全文获取类型
收费全文 | 2414篇 |
免费 | 213篇 |
国内免费 | 103篇 |
出版年
2024年 | 4篇 |
2023年 | 22篇 |
2022年 | 17篇 |
2021年 | 37篇 |
2020年 | 30篇 |
2019年 | 32篇 |
2018年 | 46篇 |
2017年 | 41篇 |
2016年 | 39篇 |
2015年 | 49篇 |
2014年 | 71篇 |
2013年 | 83篇 |
2012年 | 64篇 |
2011年 | 69篇 |
2010年 | 48篇 |
2009年 | 151篇 |
2008年 | 134篇 |
2007年 | 143篇 |
2006年 | 124篇 |
2005年 | 131篇 |
2004年 | 98篇 |
2003年 | 83篇 |
2002年 | 71篇 |
2001年 | 57篇 |
2000年 | 76篇 |
1999年 | 80篇 |
1998年 | 71篇 |
1997年 | 69篇 |
1996年 | 52篇 |
1995年 | 50篇 |
1994年 | 66篇 |
1993年 | 64篇 |
1992年 | 53篇 |
1991年 | 60篇 |
1990年 | 52篇 |
1989年 | 42篇 |
1988年 | 49篇 |
1987年 | 48篇 |
1986年 | 36篇 |
1985年 | 37篇 |
1984年 | 33篇 |
1983年 | 13篇 |
1982年 | 25篇 |
1981年 | 36篇 |
1980年 | 29篇 |
1979年 | 19篇 |
1978年 | 8篇 |
1977年 | 11篇 |
1976年 | 2篇 |
1974年 | 2篇 |
排序方式: 共有2730条查询结果,搜索用时 16 毫秒
1.
We have examined cytokeratin distribution and their nature in toe pads of the Himalayan tree-frog Philautus annandalii. Toe pads are expanded tips of digits and show modifications of their ventral epidermis for adhesion. The toe pad epidermal cells, being organized into 3–4 rows, possess keratin bundles, especially in surface nanostructures that are involved in adhesion. Immunohistochemical localization using a pan-cytokeratin antibody revealed that cytokeratin immunoreactivity is the strongest in the mid- to basal cell rows of the epidermis, which parallels our previous ultrastructural observation of dense keratin bundles present in this part of the epidermis. The remainder of the epidermis (i.e., the superficial cell layer) showed little immunoreactivity. Immunoblot analysis revealed that toe-pads possessed keratins prominently in the molecular mass of 50 kDa. Possible presence of keratin 5 in toe pad epidermis has been correlated with its usual distribution pattern in mammalian epidermis. 相似文献
2.
Trienoic fatty acids, namely -linolenic acid and hexadecatrienoic acid, present in leaf lipids are produced by -3 fatty acid desaturases located in the endoplasmic reticulum and plastid membranes. The changes in the level of trienoic fatty acids during leaf maturation were investigated in wild-type plants of Arabidopsis thaliana (L.) Heynh. and in the fad7 mutant deficient in the activity of a plastid -3 desaturase. The levels of trienoic fatty acids increased in 26 °C- and 15 °C-grown wild-type plants with maturation of leaves. The increase in trienoic fatty acids was mainly due to galactolipids enriched in plastid membranes. In addition, the relative levels of trienoic fatty acids in major glycerolipids, including phospholipids enriched in the endoplasmic reticulum membranes, also increased with leaf maturation. By contrast, when the fad7 mutant was grown at 26 °C, the relative levels of trienoic fatty acids in individual lipids decreased with leaf maturation. The decreases in the levels of trienoic fatty acids, however, were alleviated when the fad7 mutant was grown at 15 °C. These results suggest that the plastid -3 desaturase plays a major role in increasing the levels of trienoic fatty acids with leaf maturation.Abbreviations 163
hexadecatrienoic acid
- 183
-linolenic acid
- DGD
digalactosyldiacylglycerol
- MGD
monogalactosyldiacylglycerol
- PC
phosphatidylcholine
- PE
phosphatidylethanolamine
- TA
trienoic fatty acid
- WT
wild type
- -3
refers to the position of the double bond from the methyl end of a fatty acid
This research was supported in part by Grants-in-Aid for Scientific research (#07251214 and #06804050 to K.I.) from the Ministry of Education, Science and Culture, Japan, and by the research grant from Shorai Foundation. 相似文献
3.
