全文获取类型
收费全文 | 1512篇 |
免费 | 50篇 |
国内免费 | 3篇 |
出版年
2013年 | 30篇 |
2012年 | 42篇 |
2011年 | 45篇 |
2010年 | 61篇 |
2009年 | 62篇 |
2008年 | 60篇 |
2007年 | 86篇 |
2006年 | 34篇 |
2005年 | 42篇 |
2004年 | 23篇 |
2003年 | 22篇 |
2001年 | 13篇 |
2000年 | 10篇 |
1999年 | 12篇 |
1998年 | 29篇 |
1997年 | 44篇 |
1996年 | 48篇 |
1995年 | 32篇 |
1994年 | 21篇 |
1993年 | 29篇 |
1992年 | 21篇 |
1991年 | 33篇 |
1990年 | 34篇 |
1989年 | 39篇 |
1988年 | 29篇 |
1987年 | 29篇 |
1986年 | 17篇 |
1985年 | 27篇 |
1984年 | 26篇 |
1983年 | 18篇 |
1982年 | 26篇 |
1981年 | 16篇 |
1980年 | 23篇 |
1979年 | 29篇 |
1978年 | 25篇 |
1977年 | 22篇 |
1976年 | 20篇 |
1975年 | 32篇 |
1974年 | 24篇 |
1973年 | 19篇 |
1972年 | 35篇 |
1971年 | 36篇 |
1969年 | 12篇 |
1968年 | 13篇 |
1959年 | 9篇 |
1957年 | 20篇 |
1956年 | 16篇 |
1955年 | 11篇 |
1954年 | 10篇 |
1953年 | 10篇 |
排序方式: 共有1565条查询结果,搜索用时 343 毫秒
991.
992.
993.
Abstract. 1. The eggs of Rhopalosiphum insertum (Walker) showed a seasonal increase in cold-hardiness under field conditions. Their supercooling point fell from -35°C in November to below -40°C in January, then rose to-35°C or above by March.
2. Laboratory experiments demonstrated that both temperature and date affected cold-hardiness of the eggs. The supercooling points of eggs kept at 16 h photoperiod or in darkness did not, however, differ significantly.
3. Eggs brought from the field into warm, long-day conditions would not hatch until after mid-January. After this date, per cent hatch was significantly greater in 16 h photoperiod than in darkness; it did not differ between eggs kept at 5 or 0°C, but was reduced at -5°C.
4. It is concluded that eggs of Rinserturn are in diapause until mid-January, and that hatching rate and cold-hardiness are determined by separate environmental factors. 相似文献
2. Laboratory experiments demonstrated that both temperature and date affected cold-hardiness of the eggs. The supercooling points of eggs kept at 16 h photoperiod or in darkness did not, however, differ significantly.
3. Eggs brought from the field into warm, long-day conditions would not hatch until after mid-January. After this date, per cent hatch was significantly greater in 16 h photoperiod than in darkness; it did not differ between eggs kept at 5 or 0°C, but was reduced at -5°C.
4. It is concluded that eggs of Rinserturn are in diapause until mid-January, and that hatching rate and cold-hardiness are determined by separate environmental factors. 相似文献
994.
