全文获取类型
收费全文 | 319篇 |
免费 | 21篇 |
学科分类
生物科学 | 340篇 |
出版年
2014年 | 4篇 |
2013年 | 4篇 |
2011年 | 7篇 |
2010年 | 13篇 |
2009年 | 7篇 |
2008年 | 7篇 |
2007年 | 14篇 |
2006年 | 12篇 |
2005年 | 13篇 |
2004年 | 6篇 |
2003年 | 6篇 |
2002年 | 5篇 |
2001年 | 10篇 |
2000年 | 8篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 8篇 |
1996年 | 4篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 11篇 |
1979年 | 5篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1973年 | 5篇 |
1972年 | 7篇 |
1971年 | 8篇 |
1969年 | 3篇 |
1968年 | 3篇 |
1966年 | 8篇 |
1959年 | 3篇 |
1958年 | 4篇 |
1957年 | 9篇 |
1956年 | 4篇 |
1955年 | 3篇 |
1954年 | 5篇 |
1953年 | 4篇 |
1950年 | 5篇 |
排序方式: 共有340条查询结果,搜索用时 15 毫秒
1.
2.
3.
Interrelations Among Water and Energy Relations of Reptilian Eggs, Embryos, and Hatchlings 总被引:1,自引:0,他引:1
Reptilian eggs previously categorized with respect to the flexibilityof eggshells appear to fall into two groups: endohydric eggsare those that are invested, by the female parent at the timeof oviposition, with all of the water necessary to completeembryogenesis; and ectohydric eggs which need to absorb waterfrom the nest medium tocomplete embryogenesis. Eggs of the Galapagosland iguana are unusual among most lepidosaurians by havingvery permeable parchment shells, but containing a large albumen(apparently serving as a reservoir of water for the embryo).It appears that the eggs of Galapagos land iguanas can exploitan endohydric habit without the rigid, impermeable shell seenin other endohydric eggs. This ability appears to be mediatedby two factors: eggs of Galapagos land iguanas are laid in drysoils which are essentially impermeable to water, and the verylarge eggs of land iguanas have a relatively small surface areatovolume ratio which results in a relatively small exchangeof water across the eggshell. It appears too that the waterrelations of Galapagos land iguana eggs will affect the energeticsof both the contained embryo and the subsequent hatchling. Withoutadequate water, land iguana eggs will produce hatchlings thatare both smaller and possess less fat to sustain thejuvenileduring the first year of life. 相似文献
4.
The breeding biology of two island populations of the Northwestern Crow was studied in British Columbia over a period of five years. Both populations laid an average of four eggs per clutch, of which approximately 75% hatched. More young survived to fledge on Mandate than on Mitlenatch Island. The young from nests close (< 100 m) to the beach on Mitlenatch had a higher survival rate than those from further inland. The number of eggs lost or failing to hatch was lowest in clutches of four eggs. There was a non-significant trend for chick survival to be inversely related to clutch size. The net result was that clutches of five eggs produced no more young than did clutches of four eggs. It is suggested that the major limiting factor on clutch size is food availability but the decreased hatching success of any remaining eggs once hatching has started is also a contributing factor. 相似文献
5.
HOWARD E. BUHSE 《The Journal of eukaryotic microbiology》1966,13(3):429-435
SYNOPSIS. Tetrahymena vorax is a small polymorphic holotrichous ciliate capable of transforming to macrostomes when microstomes are washed and suspended with prey in distilled water. Extrinsic factors having an effect on this transformation were examined; maximum yields of macrostomes (in excess of 90%) were obtained under the following conditions: populations of both prey, T. pyriformis, and potential predator, T. vorax microstomes, were grown on Loefer's medium for 48 hours prior to washing in distilled water. The density of the prey was adjusted to 300,000 cells/ml and the predator density to 2,000–3,000 cells/ml. Five ml of prey suspension and 5 ml of T. vorax microstome suspension were mixed together in a large petri dish because a high surface-volume ratio is important for high yields of macrostomes. The pH was adjusted to 6.0 and the petri dish was placed at 20 C for 12 hr. Macrostomes then appeared about 6 hr after addition of the prey. A dialyzable, heat stable substance released by the prey which can induce the microstome-macrostome transformation was isolated. This material was effective after being stored for weeks in the cold; its activity was not affected by the protein digesting enzymes pepsin or trypsin. This factor was called stomatin because its first visible effect in producing microstome-macrostome transformation appeared to be to incite reorganization of the oral structures. 相似文献
6.
To test the hypothesis that early maturation of barley affords it some resistance to cereal aphids, the colonisation, survival, growth, reproduction and emigration of M. dirhodum on barley was measured and compared with previously published results for this aphid on wheat.
Barley plants were colonised as readily as wheat plants. Survival to maturity was similar on both hosts. Relative growth rate was higher on wheat than on barley for most of the season while reproduction was similar on both. Emigration, measured alate production, was significantly higher on mature wheat than on mature barley.
It is proposed that the increase of M. dirhodum populations on barley is normally terminated early in the year directly plant maturation and consequent drying out of the leaves, resulting in the death of aphids. This is different to the situation on wheat, which matures later, and where emigration following plant induced alate induction has been shown to be the major factor affecting the decline in population growth.
These results support the hypothesis that early maturation of barley provides some resistance to M. dirhodum. 相似文献
Barley plants were colonised as readily as wheat plants. Survival to maturity was similar on both hosts. Relative growth rate was higher on wheat than on barley for most of the season while reproduction was similar on both. Emigration, measured alate production, was significantly higher on mature wheat than on mature barley.