M. C. Press A. N. Parsons A. W. Mackay C. A. Vincent V. Cochrane W. E. Seel 《Oecologia》1993,95(1):145-151
Plant height, light-saturated rates of photosynthesis (A
max) and foliar nitrogen concentration (N
1) were measured forBartsia trixago under field conditions in Mallorca. All three variables were postively correlated, and were also positively related to the abundance of nitrogen-fixing legumes in the associated vegetation (putative host species).A
max forB. trixago ranged from 7.7 to 18.8 mol m-2 s-1; similar rates were measured for a second hemiparasiteParentucellia viscosa, and both species were within the range of rates measured for six putative hosts (10.6–19.2 mol m-2 s-1). Fertilization of unattachedB. trixago plants with inorganic nitrogen (ammonium nitrate) elicited neither the growth nor the photosynthetic responses observed in plants considered to be parasitic on legumes and in receipt of an enriched organic nitrogen supply. Both hemiparasites had high diurnal leaf conductances (g
s) (469–2291 mmol m-2 s-1) and were at the upper end of the range of those measured in putative hosts (409–879 mmol m-2 s-1). In contrast with the latter, high nocturnal rates ofg
s were also recorded for the two hemiparasites (517–1862 mmol m-2 s-1). There was no clear relationship between eitherA
max orN
1 and eitherg
s, transpiration (E) or water use efficiency (A
max/E) inB. trixago plants. The economics of water loss appear to be independent of both the supply of nitrogen from the host and autotrophic carbon fixation. 相似文献
4.
Detached leaves of Cyclamen persicum Mill. can be used as a simple source-sink system. Phloem transport in the excised material was monitored by the noninvasive 11C-technique. Assimilate movement stopped immediately when the petiole was cut off. However, within 20 min a recovery of transport was observed. The translocation rate in the detached leaf was only 13% of that in the intact plant. 14C-Xenobiotics and [3H]sucrose were injected into the upper petiole parenchyma (source). They moved downstream by a symplastic route. The stump of the petiole was inserted into a buffer solution containing ethylenediaminetetraacetic acid (sink). After 3 h, the distribution of sucrose and xenobiotics was determined in five subsequent segments of the petiole (path). The retention coefficient (r) was calculated from the ratio of radioactivity in the vascular bundle to that in the petiole parenchyma. The distribution along the vascular path was given by a geometric progression, whereas its constant was the transport coefficient (q). Values of r and q corresponded with the degree of phloem mobility and ambimobility. Four groups of compounds were classified: (i) acidic substances with log Kow = — 2 to — 2.4 (Kow is the partition coefficient octanol/water) at pH 8 (pH of sieve tube sap), retained by ion trapping and exhibiting small lateral efflux (q0.7; maleic hydrazide, dalapon); (ii) acidic substances with log Kow = — 0.7 to — 0.8 at pH 8, retained by ion trapping and subjected to a moderate lateral efflux (0.7>q> 0.5; 2,4-dichlorophenoxyacetic acid, 2-methyl-4-chlorophenoxyacetic acid, bromoxynil); (iii) nonionised substances retained by optimum permeability, exhibiting a considerable lateral leakage (q<0.5; glyphosate, amitrole); (iv) substances without basipetal transport in the phloem (atrazine, diuron). Retention of sucrose corresponded quantitatively with that shown in group (i). This classification was also supported by results of uptake and efflux experiments using the isolated conducting tissue. Theoretical translocation profiles were calculated from the determined transport coefficients (q).Abbreviations 2,4-D
2,4-dichlorophenoxyacetic acid
- Kow
partition coefficient octanol/water
- MCPA
2-methyl-4-chloro-phenoxyacetic acid
- q
transport coefficient in the vascular bundle
- r
retention coefficient in the vascular bundle
The authors gratefully acknowledge the assistance of H. Fiedler and M. Neugebauer. We are particularly grateful to K. Dutschka, G. Hudepokl, and Dr. J. Knust for producing 11CO2. 相似文献
5.
