethyl sulphoxide
- DPC
diphenyl carbazide
- MV
methyl viologen
- PS I, PS II
photosystem I, II
- TMPD
N,N,N,N-tetramethyl-p-phenylene diamine 相似文献
37.
Photosynthetic characteristics of detached barley leaves during greening in the presence of SAN 9785
The effects of the pyridazinone compound SAN 9785 on the photosynthetic competence of leaves, on the photochemical activity of isolated thylakoids and on the formation and spectral properties of chlorophyll-protein complexes were studied during a 72-h greening period of detached etiolated leaves of barley (Hordeum vulgare L. cv. Horpácsi kétsoros). It was established that i) the photosynthetic capacity of the leaves decreased considerably (by 80 and 90%, as determined by14CO2 fixation and fast fluorescence induction measurements, respectively); ii) the photochemical activity of isolated thylakoids from water to potassium ferricyanide and from dichlorophenol indophenol/ascorbate to methylviologen exhibited only slight reductions when expressed on a chlorophyll basis compared with the control; iii) the slow fluorescence induction curves of the treated leaves demonstrated the presence of a peculiar fluorescence component interrupting the quenching of fluorescence at around 1 min illumination; iv) a shortage of the chlorophyll-protein complex of photosystem I (CPI) occurred with a higher content of the monomer of the light harvesting complex in the thylakoids of treated leaves; and v) the fluorescence spectrum of the CPI band present in treated leaves indicates the destruction of the structural integrity of this complex during isolation from the membrane.Abbreviations Chl
chlorophyll
- CPI, CPII
chlorophyll-protein complexes of the reaction centres of PSI and PSII
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- DPIP
2,6-dichlorophenol indophenol
- DPIPH2
chemically reduced form of DPIP
-
F
o
fluorescence of constant yield
-
F
v
fluorescence of variable yield
-
F
i
,F
m
mitial and maximum yield of fluorescence
- LHCP3
monomer of the light-harvesting complex
- LHCP2 and LHCP1
oligomers of the light-harvesting complex LHCP3
- PSI, PSII
photosystems I, II
- SAN 9785
4-chloro-5-(dimethylamino)-2-phenyl-3(2H)-pyridazinone, also known as BASF 13-338
- SDS-PAGE
sodium dodecyl sulphate-polyacrylamide gel electrophoresis 相似文献
38.
叶龄及树冠不同部位光强对黄花梨光合速率的影响 总被引:7,自引:0,他引:7
笔者以黄花梨为试材,研究了叶龄及树冠不同部位光强对其光合速率的影响。结果表明:黄花梨叶片在展叶后约11天即有少量光合产物输出,25天时,单叶净光合速率接近最大值;密植梨树高光合叶幕厚度约125cm左右。 相似文献
39.
40.