全文获取类型
收费全文 | 5226篇 |
免费 | 270篇 |
国内免费 | 8篇 |
专业分类
5504篇 |
出版年
2021年 | 55篇 |
2020年 | 41篇 |
2019年 | 49篇 |
2018年 | 136篇 |
2017年 | 109篇 |
2016年 | 162篇 |
2015年 | 139篇 |
2014年 | 190篇 |
2013年 | 276篇 |
2012年 | 318篇 |
2011年 | 315篇 |
2010年 | 209篇 |
2009年 | 200篇 |
2008年 | 253篇 |
2007年 | 226篇 |
2006年 | 261篇 |
2005年 | 193篇 |
2004年 | 190篇 |
2003年 | 173篇 |
2002年 | 181篇 |
2001年 | 123篇 |
2000年 | 134篇 |
1999年 | 92篇 |
1998年 | 58篇 |
1997年 | 41篇 |
1996年 | 44篇 |
1995年 | 33篇 |
1994年 | 48篇 |
1993年 | 37篇 |
1992年 | 47篇 |
1991年 | 41篇 |
1990年 | 28篇 |
1988年 | 38篇 |
1987年 | 39篇 |
1985年 | 39篇 |
1984年 | 39篇 |
1983年 | 35篇 |
1982年 | 47篇 |
1981年 | 42篇 |
1980年 | 30篇 |
1979年 | 29篇 |
1978年 | 34篇 |
1977年 | 34篇 |
1976年 | 42篇 |
1975年 | 37篇 |
1974年 | 48篇 |
1973年 | 29篇 |
1972年 | 34篇 |
1969年 | 25篇 |
1968年 | 25篇 |
排序方式: 共有5504条查询结果,搜索用时 15 毫秒
71.
Summary LW13K2 cells, a clone of a spontaneously in vitro transformed derivative of embryonic Lewis rat fibroblastic cells, were studied by phase contrast cine-light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The ruffles found at the advancing edge of cells grown on glass substrates in vitro form and recede in a period of less than one min if they do not make an attachment of the substrate. If they fail to make an attachment they may form pinocytotic channels near the leading edge as described by Price (1972) and/or collapse, generally backwards, towards the cell body. The spines which appear to reinforce the membranous ruffles are the last structures to disappear, and accumulate in an irregular array behind the ruffling edge; this area is behind that in which pinocytosis occurs. In comparison with the sparse numbers of ribosomes found in the trailing edge, they are present in notable concentrations near the leading, ruffling edge of the cell. No membrane vesicles have been found in or near the ruffling edges at the ruffle-spine concentration zone. 相似文献
72.
The capacity of pollen tubes to utilize exogenous uridine during 8 h of cultivation in shaken suspension in a sugar-mineral
medium was examined by continuous and pulse labelling with3H-uridine. The increase of uptake with increasing concentration of the nucleoside indicated a saturable transport system with
an approximate Km of 9.4 × 10−6 M and 12.5 × 10−6M as determined in 1-h and 6-h cultures, respectively. Maximal uptake took place at the beginning of germination reaching
a rate of about 2 nmol h−1 per 1 mg of dry pollen at 0.1 mM external uridine. The uptake activity decreased with the time of pollen tube growth to less
than one third during the 8-h cultivation period. Moreover, the level of radioactivity taken up initially decreased later
on during continuous cultivation in the presence of3H-uridine. The uptake took place against a concentration difference and the onset and rate of uridine release depended on
its exogenous concentration.
The activity of the nucleoside incorporation into RNA increased during the first 4 h of cultivation, decreasing later on.
The proportion of RNA radioactivity in continuously labelled pollen tubes grew steadily during 6 h and reached 2.5% with respect
to soluble pool at 0.4 μM uridine. The time course of RNA labelling was independent of uridine concentration within the range
of 0.4 μM to 40 μM but this concentration rise resulted in an about fiftyfold increase of the total amount of external uridine
incorporated. 相似文献
73.
74.
