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921.
- 1. 1. Larger members of the Polychacta exhibit two contrasting life cycles: semelparous in the Nereidae, iteroparous in most others.
- 2. 2. In semelparous forms environmental interaction determines age at reproduction and fecundity in the single spawning event whereas in iteroparous forms such interaction influences the variable age specific reproductive effort.
- 3. 3. Development of aquaculture has created conditions where organisms are grown under conditions of optimum temperature for growth and unlimited food.
- 4. 4. We present data on the life history responses (reaction norms) of the semelparous Nereis virens in which age at death in natural populations varies between 3 to 8+ years.
- 5. 5. In Nereis virens minimum life span (= generation time) in culture is one year but the lifespan remains modular 12 months without manipulation of photoperiod.
- 6. 6. Environmental temperature plays two roles: i) in conjunction with energy availability to determine “age at first/only reproduction” and secondly as an element (with photoperiod) in the control of gametogenic processes imposing seasonality on the life cycle.
- 7. 7. The observations suggest that long generation time in natural populations of N. virens is associated with reduced growth rate and that low growth rate is associated with reproduction at a larger size.
922.
花粉通量与气象要素的关系及其在预报小麦、玉米产量上的前景 总被引:3,自引:0,他引:3
应用花粉通量测定新技术,于1992~1994年在小麦(Triticum aestivum L.)、玉米(Zea mays L.)传粉期间,应用库尔式收集仪收集和测定空气中的花粉,记录气象资料,进行统计分析,确定小麦、玉米产量与它们最大传粉期空气中花粉浓度的相对应数据,根据这些数据,试图建立禹城实验区小麦和玉米产量预报图式。 相似文献
923.
Holding temperature during the breeding season influences final maturation and egg quality in common wolffish 总被引:3,自引:0,他引:3
Mature common wolffish, Anarhichas lupus L., previously held at 8°C for several months, were exposed to 4, 8 or 12°C throughout the breeding season (November-January). Regardless of temperature, some females ovulated in early-November, but thereafter ovulation was delayed in the 8 and 12°C groups compared with the 4°C group. Temperature experienced by the adult fish also affected egg development and survival even though eggs were incubated at a common temperature of 6°C. The percentages of normally cleaved eggs (at the 32-cell stage) and egg survival to the eyed stage were significantly lower in the 12°C group (8%) than in the 4°C (73%) and 8°C (48%) groups. The differences in egg survival were also significant between the 4 and 8°C groups. Egg survival was also influenced by the length of time the broodfish had been exposed to the different temperatures: at 4 and 12°C there were significant positive and negative relationships, respectively, between egg survival and the number of days for which the female had been exposed to the given temperature prior to ovulation. Temperature experienced by the female did not affect egg size, relative egg weight, fertilization rate, relative fecundity or the number of ovulating individuals. The results indicate that both the timing of ovulation and egg quality of common wolffish are affected by the temperature experienced during the breeding season. 相似文献
924.
InLarix pollen captured by the ovule and rested at the distal end of the micropylar canal is transferred upward to the nucellus before
it develops a pollen tube. This upward movement occurs after the canal is filled with secreted fluid, despite the fact that
the pollen sinks in the fluid. We examined the mechanism of the movement based on the morphology of the canal and its simulation
using pipettes. When a water column moves upward in a waxed pipette, suspended particles also move upward carried by the meniscus.
InL. x eurolepis the inner surface of the integument lining the micropylar canal is coated by a cuticle layer. This layer is further coated
by an integumentary membrane before the fluid is secreted. This membrane, however, becomes distorted or disappears during
fluid secretion. The exposed cuticle and the degenerated hydrophilic nucellar apex may facilitate the movement of the meniscus
toward the nucellus as in the simulated pipette. Pollen is interpreted to move by being carried by the meniscus when the fluid
recedes. 相似文献
925.
Transforming growth factor-β (TGF-β) affects B cells at all stages in development. It appears to be involved in lymphopoiesis and is required for the development of plasma cells secreting all secondary isotypes. Its ability to inhibit proliferation and stimulate apoptosis suggest that it may be involved both in germinal center development and regulation of B-cell proliferation at sites of high antigen load such as the gastrointestinal tract. Although TGF-β appears to be required for the generation of B cells secreting secondary isotypes, it inhibits secretion of IgM and IgA from cells expressing those isotypes. In this regard, TGF-β may alter the level of RNA processing factors either directly or indirectly by inhibiting progression through the cell cycle. One of the best characterized effects of TGF-β is its ability to stimulate isotype switching to IgA in both mouse and man. There is some controversy concerning its mechanism of action in this process, but its critical role is without question. The controversy may stem in part from an inability to separate the effects of endogenous and exogenous TGF-β in the multiple models of isotype switching. The influence of endogenous TGF-β is perhaps best exemplified by analysis of production of the different classes of IgG in mouse strains producing different levels of TGF-β. 相似文献
926.