It is proposed that the increase of M. dirhodum populations on barley is normally terminated early in the year directly plant maturation and consequent drying out of the leaves, resulting in the death of aphids. This is different to the situation on wheat, which matures later, and where emigration following plant induced alate induction has been shown to be the major factor affecting the decline in population growth.
These results support the hypothesis that early maturation of barley provides some resistance to M. dirhodum. 相似文献
7.
Leaf expansion of four sunflower (Helianthus annuus L) cultivars in relation to water deficits. I. Patterns during plant development 总被引:3,自引:3,他引:0
Abstract. The influence of a slow stress and recovery cycle on the pattern of leaf expansion in four diverse sunflower cultivars ( Helianthus annuus L. cvs. Hysun 31, Havasupai, Hopi and Seneca) was studied in a glasshouse. Stress had no significant effect on the time of flower bud emergence and anthesis, or on final leaf number, but delayed the appearance of leaves at high insertions in all cultivars except Hysun 31.
Leaf expansion was markedly reduced as the predawn leaf water potential decreased from −0.35 to −0.60 MPa, and the predawn turgor pressure decreased from 0.3 to 0.2 MPa, and expansion ceased at a predawn leaf water potential of about −1.0 MPa, i.e. when the predawn turgor pressure reached zero.
The leaves most reduced in final size when water was withheld were those at the insertions which grew the most rapidly in unstressed plants. The maximum reduction in final leaf size of 25–35% was similar in all cultivars and was due to retardation of the rate of leaf expansion: the duration of leaf expansion was actually increased by stress. However, leaves that were initiated during stress, but emerged after rewatering, had final leaf areas at least equal to those in the unstressed plants: in the cultivar Seneca, the final size of leaves of high insertion was significantly greater in stressed than unstressed plants, whereas in the three other cultivars the final leaf sizes were similar in both treatments. All four cultivars examined adjusted osmotically to the same degree, but leaf water potentials in one, Seneca, increased more rapidly after rewatering than in the other three, and this may have contributed to the greater relative leaf size in the leaves of high insertion in this cultivar. 相似文献
Leaf expansion was markedly reduced as the predawn leaf water potential decreased from −0.35 to −0.60 MPa, and the predawn turgor pressure decreased from 0.3 to 0.2 MPa, and expansion ceased at a predawn leaf water potential of about −1.0 MPa, i.e. when the predawn turgor pressure reached zero.
The leaves most reduced in final size when water was withheld were those at the insertions which grew the most rapidly in unstressed plants. The maximum reduction in final leaf size of 25–35% was similar in all cultivars and was due to retardation of the rate of leaf expansion: the duration of leaf expansion was actually increased by stress. However, leaves that were initiated during stress, but emerged after rewatering, had final leaf areas at least equal to those in the unstressed plants: in the cultivar Seneca, the final size of leaves of high insertion was significantly greater in stressed than unstressed plants, whereas in the three other cultivars the final leaf sizes were similar in both treatments. All four cultivars examined adjusted osmotically to the same degree, but leaf water potentials in one, Seneca, increased more rapidly after rewatering than in the other three, and this may have contributed to the greater relative leaf size in the leaves of high insertion in this cultivar. 相似文献
8.
LANG JUDITH C.; LASKER HOWARD R.; GLADFELTER ELIZABETH H.; HALLOCK PAMELA; JAAP WALTER C.; LOSADA FREDDY J.; MULLER ROBERT G. 《Integrative and comparative biology》1992,32(6):696-706
Western Atlantic coral reefs were differentially affected bya mass bleaching (discoloration) event in 1987. We periodicallyassessed the "appearance" of zooxanthellate organisms betweenDecember 1987 and June 1988 at nine conspicuously affected sitesin the Bahamas, Florida, St. Croix, and Venezuela, using a standardizedpoint-count technique. Three to four months after the localinitiation of the event, the "bleached" state was still presentin one to three of the most abundant reef coral taxa and ina few of the less common species (n = 5 sites). "Recovery" occurredsomewhat faster at shallower depths, at least in the Bahamasand Florida. Scleractinian corals which were "prolonged bleachers"had foliaceous or massive, rather than branching, morphologies."Bleached" points disappeared from the point counts after $6to $8 months. Long-term field data on spatial and temporal variability inthe dynamics of zooxanthellate organisms would help us to understandthe ecological consequences of bleaching. More generally, weneed to distinguish anthropogenic changes in the structure andfunctioning of reef ecosystems from those which occur naturally.Point-count techniques are well suited for collaborative studiesinvolving rapid quantification of coloration states and healthin reef corals. 相似文献
9.
The various techniques in common use for conservation and restorationof depleted sea turtle populations are reviewed, namely: banninginternational commerce; operating artificial hatcheries, bothin the natural beach environment and in styrofoam and othertypes of incubators; "head-starting" of hatchlings in captivity;protection of nesting females by means of beach patrols; andtranslocation of eggs or hatchlings to distant areas from whichturtles have been extirpated or to which it is desired to introducenew colonies. The difficulties of monitoring the results ofall of these techniques are discussed, and potential dangersor disadvantages of each approach are reviewed. It is concludedthat, until unequivocal data become available, turtle conservationistsshould continue to pursue common sense or logically sound restorationprograms, but should constantly re-evaluate their actions inthe light of the latest available knowledge and modify or desistfrom current approaches as necessary. 相似文献
10.