A new type of flow bioreactor designed to remove nitrate from water was developed. Denitrification activity of native Paracoccus denitrificans cells was used, the cells being separated from the refined medium by a semipermeable membrane. Relationships between the degree of nitrate conversion and the denitrification rate, on the one hand, and the volume flow rate and the amount of biomass, on the other, together with the results concerning denitrification during closed-circuit recirculation of the medium are discussed. 相似文献
75.
A method for the preparation and regeneration of protoplasts of Streptomyces lincolnensis is described. Mycelium in the early exponential phase appeared to be most suitable for this purpose and yielded up to 25% regenerated intact cells. Transformation of S. lincolnensis protoplasts was achieved using broad-host-range streptomycete plasmid vectors pIJ622, pMP66, pRS410 and pIJ943 constructed from replicons pIJ101, pSLG33 and SCP2. The efficiency of transformation was 3.10(3) transformants per micrograms plasmid DNA when (2-5).10(7) recipient protoplasts were used. Interspecific transformations showed that there is no efficient restriction system in S. lincolnensis that would limit the transfer of genetic information from S. lividans or E. coli. 相似文献
76.
Adult resident males of one-male-multi-female primate groups housed at the Hannover Zoo exhibited aggression, when confronted
with nonadult individuals, which were fathered by other males: (1) a new adult resident male in a group of blue monkeys killed
a 5.8-month-old female infant: (2) a new adult resident male in a group of white collared mangabeys injured a 24.0-month-old
female and an 18.9-month-old male severely; they would have died without veterinary care; and (3) the resident male of a group
of drills threatened an 1.8-month-old foreign female infant seriously; efforts to introduce the infant were discontinued.
Pathological explanations are unlikely because the adult males showed no aggression towards own nonadult offspring under the
same captive conditions. By and large, the events support the theory that infanticide is the result of sexual selection among
males. 相似文献
77.
Callus was induced from mature leaf lamina, petiole, stem, and main and branch root of Panax ginseng and maintained under different nutrient and light conditions. Heterogenous callus cultures differentiated organoids and/or embryoids. Embryoids that arose from callus cultures of leaf origin germinated into shoots. The occurrence of the early stages of different regenerated structures was proved on histological level. 相似文献
78.
Spring barley seedling were grown in the dark for 21 d and respiration rates of the whole plant (including the seed), of the
shoots, and of the roots were determined. A function describing the growth and maintenance components of respiration was interpolated
through the experimental points and its parameters in plants under different mineral nutrition were compared. The plants grown
in a complete nutrient solution showed the highest growth rate in the initial phase of development and thus reached the maximum
respiration rate earlier than plants in the other variants. The highest proportion of substrate was respired in the shoot.
Plants grown under deficiency of phosphorus and magnesium had a slower respiration rate than plants grown in the complete
nutrient solution (NP), whereas the amount of respired substrate in plant parts was similar to that recorded in the NP plants.
Plants grown in distilled water showed the lowest growth efficiency and respirated the highest proportion of substrate in
the root. 相似文献
79.
Summary Four monoclonal antibodies that discriminate between structural domains of alpha-(TU-01, TU-04) or beta-(TU-06, TU-12) tubulin and a polyclonal anti-tubulin antibody were used for immunostaining of human spermatozoa using immunofluorescence microscopy. Specificity of antibodies was confirmed by immunoblotting experiments. Antibodies TU-01 and TU-06 uniformly stained the whole tail and the neck, whereas antibodies TU-04, TU-12 showed differential distribution of corresponding epitopes in the stable arrays of flagellar microtubules. Of the monoclonal antibodies used, only TU-12 against the antigenic determinant on C-terminal domain of -tubulin showed strong reactivity with the equatorial segment of the head. The results document a differential exposure of tubulin epitopes at the single-cell level and suggest the existence of distinct tubulin populations in various structural compartments of the human spermatozoon. 相似文献
80.
The paper summarizes data concerning the biology and ecology ofOenanthe aquatica. The species is commonly distributed all over Czechoslovakia, from lowlands to the submontane belt, especially in fishpond and river basins.O. aquatica shows no special relationship to the chemical and physical soil properties, and is well adapted to habitats with a changing water level. The reproduction ofO. aquatica depends on emerging of the bottom. The most developed populations are formed by biennial plants and arise in the year following summer or autumn drainage. 相似文献