为了解罗汉果(Siraitia grosvenorii(Swingle)C.Jeffrey)中的化学成分,利用溶剂萃取和色谱分离手段,从罗汉果叶中分离得到9个化合物。通过光谱分析,分别鉴定为:山奈酚-3,7-O-α-L-二鼠李糖苷(1)、山奈酚-3-O-α-L-鼠李糖苷(2)、阿魏酸(3)、4′-甲氧基二氢槲皮素(4)、大黄素(5)、芦荟大黄素(6)、槲皮素(7)、山奈酚(8)、山奈酚-7-O-α-L-鼠李糖苷(9),其中化合物3~6为首次从罗汉果叶中分离得到。 相似文献
6.
Latitudinal comparison of altitudinal changes in forest structure,leaf-type,and species richness in humid monsoon Asia 总被引:10,自引:0,他引:10
M. Ohsawa 《Plant Ecology》1995,121(1-2):3-10
A new template for mountain vegetation zonation along latitudinal gradients is proposed for examining geographical pattern of various forest attributes in humid monsoon Asia. The contrasting temperature regime in tropical and temperate mountains, i.e., the former is a non-seasonal, temperature-sum controlled environment, and the latter is a seasonal, low temperature limiting environment, leads to different altitudinal patterns of tree height distribution and species richness. In the tropical mountains, both tree height and species richness decrease steeply, and the tree height often stepwise. The decline of tree height and species diversity in the temperate mountains is far less pronounced except near the forest limit. Both trends are explained by their temperature regime. 相似文献
7.
Epidermal topography was examined, including papillate ridges, grooves and ciliated sensory papillae of Craspedella sp. from the branchial chamber of redclaw crayfish, Cherax quadricarinatus, from Queensland, Australia. Rhandites were observed to discharge from ducts opening mainly in a small distal region of the ventral epidermis of the three central (of five) tentacles. These regions, devoid of ciliated sensory papillae, serve to adhere the anterior end of the worms during locomotion. Secretions from glands associated with the posterior attachment organ were observed to discharge from pores on the outside region of the ventral surface of the disc.A comparison of various scanning electron microscopy (SEM) fixation techniques showed that (1) hot fixatives at 90 °C provide most information on the largest number of epidermal structures and (2) different fixation regimes highlight different epidermal features. 相似文献
8.
Abstract. Gas exchange, leaf-nitrogen concentration and water potential were measured in early and late spring in early successional herbaceous plants occurring after cutting and after fire, and in mature woody species from the Mediterranean climax community Quercetum ilicis in central Italy. Net photosynthesis peaked in early spring in all species studied when values for temperature and light were lower but leaf-nitrogen content was higher as compared to late spring, suggesting that nitrogen more than energy input controlled photosynt-hetic rates. Herbaceous pioneer species occurring after cutting showed higher field photo synthetic capacity than evergreen climax trees and shrubs. By contrast, net photosynthesis of herbaceous species occurring in a persistent stage after fire, was in the same range as that of climax trees. This evidence suggests that carbon-gaining appears to be partly related to the dynamic stage of succession and not solely to the growth form. 相似文献
9.
We studied the spatial distribution of fruits and plants, mortality and growth rates ofScaphium macropodum (Sterculiaceae) in four 1-ha plots in a tropical rain forest in West Kalimantan, Indonesia. The species is a large deciduous
tree and produces wind-dispersed fruits on defoliated twigs. The density of dispersed fruits on the ground decreased with
increasing distance from a parent tree. The area under the parent's crown had the highest density of the fruits and the highest
mortality of the seedlings immediately after germination. Consequently, the density of the established seedlings peaked 14
m from the tree which is outside its crown. Thick litter mainly from the parent tree seemed to physically prevent the seedlings'
root from reaching the soil surface and caused the high mortality. Juvenile and mature trees distributed exclusively, suggesting
that regeneration is the most successful outside of the crown of mature trees. Saplings under canopy shade did not grow well.Scaphium macropodum is hypothesized to require a gap for seedling growth and successful regeneration, whereas it can germinate and last under
closed canopies as suppressed seedlings or saplings. 相似文献
10.