全文获取类型
收费全文 | 4056篇 |
免费 | 304篇 |
国内免费 | 1篇 |
出版年
2023年 | 42篇 |
2022年 | 75篇 |
2021年 | 143篇 |
2020年 | 111篇 |
2019年 | 114篇 |
2018年 | 137篇 |
2017年 | 132篇 |
2016年 | 168篇 |
2015年 | 231篇 |
2014年 | 268篇 |
2013年 | 299篇 |
2012年 | 360篇 |
2011年 | 316篇 |
2010年 | 200篇 |
2009年 | 187篇 |
2008年 | 216篇 |
2007年 | 225篇 |
2006年 | 163篇 |
2005年 | 172篇 |
2004年 | 177篇 |
2003年 | 124篇 |
2002年 | 143篇 |
2001年 | 36篇 |
2000年 | 19篇 |
1999年 | 32篇 |
1998年 | 27篇 |
1997年 | 40篇 |
1996年 | 20篇 |
1995年 | 21篇 |
1994年 | 17篇 |
1993年 | 15篇 |
1992年 | 8篇 |
1991年 | 6篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 9篇 |
1985年 | 6篇 |
1984年 | 12篇 |
1983年 | 5篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1977年 | 9篇 |
1976年 | 5篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1961年 | 3篇 |
排序方式: 共有4361条查询结果,搜索用时 787 毫秒
31.
Alejandro Vazquez-Tello Makoto Hidaka Takeshi Uozumi 《Plant Cell, Tissue and Organ Culture》1995,40(2):169-177
Embryogenic cell suspensions of Lavatera thuringiaca L. were established from leaf petiole and shoot regeneration was achieved when cells were plated on medium without growth regulators. We tested three methods for protoplast culture, isolated from a one-year old embryogenic cell suspension, to determine the best conditions for L. thuringiaca protoplast culture and shoot regeneration. The highest protoplast plating efficiency was obtained with the agaroseembedded method, reaching 30%, while the nursing culture method gave 5% when the protoplasts were plated over Whatman paper No. 2. However, the same nursing culture failed to produce protoplast-derived microcalluses when the protoplasts were plated on a nitrocellulose filter. The liquid thin layer method gave the lowest plating efficiency with only 0.5%. Shoot regeneration from protoplast-derived microcalluses was achieved in two steps; first, globular embryo development was favored in medium low in auxin (2,4-d and BA at 0.01 and 0.05 mg 1-1, respectively), second, the globular embryos further differentiate into shoots in medium without growth regulators or in medium containing GA3 (0.5 to 1.0 mg 1-1).Abbreviations 2,4-d
2,4-dichlorophenoxyacetic acid
- BA
benzyladenine
- GA3
gibberellic acid
- NAA
-naphthaleneacetic acid
- IBA
indole-3-butyric acid 相似文献
32.
Coloration of phyllopods varies from place to place and from one life stage to another. It ranges from translucent or whitish through gray, blue, green, orange, and reddish. Here, we present experimental evidence for a food- dependent color pattern inThamnocephalus platyurus Packard. The presence or absence of the synthetic pigment trans — — carotene in a baker's yeast diet was the controlling factor. All the 24 old larvae used in the experiment were whitish in color. From day 6 until the end the experiment (day 11), 100% of the shrimps under a diet with synthetic trans — — carotene (treatment 1) exhibited a characteristic color pattern which consisted of an orange color in the cercopods, and in all theracopods; the rest of the body exhibited no particular color. In comparison, 100% of the shrimps under a diet without synthetic trans — — carotene (treatment 2) were whitish throughout the body. In females from treatment 1, the ovaries and oocytes were green-bluish, while in females from treatment 2 the ovaries and oocytes were whitish. No significant differences in survival and growth were found, except that at day 9, there was a significant difference in growth, the females with the synthetic trans — — carotene group growing faster. 相似文献
33.
We argue that the Würm III glaciation eradicated possible European populations ofS. torvicornis, and that today, a reconquest of Europe takes place on two fronts.A western wave has reached the Pyrenees, an eastern one now occupies most of eastern and northern Europe. The western route probably started in the Maghreb, the eastern one in the Levant and the Ponto-Caspian. Animals in the west had to move north by crossing, at right angles, a series of east-west oriented river valleys and progressed slowly; animals in the east could move up river valleys extending north-east, and moved quickly. Italy was not occupied, becauseS.torvicornis is a warm stenotherm, and by the time the climate had warmed sufficiently for it to reach the southern shore of the mediterranean (ca 6000 BP), the gap with Italy was probably too large for a crossing. Cold-loving species (of the generaBranchipus, Chirocephalus) conversely, and could freely flow across the Central Mediterranean at low sea-levels (ca 12000 BP), and now occur in Italy (and the rest of Europe) as well as in Northern Africa.A prediction of our hypothesis is that the pioneer populations in Spain and Central Europe should have been isolated longest. Thisis tested and confirmed by their comparative morphology, and two subspecies,S. t. torvicornis andS. t. bucheti are reinstated.A gap across the Nile Valley where onlyS. rubricaudatus seems to occur, deserves further study. 相似文献
34.