Fopius arisanus is unusual among hymenopterous parasitoids in males having an obligatory premating period. We confirmed Hagen's (1953) view that sperm takes several days to migrate from the testes to the seminal vesicles. Males mated for the first time only 4 days after emergence, the time that sperm was first ever recorded in the seminal vesicles. In the field, we studied the sexual maturation of F. arisanus males in relation to their behavior. In general, sexually immature males were found in male-only swarms that persisted over several days in host tree canopies. Mature males were usually found in loose aggregations in the vegetation beneath host trees. Females entered these loose aggregations and were mated, whereas they were generally absent from swarms in the canopy. Swarming is therefore apparently not a primary component of the sexual communication system of the species. We describe, for the first time in F. arisanus, the behavioral sequence that accompanies the intersexual communication that leads to mating. Males probably release a volatile chemical that attracts females from a distance, but we have only circumstantial evidence for this. We also postulate that males may form aggregations to amplify the effects of this distance attractant. In the vicinity of males, females release a pheromone that attracts males, a process we demonstrated with female-baited sticky traps. 相似文献
927.
Danilo D. Fernando J. N. Owens Patrick von Aderkas Tokushiro Takaso 《Sexual plant reproduction》1997,10(4):209-216
Pollen tube and female gametophyte interactions in Douglas fir (Pseudotsuga
menziesii) were examined in vitro. Formation of pollen tubes in Douglas fir occurred on a modified Murashige and Skoog medium in which
concentrations of H3BO3 and Ca(NO3)2 were altered and supplemented with sucrose and polyethylene glycol. Addition of 100 μg/ml H3BO3 and 300 μg/ml Ca(NO3)2 resulted in optimum pollen viability. Lack of H3BO3 inhibited pollen tube formation. Addition of H3BO3 and Ca(NO3)2 significantly increased pollen tube formation within one week in culture. Using a medium supplemented with mannitol, viability
of Douglas fir pollen can be sustained for 7 weeks in culture, about the same length of time as in vivo. However, pollen tubes
are not formed. This suggests that the factors responsible for tube formation reside in the external environment of the pollen.
Culture of female gametophytes to examine egg viability and longevity had not been done previously. We found that egg viability
in culture is short-lived, and therefore the window to study and manipulate events of fertilization in Douglas fir is very
limited. In spite of this, about 7% of the female gametophytes that were co-cultured became penetrated by pollen tubes. In
vitro archegonial penetration has been repeatedly achieved, but pollen tubes also penetrated other parts of the female gametophytes.
Pollen tubes also penetrated non-viable eggs. Most female gametophytes were not penetrated because of pollen tube branching
and swelling, failure of tubes to orient towards the female gametophytes, or premature pollen tube death due to plasmolysis.
This report outlines the first attempt towards in vitro fertilization in conifers.
Received: 13 March 1997 / Revision accepted: 6 June 1997 相似文献
928.
929.
Associations in the timing of emergence among the permanent teeth of Boston children were obtained from a mixed longitudinal growth study of 414 Caucasian twin pairs examined annually. Correlations were estimated by the method of maximum likelihood for corresponding left-right teeth, upper-lower teeth, and all paired combinations from central incisor to second molar in a jaw quadrant of each sex. Strong positive correlations in emergence timing prevailed throughout the dentition. Principal component analyses on correlation matrices of jaw quadrant relations for boys and girls in the maxilla and mandible showed that three components effectively explained the emergence associations among the seven permanent teeth in each jaw quadrant. Factor analytic techniques further illustrated the nature of the three components and showed the emergence relations to be essentially the same in the maxilla and mandible for both sexes. The first component was a general maturation factor influencing all of an individual's teeth to be simultaneously early or late in emerging. The remaining two components were a molar factor, affecting almost exclusively the emergence timing of the permanent first and second molars, and a duration factor that affected the duration of the emergence process for non-molar teeth, contrasting particularly the incisors and premolars. 相似文献
930.
Growth of body height and weight and skeletal maturation are discussed, based on 49 male and 61 female Hottentot children aged 3 to 17 years from Warmbad, Namibia (South West Africa) and 124 boys and 113 girls aged 1 to 21 years of related populations, the Rehoboth Basters of Namibia and Cape Coloreds from Cape Town, South Africa. The related populations are taller and heavier than the Hottentots, and have almost the same body height as American blacks and whites at least after the age of 18 years. In the Hottentots and Rehoboth Basters, the mean TW2 skeletal age is always less than the British standard by one or two years in both sexes. In general, the Rehoboth Basters have a skeletal age that is intermediate between Hottentot and British children. In both Hottentots and Rehoboth Basters, the increase in body height shows a linear relation to the skeletal age, and the regression curves are almost parallel in both sexes. The differences in body height and weight between the Hottentots and Rehoboth Basters become greater after the skeletal ages of 15 years for boys and 13 years for girls. 相似文献