Each of 8 snails in 2 groups of Bulinus (Physopsis) globosus,1 group raised in isolation and 1 group raised in community,were paired for 14 consecutive days with a male-acting partnersnail. In each group, the experimental snails, which were notallowed to act as males, were able to copulate as females onapproximately 94% of the days paired. Two copulations as female,with the same male partner, occurred on 50% of the days thatthe snails were paired, in the 2 groups combined. Non-receptivefemale behaviour by the experimental snails occurred frequently,and copulation was prevented by such behaviour during 6 pairings,3 in each group. Young B. (P.) africanus first copulated as females when theywere 3133 days old. The accessory sex glands of the femalereproductive tracts of these young female-acting snails containedmoderate to large amounts of secretion. B. (P.) africanus, which were raised in pairs, laidcross-fertilizedeggs in isolation for an average of 76 days, and 1559 eggs/snailwere deposited before cross-fertilization ceased. Cross-fertilizedeggs were produced for as long as 120 days. After 1 copulation as female, virgin B. (P.) africanus laidcross-fertilized eggs for an average of 78 days and deposited3654 eggs/snail before crossfertilization ceased. Cross-fertilizedeggs were produced for as long as 113 days. After 2 copulationsas female, 1 copulation on each of 2 consecutive days, virginB. (P.) africanus laid cross-fertilized eggs for an averageof 102 days and produced 4397 eggs/snail before cross-fertilizationceased. Cross-fertilized eggs were produced for as long as 123days. Snails which were homozygous for an allele governing mantlepigment pattern were raised with a partner which was homozygousfor a different pigment pattern. Young produced in a 4-day periodafter the snails were isolated were 100% heterozygous. The snailswere then rearranged into pairs with a partner of the same genotypefor 4 days, during which time 26% of the young produced werehomozygous. The snails were again isolated for 4 days, and 49%of the young produced during this 4-day period were homozygous.The results of this experiment strongly suggest that multipleoutcrossing occurred. In B. (P.) africanus, stored allosperm were used to fertilizeeggs after 1, 4 and 7 weeks of starvation; after 1 and 4 weeksof 15°C low temperature and 4 weeks of 15°C + 4 weeksof 10°C low temperature; and after 1 and 4 weeks of desiccation.After 8 weeks of desiccation, 2 of 3 surviving snails reproducedby self-fertilization and 1 snail did not reproduce. Too fewsnails survived 8 weeks of desiccation for a conclusion to bereached on the ability of allosperm to survive. (Received 1 June 1984; 相似文献
995.
996.
A captive colony of green sea turtles, Chelonia mydas, has beenmaintained and observed at a commercial sea turtle farm on GrandCayman Island, B.W.I., since 1973. Observations of this breedingcolony show that the mating and nesting behaviour of the captivegreen sea turtle is similar to that observed in wild populations.Evidence indicates that mating observed prior to a female'snesting in a given season determines the hatchabilityof thatseason's egg production. Annual per female egg production ofthe captive colony appears to be two to five times greater thanthat reported for wild colonies. Observations on the reproductivebiology of green sea turtles hatched and raised under farm conditionssuggests that the minimum age of sexual maturity is eight tonine years of age. The number of eggs per nest, the number ofnests per season per female and hatch rate tend to increasewith successive seasons nesting for these turtles reaching sexualmaturity. 相似文献
997.
998.
In a survey of shell-boring polychaeles of New England, representativesof five families were found: Spionidae, Cirratulidae, Capitellidae,Terebellidae, and Sabellidae. The five spionid species were studied extensively. These includePolydora commensalis Andrews, found only in shells occupiedby hermit crabs, P. concharum Verrill, P. socialis (Schmarda), P. websleri Hartman, and Boccardia hamata (Webster) , foundin various types of shell. Breeding periods were defined andthe larval development described for each species. Polydoraconcharum deposits egg capsules inwinter months, while the oilierpolydorids spawn in spring or summer. Morphology of planktoniclarvae is distinct in each species, although P. websteri larvaemay be confused with non-boring species such as P. ligni Webster.Polydora socialis adults are found in both shells and sedimentand have a unique grinding apparatus, a gizzard, between theesophagus and intestine. The cirratulid, Dodecaceria sp., follows an asexual mode ofreproduction. The syslematics of Dodecaceria is complex owingto multiple modes of reproduction. No sexually mature individualswere observed during the course of this study. Asexual budswerefound in the Fall. A sabellid, Pseudopolamilla reniformis (Miiller), is commonlyfound in theshells of Placopeclen magellanicits (Gmelin) inMaine waters. Its reproductionis unknown. 相似文献
999.
JERE H. LIPPS JAMES W. VALENTINE 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1970,3(3):279-286
Foraminifera are recorded as feeding chiefly upon bacteria, small diatoms, and nannoplankton in a wide variety of marine environments. Thus their food items are usually below 50 μ and commonly below 25 μ in size. Predators upon Foraminifera range from highly specialized microcarnivores that feed largely on Foraminifera to less selective ones that include Foraminifera in a mixed diet and to generalized feeders that ingest Foraminifera along with much other material.
Foraminifera thus form part of a key link in marine food chains, assimilating energy available from minute autotrophs and also retrieving energy available during the final stages of degradation of organic debris. In turn, they support a variety of larger organisms and thus contribute to the diversity and secondary productivity of ecosystems. 相似文献
Foraminifera thus form part of a key link in marine food chains, assimilating energy available from minute autotrophs and also retrieving energy available during the final stages of degradation of organic debris. In turn, they support a variety of larger organisms and thus contribute to the diversity and secondary productivity of ecosystems. 相似文献
1000.