35.
Diversity, abundance or rare species as a target for the conservation of mammalian carnivores: a case study in Southern Spain 总被引:6,自引:2,他引:4
Alejandro Travaini Miguel Delibes Pablo Ferreras Francisco Palomares 《Biodiversity and Conservation》1997,6(4):529-535
Management goals in protected areas and/or communities usually include diversity as one of the most valuable and confident criteria. Nevertheless, the use of diversity and related indices as a means of evaluating successful management practices could produce conflicting results. Here we report a case study in one of the most important European protected areas. After 6 years of intensive conservation management of the Don~ana National Park, the general abundance and numbers of the target single-species conservation plan (the Iberian lynx) increased, although carnivore community diversity and evenness decreased. This was a result of a disproportionate increase of an oportunistic native species, the red fox. We propose the combined use of diversity, richness and evenness indices when monitoring management practices such as those reported here. 相似文献
36.
Maeda-Martnez Alejandro M. Obregn-Barboza Hortencia Garca-Velazco Humberto 《Hydrobiologia》1997,359(1-3):63-69
This paper reports new distribution records of large branchiopods for Mexico following a three year survey of the Baja California
peninsula. The occurance of the anostracans Thamnocephalus mexicanus (Linder, 1941) and T. platyurus (Packard, 1877), the
notostracan Lepidurus lemmoni (Holmes, 1894), and the spinicaudatans Eulimnadia cylindrova (Belk, 1989) and E. texana (Packard,1871),
all represent the first records for the peninsula. An undescribed species of the anostracan genus Streptocephalus is recorded
from the state of Baja California (Norte). The occurrence of the notostracan genus Triops and four other anostracan species
on the peninsula is also confirmed. The conchostracan Cyclestheria hislopi (Baird, 1859) is recorded from the state of Quintana
Roo. The collections of Lepidurus and Cyclestheria are the first records for México. These records increase the number of
species of large branchiopods reported fromMéxico to 36: 20 Anostraca, 3 Notostraca, 11 Spinicaudata, and 2Laevicaudata.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
37.
A review on salinity adaptation of marine molluscsbased on mainly Russian scientific literature ispresented. The existence of two relativelyindependent systems of adaptation to extreme(resistance level) and moderate (tolerance level)changes of environmental salinity was shown. Theresistance of molluscs is based mainly on an impededwater-salt exchange with the external medium due tomantle cavity hermetization. The tolerance ofmolluscs is determined by cellular mechanisms ofadaptation. Reversible changes of protein and RNAsynthesis, alteration of the pattern of multiplemolecular forms of different enzymes, and theregulation of ionic content and cell volume wereshown to be of importance for the above mentionedmechanisms. The efficiency of resistance andtolerance adaptations to salinity changes may varyin different species and in different colourphenotypes of the same species (intrapopulationalpolymorphism). Parasites (trematodes) may suppressthe resistance of the mollusc-host to extremesalinity changes without effecting the host'scapacity for adaptive changes in salinitytolerance. 相似文献
38.
39.
Summary A large pedigree with a satellited Yq chromosome is described, Q, C, and NOR banding were performed. Family C proband suffers from a Klinefelter syndrome. 相似文献
40.
Patterns of predation in a free-ranging troop of stumptail macaques (Macaca arctoides): Relations to the ecology II 总被引:2,自引:2,他引:0
Data on patterns of systematic and ocassional hunting of birds, mammals, reptiles, mollusks and insects by Stumptail macaques
are reported for a period of ten months. Systematic hunting of water snails, terrestrial spiders, and land worms was conducted
by all age classes, except infants, and both sexes. Of the occasional hunting of birds, large lizards, large frogs, and field
mice, the adult females conducted 70%, the adult males 12%, the two year old females 12%, and the two year old males 6%. The
differences between males and females were statistically significant (.05 confidence level). Females dominated the hunt and
were more interested in meateating than the males. This contrasts strikingly with the data reported for baboons and chimpanzees
in which the males dominate the hunt.
Of all the prey hunted ocassionally, 76% was shared. The differences between shared and not sared prey were statistically
significant (.05 confidence level). All age classes, including infants, participated in meat-sharing. Three types of meat-sharing
are described: mother-offspring, hunter-close-friend, and piece-dropping. The prey was shared with genetic relatives, and
with close and sistant friends in this order. Pearson’s correlation coeficients between rank of hunter and number of hunts
and between rank of hunter and number of individuals with whom the prey was shared yielded +.866 and +.890 respectively. Meat-sharing
seems to be similar to that observed for baboons but some differences exist between baboons and chimpanzees on the one hand
and Stumptails on the other. Dominance relations in our Stumptails seem to act as the context determining the direction and
the type of sharing.
An increase in hunting activity during the study period is suggested to be the result of the prey’s migratory and breeding
patterns, of environmental changes, and of the high activity scores and physiological states of the adult-females in the troop.
Although not hunted, reactions to snakes, iguanas, scorpions, and gulls are also described.
This work was supported by grants from Behavioral Sciences Foundation and by NSF Grant No. GB-42235. 相似文献